The five great lakes

The Great Lakes are a collection of freshwater lakes located in eastern North America, on the Canada – United States border. Consisting of Lakes Superior, Michigan, Huron, Erie, and Ontario, they form the largest group of freshwater lakes on Earth by surface.[1][2] They are sometimes referred to as the "Third Coast" by some citizens of the United States. Because of their size, types of ecosystems, and large abundances of beaches and coastal wetlands along their coasts, some regard them as inland seas or as one sea.[citation needed]

The Great Lakes hold 20% of the world's fresh water.[citation needed]

[edit] Geography
The Great Lakes region contains not only the five main lakes themselves, but also numerous minor lakes and rivers, as well as approximately 35,000 islands.

[edit] Bathymetry
Lake Erie Lake Huron Lake Michigan Lake Ontario Lake Superior
Surface area 9,940 sq mi (25,700 km2) 23,010 sq mi (59,600 km2) 22,400 sq mi (58,000 km2) 7,540 sq mi (19,500 km2) 31,820 sq mi (82,400 km2)
Water volume 116 cu mi (480 km3) 849 cu mi (3,540 km3) 1,180 cu mi (4,900 km3) 393 cu mi (1,640 km3) 2,900 cu mi (12,000 km3)
Elevation[3] 571 ft (174 m) 577 ft (176 m) 577 ft (176 m) 246 ft (75 m) 600 ft (180 m)
Average depth[4] 62 ft (19 m) 195 ft (59 m) 279 ft (85 m) 283 ft (86 m) 483 ft (147 m)
Maximum depth 210 ft (64 m) 770 ft (230 m) 923 ft (281 m) 808 ft (246 m) 1,332 ft (406 m)
Major settlements[5] Buffalo, NY
Cleveland, OH
Erie, PA
Toledo, OH Sarnia, ON
Owen Sound, ON
Alpena, MI
Port Huron, MI
Bay City, MI Chicago, IL
Gary, IN
Michigan City, IN
Muskegon, MI
Green Bay, WI
Milwaukee, WI Hamilton, ON
Kingston, ON
Oshawa, ON
Rochester, NY
Toronto, ON
Mississauga, ON Duluth, MN
Sault Ste. Marie, ON
Sault Ste. Marie, MI
Thunder Bay, ON
Marquette, MI
Superior, WI
Relative elevations, average depths, maximum depths, and volumes of the Great Lakes.

Notes: The area of each rectangle is proportionate to the volume of each lake. All measurements at Low Water Datum.
Source: EPA[3]

[edit] Lake Michigan-Huron
Lakes Michigan and Huron are hydrologically a single lake, sometimes called Lake Michigan-Huron; they have the same surface elevation of 577 feet (176 m),[6] and are connected not by a river but by the 295-foot (90 m) deep Straits of Mackinac.[4]

[edit] Rivers

Sarnia, ON, the largest city on Lake Huron, and the St. Clair River shoreline. The smokestacks of Chemical Valley along the river are visible in the background.The St. Marys River connects Lake Superior to Lake Huron.
The St. Clair River connects Lake Huron to Lake St. Clair.
The Detroit River connects Lake St. Clair to Lake Erie. Through its middle is the border between Canada and the United States.
The Niagara River, including Niagara Falls, connects Lake Erie to Lake Ontario.
The St. Lawrence River connects Lake Ontario to the Atlantic Ocean.
[edit] Other bodies of water
Georgian Bay is a large bay located within Lake Huron, separated by the Bruce Peninsula and Manitoulin Island. It contains the majority of the islands of the Great Lakes, with a count of approximately 30,000. The North Channel, a narrower westerly extension of Georgian Bay, separates Manitoulin Island from mainland Northern Ontario.
The Straits of Mackinac connects Lake Michigan to Lake Huron.
The Welland Canal connects Lake Erie to Lake Ontario, bypassing the Niagara River which cannot be fully navigated due to the presence of Niagara Falls.
Lake St. Clair is the smallest lake in the Great Lake system but due to its relatively small size (compared to the five "Great Lakes"), it is rarely, if ever, considered a Great Lake.
Lake Nipigon to the north of Lake Superior was formed by an extension or aulacogen of the Midcontinent Rift System which also formed Lake Superior, so the two lake beds are connected by shared geology. Lake Nipigon is sometimes called the sixth Great Lake.

The shoreline of a beach in the Apostle Islands, Lake Superior[edit] Islands
Dispersed throughout the Great Lakes are approximately 35,000 islands. The largest among them is Manitoulin Island in Lake Huron, the largest island in any inland body of water in the world. The second-largest island is Isle Royale in Lake Superior. Both of these islands are large enough to contain multiple lakes themselves — Manitoulin Island's Lake Manitou is listed in the Guinness Book of World Records as the world's largest lake located on a freshwater island.

[edit] Connection to ocean and open water
The Saint Lawrence Seaway and Great Lakes Waterway opened the Great Lakes to ocean-going vessels. The move to wider ocean-going container ships — which do not fit through the locks on these routes — has limited shipping on the lakes. Despite their vast size, large sections of the Great Lakes freeze over in winter, interrupting most shipping. Some icebreakers ply the lakes.

The Great Lakes are also connected to the Gulf of Mexico by way of the Illinois River (from Chicago) and the Mississippi River. An alternate track is via the Illinois River (from Chicago), to the Mississippi, up the Ohio, and then through the Tennessee-Tombigbee Waterway (combination of a series of rivers and lakes and canals), to Mobile Bay and the Gulf. Commercial tug-and-barge traffic on these waterways is heavy.

Pleasure boats can also enter or exit the Great Lakes by way of the Erie Canal and Hudson River in New York. The Erie Canal connects to the Great Lakes at the east end of Lake Erie (at Buffalo, NY) and at the south side of Lake Ontario (at Oswego, NY).

[edit] Boundaries
The lakes are bound by the Canadian province of Ontario and the U.S. states of Minnesota, Wisconsin, Michigan, Illinois, Indiana, Ohio, Pennsylvania, and New York. However, not all of the lakes border on all of these regions. Four of the five lakes form part of the Canada-United States border; the fifth, Lake Michigan, is contained entirely within the United States. The Saint Lawrence River, which marks the same international border for a portion of its course, is the primary outlet of these interconnected lakes, and flows through Quebec and past the Gaspé Peninsula to the northern Atlantic Ocean.

[edit] Great Lakes Circle Tour
The Great Lakes Circle Tour is a designated scenic road system connecting all of the Great Lakes and the St. Lawrence River.[7]

[edit] Name origins
Lake Erie Lake Huron Lake Michigan Lake Ontario Lake Superior
Origins of Name Erie (tribe); shorten form of Iroquoian word Erielhonan or “long tail” Named by French explorers for inhabitants in the area, Wyandot or “Hurons” Likely from the Ojibwa word mishigami meaning “great water” Wyandot (Huron) word ontarío meaning “Lake of Shining Waters” (Ontara = beautiful, Ontario = beautiful lake) English translation of French term “lac supérieur” ("upper lake"), referring to its position above Lake Huron, Ojibwe called it "Gitchigumi"

[edit] Statistics
The Great Lakes contain roughly 22% of the world’s fresh surface water: 5,472 cubic miles (22,810 km3), or 6.0×1015 U.S. gallons (2.3×1016 liters). This is enough water to cover the 48 contiguous U.S. states to a uniform depth of 9.5 feet (2.9 m). However, only 2% of this volume is replaced each year, causing water levels to fall in recent years as the water undergoes heavy human use[citation needed]. Although the lakes contain a large percent of the world's fresh water, the Great Lakes supply only a small portion of America's drinking water (roughly 4.2%).[citation needed]

The combined surface area of the lakes is approximately 94,250 square miles (244,100 km2)—nearly the same size as the United Kingdom, and larger than the U.S. states of New York, New Jersey, Connecticut, Rhode Island, Massachusetts, Vermont and New Hampshire combined.

The Great Lakes coast measures approximately 10,500 miles (16,900 km);[4] however, the length of a coastline is impossible to measure exactly and is not a well-defined measure (see Coastline paradox).

[edit] Geology

A diagram of the formation of the Great Lakes.It has been estimated that the foundational geology which created the conditions shaping the present day upper Great Lakes was laid from 1.1 to 1.2 billion years ago,[4][8] when two previously fused tectonic plates split apart and created the Midcontinent Rift. A valley was formed providing a basin that eventually became modern day Lake Superior. When a second fault line, the Saint Lawrence rift, formed approximately 570 million years ago,[4] the basis for Lakes Ontario and Erie were created, along with what would become the St. Lawrence River.

The Great Lakes are estimated to have been formed at the end of the last ice age (about 10,000 years ago), when the Laurentide ice sheet receded. The retreat of the ice sheet left behind a large amount of meltwater (see Lake Agassiz) which filled up the basins that the glaciers had carved, thus creating the Great Lakes as we know them today.[9] Because of the uneven nature of glacier erosion, some higher hills became Great Lakes islands. The Niagara Escarpment follows the contour of the Great Lakes between New York and Wisconsin. Land below the glaciers "rebounded" as it was uncovered.[10] Because the glaciers covered some areas longer than others, this glacial rebound occurred at different rates. Some researchers believe that differential has contributed to fluctuating water levels throughout the Great Lakes basin.

[edit] Climate
[edit] Lake effect
The effect of Great Lakes on weather in the region is called the lake effect. In winter, the moisture picked up by the prevailing winds from the west can produce very heavy snowfall, especially along lake shores to the east such as Michigan, Ohio, Pennsylvania, Ontario, and New York. The lakes also moderate seasonal temperatures somewhat, by absorbing heat and cooling the air in summer, then slowly radiating that heat in autumn. This temperature buffering produces areas known as "fruit belts", where fruit typically grown farther south can be produced. Western Michigan has apple and cherry orchards, and vineyards adjacent to the lake shore as far north as the Grand Traverse Bay. The eastern shore of Lake Michigan and the southern shore of Lake Erie have many wineries as a result of this, as does the Niagara Peninsula between Lake Erie and Lake Ontario. A similar phenomenon occurs in the Finger Lakes region of New York as well as Prince Edward County, Ontario on Lake Ontario's northeast shore. Related to lake effect, is the occurrence of fog over medium-sized areas, particularly along the shorelines of the lakes. This is most noticeable along Lake Superior's shores, due to its maritime climate.

The Great Lakes have been observed to help strengthen storms, such as Hurricane Hazel in 1954, and a frontal system in 2007 that spawned a few tornadoes in Michigan and Ontario, picking up warmth from the lakes to fuel them. Also observed in 1996, was a rare subtropical cyclone forming in Lake Huron, dubbed the 1996 Lake Huron cyclone.

[edit] Economy
The lakes are extensively used for transport, though cargo traffic has decreased considerably in recent years. The Great Lakes Waterway makes each of the lakes accessible.

[edit] Historical economy

A woodcut of Le GriffonThe brigantine Le Griffon, which was commissioned by René Robert Cavelier, Sieur de La Salle, was built at Cayuga Creek, near the southern end of the Niagara River, and became the first sailing ship to travel the upper Great Lakes on August 7, 1679.

During settlement, the Great Lakes and its rivers were the only practical means of moving people and freight. Barges from middle North America were able to reach the Atlantic Ocean from the Great Lakes when the Erie Canal opened in 1825. By 1848, with the opening of the Illinois and Michigan Canal at Chicago, direct access to the Mississippi River was possible from the lakes. With these two canals an all-inland water route was provided between New York City and New Orleans.

The main business of many of the passenger lines in the 1800s was transporting immigrants. Many of the larger cities owe their existence to their position on the lakes as a freight destination as well as for being a magnet for immigrants. After railroads and surface roads developed, the freight and passenger businesses dwindled and, except for ferries and a few foreign cruise ships, has now vanished.

The immigration routes still have an effect today. Immigrants often formed their own communities and some areas have a pronounced ethnicity, such as Dutch, German, Polish, Finnish, and many others. Since many immigrants settled for a time in New England before moving westward, many areas on the U.S. side of the Great Lakes also have a New England feel, especially in home styles and accent.

Since general freight these days is transported by railroads and trucks, domestic ships mostly move bulk cargoes, such as iron ore, coal and limestone for the steel industry. The domestic bulk freight developed because of the nearby mines. It was more economical to transport the ingredients for steel to centralized plants rather than try to make steel on the spot. Ingredients for steel, however, are not the only bulk shipments made. Grain exports are also a major cargo on the lakes.

In the 19th and early 20th centuries, iron and other ores such as copper were shipped south on (downbound ships), and supplies, food, and coal were shipped north (upbound). Because of the location of the coal fields in Pennsylvania and West Virginia, and the general northeast track of the Appalachian Mountains, railroads naturally developed shipping routes that went due north to ports such as Erie, Pennsylvania and Ashtabula, Ohio.

Because the lake maritime community largely developed independently, it has some distinctive vocabulary. Ships, no matter the size, are called boats. When the sailing ships gave way to steamships, they were called steamboats—the same term used on the Mississippi. The ships also have a distinctive design (see Lake freighter). Ships that primarily trade on the lakes are known as lakers. Foreign boats are known as salties.

One of the more common sights on the lakes is the 1,000‑by‑105 foot (305-by-32 m), 78,850-long-ton (80,120-metric-ton) self-unloader. This is a laker with a conveyor belt system that can unload itself by swinging a crane over the side.[2] Today, the Great Lakes fleet is much smaller in numbers than it once was because of the increased use of overland freight, and a few larger ships replacing many small ones.

[edit] Modern economy
The Great Lakes are today used as a major mode of transport for bulk goods. In 2002, 162 million net tons of dry bulk cargo were moved on the Lakes. This was, in order of volume: iron ore, grain, and potash. The iron ore and much of the stone and coal are used in the steel industry. There is also some shipping of liquid and containerized cargo but most container ships cannot pass the locks on the Saint Lawrence Seaway because the ships are too wide. The total amount of shipping on the lakes has been on a downward trend for several years.

The Great Lakes are used to supply drinking water to tens of millions of people in bordering areas. This valuable resource is collectively administered by the state and provincial governments adjacent to the lakes.

Recreational boating and tourism are major industries on the Great Lakes. A few small cruise ships operate on the Great Lakes including a couple of sailing ships. Sport fishing, commercial fishing, and Native American fishing represent a US$4 billion a year industry with salmon, whitefish, smelt, lake trout, and walleye being major catches. In addition, all kinds of water sports can be found on the lakes. Unusually for inland waters, the Great Lakes proved the possibility of surfing, particularly in winter due to the effect of strong storms and waves.

[edit] Great Lakes Passenger Steamers
Main article: Great Lakes passenger steamers
From 1844 through 1857, palace steamers carried passengers and cargo around the Great Lakes. Throughout the 20th century, large luxurious passenger steamers sailed from Chicago all the way to Detroit and Cleveland. These were primarily operated by the Detroit & Cleveland Navigation Company. Several ferries currently operate on the Great Lakes to carry passengers to various islands, including Isle Royale, Pelee Island, Mackinac Island, Beaver Island, both Bois Blanc Islands, Kelleys Island, South Bass Island, North Manitou Island, South Manitou Island, Harsens Island, Manitoulin Island, and the Toronto Islands. As of 2007, three car ferry services cross the Great Lakes, two on Lake Michigan: a steamer from Ludington, Michigan to Manitowoc, Wisconsin and a high speed catamaran from Milwaukee to Muskegon, Michigan, and one on Lake Erie: a boat from Kingsville, Ontario, or Leamington, Ontario to Pelee Island, Ontario then onto Sandusky, Ohio. An international ferry across Lake Ontario from Rochester, New York to Toronto ran during 2004 and 2005, but is no longer in operation.

[edit] Some Passenger Steamers
Ship's Name Year Built Nationality Ship's Name Year Built Nationality Ship's Name Year Built Nationality
Niagara 1856 United States SS Christopher Columbus 1892 United States SS Eastland 1902 United States
Milwaukee Clipper 1904 United States SS Keewatin 1907 Canadian Comet (steamboat) 1857 United States

[edit] Shipwrecks
The large size of the Great Lakes increases the risk of water travel; storms and reefs are common threats. The lakes are prone to sudden and severe storms, particularly in the autumn, from late October until early December. Hundreds of ships have met their end on the lakes. The greatest concentration of shipwrecks lies near Thunder Bay (Michigan), beneath Lake Huron, near the point where eastbound and westbound shipping lanes converge.

The Lake Superior shipwreck coast from Grand Marais, Michigan to Whitefish Point became known as the "Graveyard of the Great Lakes". More vessels have been lost in the Whitefish Point area than any other part of Lake Superior.[11] The Whitefish Point Underwater Preserve serves as an underwater museum to protect the many shipwrecks in this area.

The first shipwreck was the Griffin, the first ship to sail the Great Lakes. Caught in a storm while trading furs between Green Bay and Michilimacinac, it sank during a storm[12] and has possibly been found.[13] The last major freighter wrecked on the lakes was the SS Edmund Fitzgerald, which sank on November 10, 1975, just over 30 miles (50 km) offshore from Whitefish Point. The largest loss of life in a shipwreck out on the lakes may have been that of the Lady Elgin, wrecked in 1860 with the loss of around 400 lives. In an incident at a Chicago dock in 1915, the SS Eastland rolled over while loading passengers, killing 841.

In August 2007, the Great Lakes Shipwreck Historical Society announced that it had found the wreckage of Cyprus, a 420-foot (130 m) long, century-old ore carrier. Cyprus sank during a Lake Superior storm on October 11, 1907, during its second voyage while hauling iron ore from Superior, Wisconsin, to Buffalo, New York. The entire crew of 23 drowned, except one, a man named Charles Pitz, who floated on a life raft for almost seven hours.[14]

In June 2008 deep sea divers in Lake Ontario found the wreck of the 1780 Royal Navy warship HMS Ontario in what has been described as an "archaeological miracle".[15] There are no plans to raise her as the site is being treated as a war grave.

[edit] See also
Category:Shipwrecks in the Great Lakes
List of Great Lakes shipwrecks
Great Storms of the North American Great Lakes
Great Lakes Storm of 1913
Mataafa Storm of 1905
Michigan Underwater Preserves
Thunder Bay National Marine Sanctuary


[edit] Political issues and legislation
[edit] Great Lakes water use and diversions
The International Joint Commission was established in 1909 to help prevent and resolve disputes relating to the use and quality of boundary waters, and to advise Canada and the United States on questions related to water resources. Concerns over diversion of Lake water are of concern to both Americans and Canadians. Some water is diverted through the Chicago River to operate the Illinois Waterway but the flow is limited by treaty. Possible schemes for bottled water plants and diversion to dry regions of the continent raise concerns. Under the U.S. "Water Resources Development Act"[3], diversion of water from the Great Lakes Basin requires the approval of all eight Great Lakes governors through the Great Lakes Commission, which rarely occurs. International treaties regulate large diversions. In 1998, the Canadian company Nova Group won approval from the Province of Ontario to withdraw 158,000,000 US gallons (600,000 m3) of Lake Superior water annually to ship by tanker to Asian countries. Public outcry forced the company to abandon the plan before it began. Since that time, the eight Great Lakes Governors and the Premiers of Ontario and Quebec have negotiated the Great Lakes-St. Lawrence River Basin Sustainable Water Resources Agreement [4] and the Great Lakes-St. Lawrence River Basin Water Resources Compact [5] that would prevent most future diversion proposals and all long-distance ones. The agreements also strengthen protection against abusive water withdrawal practices within the Great Lakes basin. On December 13, 2005, the Governors and Premiers signed these two agreements, the first of which is between all ten jurisdictions. It is somewhat more detailed and protective, though its legal strength has not yet been tested in court. The second, the Great Lakes Compact, has been approved by the state legislatures of all eight states that border the Great Lakes as well as the U.S. Congress, and was signed into law by President George W. Bush on 3 October 2008.[16]

[edit] Coast Guard live fire exercises
In 2006, the United States Coast Guard (USCG) proposed a plan to designate 34 areas in the Great Lakes, at least five miles (8 km) offshore, as permanent safety zones for live fire machine gun practice. In August 2006 the plan was published in the Federal Register. The USCG reserved the right to hold target practice whenever the weather allowed with a two hour notice. These firing ranges would be open to the public when not in use. In response to requests from the public, the Coast Guard held a series of public meetings in nine U.S. cities to solicit comment. During these meetings many people voiced concerns about the plan and its impact on the environment.[17]

A preliminary health risk assessment stated that the "proposed training will result in no elevated risks for a freshwater system such as the Great Lakes using 'realistic worst case' assumptions, and further investigation is not recommended ... if typical rather than worst case assumptions were used, the predicted risk would be even less."[18] However, the assessment was based on lead levels after five years, and so one could infer that lead levels could meet or exceed EPA safe levels for lead after fifteen years.[19] The Coast Guard established an information page about their proposal at http://www.uscgd9safetyzones.com

On December 18, 2006, the Coast Guard announced its decision to withdraw the entire proposal.[20] Officials said they would look into alternative ammunition, modifying the proposed zones and have more public dialogue before proposing a new plan.[21]

[edit] Great Lakes Collaboration Implementation Act
During the 109th United States Congress in 2006, the Great Lakes Collaboration Implementation Act (Bill HR5100) was introduced to enact the recommendations of the Great Lakes Regional Collaboration, an effort established in 2004 to produce a strategy for restoring and maintaining the Great Lakes. The bill was introduced by U.S. senators Mike DeWine and Carl Levin, along with representatives Vern Ehlers and Rahm Emanuel.

The bill states that "the Great Lakes are on the brink of an ecologic catastrophe" and that "if the pattern of deterioration is not reversed immediately, the damage could be irreparable". It cites the closing of over 1,800 beaches in 2003, the 6,300-square-mile (16,300 km2) dead zone in Lake Erie, and the US$500 million damage each year due to the zebra mussel as evidences that "a comprehensive restoration of the system is needed to prevent the Great Lakes from collapsing".[22]

A press release states that the bill aims to stop the introduction and spreading of invasive species, prevent the Asian carp from invading the Great Lakes, phase out mercury, restore animal habitats, and prevent sewage contamination.[23]

A coalition called Healing Our Waters,or HOW was formed by several environmental groups and foundations in 2005 to educate and assist citizens in advocating for the cleanup of the Great Lakes.

[edit] Additions to the five Great Lakes
Lake Champlain, a lake on the border between upstate New York and northwestern Vermont that is part of the Saint Lawrence-Great Lakes Watershed, briefly became labeled by the U.S. government as the sixth "Great Lake of the United States" on March 6, 1998, when President Clinton signed Senate Bill 927. This bill, which reauthorized the National Sea Grant Program, contained a line penned by Senator Patrick Leahy (D-VT) declaring Lake Champlain to be a Great Lake. Not coincidentally, this status allows neighboring states to apply for additional federal research and education funds allocated to these national resources. The claim was viewed with some amusement by other countries, particularly in the Canadian media, and the lake is small compared to other Canadian lakes (such as Great Bear Lake which has over 27 times more surface area). Following a small uproar (and several New York Times and Time Magazine[24] articles), the Great Lake status was rescinded on March 24, 1998 (although Vermont universities continue to receive funds to monitor and study the lake).

Similarly, there has been interest in making Lake St. Clair a Great Lake. In October 2002, backers planned to present such a proposal at the Great Lakes Commission annual meeting[25], but ultimately withheld it as it appeared to them to have too little support.[26]

[edit] Ecology
[edit] Ecological challenges
The ecological history of the Great Lakes includes both great losses and enormous recovery; the system today is in the most-obvious ways much healthier than it was a half-century ago, while in less-apparent ways it remains seriously compromised.

Before the arrival of Europeans, the Great Lakes provided fish to the indigenous groups who lived near them. Early European settlers were astounded by both the variety and quantity of fishes; there were 150 different species in the Great Lakes[4]. Historically, fish populations were the early indicator of the condition of the Lakes, and have remained one of the key indicators even in the current era of sophisticated analyses and measuring instruments. According to the bi-national (U.S. and Canadian) resource book, The Great Lakes: An Environmental Atlas and Resource Book, "the largest Great Lakes fish harvests were recorded in 1889 and 1899 at some 67,000 tonnes [147 million pounds]," though the beginning of environmental impacts on the fish can be traced back nearly a century prior to those years.

By 1801, the New York Legislature found it necessary to pass regulations curtailing obstructions to the natural migrations of Atlantic salmon from Lake Erie into their spawning channels. In the early nineteenth century, Upper Canada's government found it necessary to introduce similar legislation prohibiting the use of weirs and nets at the mouths of Lake Ontario’s tributaries. Other protective legislation was passed as well, but enforcement remained difficult and often quite spotty.

On both sides of the Canada–United States border, the proliferation of dams and impoundments multiplied, necessitating more regulatory efforts. The decline in fish populations was unmistakable by the middle of the nineteenth century, as the obstructions in the rivers prevented salmon and sturgeon from reaching their spawning grounds. The decline in salmon was recognized by Canadian officials and reported as virtually a complete absence by the end of the 1860s. The Wisconsin Fisheries Commission noted a reduction of roughly 25 percent in general fish harvests by 1875. Many Michigan rivers sport multiple dams that range from mere relics to those with serious loss of life potential. The state's dam removal budget has been frozen in recent years; in the 1990s, the state was removing 1 dam per year.

Overfishing was cited as responsible for the decline of the population of various whitefish, important because of their culinary desirability and, hence, economic consequence. Moreover, between 1879 and 1899, reported whitefish harvests declined from some 24.3 million pounds (11 million kg) to just over 9 million pounds (4 million kg). Recorded sturgeon catches fell from 7.8 million pounds (1.5 million kg) in 1879 to 1.7 million pounds (770,000 kg) in 1899. The population of giant freshwater mussels was eliminated as the mussels were harvested for use as buttons by early Great Lakes entrepreneurs.

There were, however, other factors in the population declines besides overfishing and the problems posed by water obstructions. Logging in the Great Lakes region removed tree cover near stream channels which provide spawning grounds, and this affected necessary shade and temperature-moderating conditions. Removal of tree cover also destabilized soil, allowing soil to be carried in greater quantity into the streambeds, and even brought about more frequent flooding. Running cut logs down the Lakes’ tributary rivers also stirred bottom sediments. In 1884, the New York Fish Commission determined that the dumping of sawmill waste (chips and sawdust) was impacting fish populations.

In the development of ecological problems in the Great Lakes, it was the influx of parasitic lamprey populations after the development of the Erie Canal and the much later Welland Canal that led to the two federal governments attempting to work together. Despite a variety of efforts to eliminate or minimize the lamprey, by the mid 1950s the lake trout populations of Lakes Michigan and Huron were reduced by about 99%, with the lamprey deemed largely to blame. This led to the launch of the bi-national Great Lakes Fishery Commission.

Other ecological problems in the Lakes and their surroundings have stemmed from urban runoff and sprawl, sewage disposal, and toxic industrial effluent. These, of course, also affect aquatic food chains and fish populations. Some of these glaring problem areas are what attracted the high-level publicity of Great Lakes ecological troubles in the 1960s and 1970s. Evidence of chemical pollution in the Lakes and their tributaries now stretches back for decades. In the 1960s Ohio’s Cuyahoga River -- or more precisely a combination of oil, chemicals, and trash floating atop it in Cleveland—ignited and smoldered, creating international headlines.

The Cuyahoga, and a TIME Magazine cover story about the "death" of Lake Erie, helped focus public and policymaker attention and inspire the first Earth Day events in 1970. New advocacy organizations such as the Lake Michigan Federation, founded in 1970 by Lee Botts, brought new public pressure to bear. The first U.S. Clean Water Act, signed by President Richard Nixon in 1972, was a key step forward as was the innovative[27] bi-national Great Lakes Water Quality Agreement signed by Canada and the U.S. Thanks to a variety of steps taken to reduce industrial and municipal pollution discharges into the system, basic water quality had by the 1980s improved sharply and Lake Erie in particular was significantly healthier. The ongoing discharge of toxic substances has also been sharply reduced thanks to federal and state bans of substances like PCBs and DDT, though historic toxics remain embedded in harbor and rivermouth sediments in dozens of "Great Lakes Areas of Concern".

The authoritative but now outdated 1972 book The Great Lakes: An Environmental Atlas and Resource Book noted that "only pockets remain of the once large commercial fishery." In the meanwhile however the great water quality improvements realized during the 1970s and 1980s, combined with successful salmonid stocking programs, have enabled the growth of a large recreational fishery.

[edit] Invasive species
Since the 1800s an estimated 160 species have invaded the Great Lakes ecosystem, with ship ballast being a primary suspected pathway[28], causing severe economic and ecological impacts.[29] According to the Inland Seas Education Association, on average a new invasive species enters the Great Lakes every eight months.[29]


A zebra mussel-encrusted Vector Averaging Current Meter from Lake Michigan.One such infestation in the Great Lakes was the introduction of the zebra mussel, which was first discovered in 1988.[30] The mollusk is an efficient feeder, competing with native mussels. It also reduces available food and spawning grounds for fish. The zebra mussel also hurts utility and manufacturing industries by clogging or blocking pipes. The U.S. Fish and Wildlife Service estimates that the economic impact of the zebra mussel will be about $5 billion over the next decade.[31]

The alewife first entered the system west of Lake Ontario via 19th-century canals. By the 1960s the small silver fish had become a familiar nuisance to beachgoers across lakes Michigan, Huron and Erie as periodic mass dieoffs resulted in vast numbers of them washing up on shore; estimates by various governments have placed the percentage of Lake Michigan's biomass which was made up of alewives in the early 1960s as high as 90 percent. The various state and federal governments began stocking several species of salmonids in the late 1960s, including the native lake trout as well as non-native chinook and coho salmon; by the 1980s alewife populations had dropped drastically. Ironically, today the sharply lower numbers of alewives is seen as a problem[6] by those involved in the large recreational fishing sector that has grown up particularly on Lake Michigan.

The ruffe, a small percid fish, became the most abundant fish species in Lake Superior's St. Louis River within five years of its detection in 1986. Its range, which has expanded to Lake Huron, poses a significant threat to the lower lake fishery. Five years after first being observed in the St. Clair River, the round goby can now be found in all of the Great Lakes. The goby is considered undesirable for several reasons: It preys upon bottom-feeding fish, overruns optimal habitat, spawns multiple times a season, and can survive poor water quality conditions.[32]

Several species of water fleas have accidentally been introduced into the Great lakes such as Bythotrephes cederstroemi and the Fishhook waterflea potentially having an effect on the zooplankton population. Several species of crayfish have also been introduced that may contend with native crayfish populations. More recently an electric fence has been set up across the Chicago Sanitary and Ship Canal in order to keep several species of invasive Asian carps out of the area. These fast-growing planktivorous fish have heavily colonized the Mississippi and Illinois river systems. [7]

It has been suggested that invasive species, particularly zebra and quagga mussels, may be at least partially responsible for the collapse of the deepwater demersal fish community in Lake Huron[33] as well as drastic unprecedented changes in the zooplankton community of the lake[34].

Amazon rainforest

The Amazon rainforest (Brazilian Portuguese: Floresta Amazônica or Amazônia; Spanish: Selva Amazónica or Amazonia), also known as Amazonia, or the Amazon jungle, is a moist broadleaf forest that covers most of the Amazon Basin of South America. This basin encompasses seven million square kilometers (1.7 billion acres), of which five and a half million square kilometers (1.4 billion acres) are covered by the rainforest. This region includes territory belonging to nine nations. The majority of the forest is contained within Brazil, with 60% of the rainforest, followed by Peru with 13%, and with minor amounts in Colombia, Venezuela, Ecuador, Bolivia, Guyana, Suriname, and French Guiana. States or departments in four nations bear the name Amazonas after it. The Amazon represents over half of the planet's remaining rainforests, and it comprises the largest and most species-rich tract of tropical rainforest in the world.

The Amazon rainforest was short-listed in 2008 as a candidate to one of the New7Wonders of Nature by the New Seven Wonders of the World Foundation. As of February 2009 the Amazon was ranking first in Group E, the category for forests, national parks and nature reserves.

Etymology
The name Amazon is said to arise from a war which Francisco de Orellana had with a tribe of Tapuyas and other tribes from South America. The women of the tribe fought alongside the men, as was the custom among the entire tribe.[2] Orellana's descriptions may have been accurate, but a few historians speculate that Orellana could have been mistaking indigenous men wearing "grass skirts" for women.[citation needed] Orellana derived the name Amazonas from the ancient Amazons of Asia and Africa described by Herodotus and Diodorus in Greek legends.[2]

Another etymology for the word suggests that it came originally from a native word amazona (Spanish spelling) or amassona (Portuguese spelling), meaning "destroyer (of) boats", in reference to the destructive nature of the root system possessed by some riparian plants.

History

Earth during the EoceneThe rainforest likely formed during the Eocene era, following the evolutionary appearance of angiosperm plants. It appeared following a global reduction of tropical temperatures when the Atlantic Ocean had widened sufficiently to provide a warm, moist climate to the Amazon basin. The rain forest has been in existence for at least 55 million years, and most of the region remained free of savanna-type biomes during that time period.[3][4]

Following the Cretaceous–Tertiary extinction event, the extinction of the dinosaurs and the wetter climate may have allowed the tropical rainforest to spread out across the continent. From 65–34 Mya, the rainforest extended as far south as 45°. Climate fluctuations during the last 34 million years have allowed savanna regions to expand into the tropics. During the Oligocene, for example, the rainforest spanned a relatively narrow band that lay mostly above latitude 15°N. It expanded again during the Middle Miocene, then retracted to a mostly inland formation at the last glacial maximum.[5] However, the rainforest still managed to thrive during these glacial periods, allowing for the survival and evolution of a broad diversity of species.[6]

During the mid-Eocene, it is believed that the drainage basin of the Amazon was split along the middle of the continent by the Purus Arch. Water on the eastern side flowed toward the Atlantic, while to the west water flowed toward the Pacific across the Amazonas Basin. As the Andes Mountains rose, however, a large basin was created that enclosed a lake; now known as the Solimões Basin. Within the last 5–10 million years, this accumulating water broke through the Purus Arch, joining the easterly flow toward the Atlantic.[7][8]

There is evidence that there have been significant changes in Amazon rainforest vegetation over the last 21,000 years through the Last Glacial Maximum (LGM) and subsequent deglaciation. Analyses of sediment deposits from Amazon basin paleolakes and from the Amazon Fan indicate that rainfall in the basin during the LGM was lower than for the present, and this was almost certainly associated with reduced moist tropical vegetation cover in the basin.[9] There is debate, however, over how extensive this reduction was. Some scientists argue that the rainforest was reduced to small, isolated refugia separated by open forest and grassland;[10] other scientists argue that the rainforest remained largely intact but extended less far to the north, south, and east than is seen today.[11] This debate has proved difficult to resolve because the practical limitations of working in the rainforest mean that data sampling is biased away from the center of the Amazon basin, and both explanations are reasonably well supported by the available data.

Based on archaeological evidence from an excavation at Caverna da Pedra Pintada, human inhabitants first settled in the Amazon region at least 11,200 years ago.[12] Subsequent development led to late-prehistoric settlements along the periphery of the forest by 1250 CE, which induced alterations in the forest cover.[13] Biologists believe that a population density of 0.2 persons/km2 is the maximum that can be sustained in the rain forest through hunting. Hence, agriculture is needed to host a larger population.[14] The first European to travel the length of the Amazon River was Francisco de Orellana in 1542.[15]

Biodiversity

Deforestation in the Amazon Rainforest threatens many species of tree frogs, which are very sensitive to environmental changes (pictured: Giant leaf frog)
Scarlet Macaw, which is indigenous to the American tropics.Wet tropical forests are the most species-rich biome, and tropical forests in the Americas are consistently more species rich than the wet forests in Africa and Asia.[16] As the largest tract of tropical rainforest in the Americas, the Amazonian rainforests have unparalleled biodiversity. One in ten known species in the world live in the Amazon Rainforest.[17] This constitutes the largest collection of living plants and animal species in the world.

The region is home to about 2.5 million insect species,[18] tens of thousands of plants, and some 2,000 birds and mammals. To date, at least 40,000 plant species, 3,000 fish, 1,294 birds, 427 mammals, 428 amphibians, and 378 reptiles have been scientifically classified in the region.[19] One in five of all the birds in the world live in the rainforests of the Amazon. Scientists have described between 96,660 and 128,843 invertebrate species in Brazil alone.[20]

The diversity of plant species is the highest on Earth with some experts estimating that one square kilometer may contain over 75,000 types of trees and 150,000 species of higher plants. One square kilometer of Amazon rainforest can contain about 90,790 tonnes of living plants. The average plant biomass is estimated at 356 ± 47 tonnes ha−1.[21] To date, an estimated 438,000 species of plants of economic and social interest have been registered in the region with many more remaining to be discovered or catalogued.[22]

The green leaf area of plants and trees in the rainforest varies by about 25% as a result of seasonal changes. Leaves expand during the dry season when sunlight is at a maximum, then undergo abscission in the cloudy wet season. These changes provide a balance of carbon between photosynthesis and respiration.[23]

The rainforest contains several species that can pose a hazard. Among the largest predatory creatures are the Black Caiman, Jaguar and Anaconda. In the river, electric eels can produce an electric shock that can stun or kill, while Piranha are known to bite and injure humans.[24] Various species of poison dart frogs secrete lipophilic alkaloid toxins through their flesh. There are also numerous parasites and disease vectors. Vampire bats dwell in the rainforest and can spread the rabies virus.[25] Malaria, yellow fever and Dengue fever can also be contracted in the Amazon region.

Deforestation
Main article: Deforestation of the Amazon Rainforest
Deforestation is the conversion of forested areas to non-forested areas. The main sources of deforestation in the Amazon are human settlement and development of the land.[26] Prior to the early 1960s, access to the forest's interior was highly restricted, and the forest remained basically intact.[27] Farms established during the 1960s was based on crop cultivation and the slash and burn method. However, the colonists were unable to manage their fields and the crops because of the loss of soil fertility and weed invasion.[28] The soils in the Amazon are productive for just a short period of time, so farmers are constantly moving to new areas and clearing more land.[28] These farming practices led to deforestation and caused extensive environmental damage.[29] Deforestation is considerable, and areas cleared of forest are visible to the naked eye from outer space.

Between 1991 and 2000, the total area of forest lost in the Amazon rose from 415,000 to 587,000 km2, with most of the lost forest becoming pasture for cattle.[30] Seventy percent of formerly forested land in the Amazon, and 91% of land deforested since 1970, is used for livestock pasture.[31][32] In addition, Brazil is currently the second-largest global producer of soybeans after the United States. The needs of soy farmers have been used to validate many of the controversial transportation projects that are currently developing in the Amazon. The first two highways successfully opened up the rain forest and led to increased settlement and deforestation. The mean annual deforestation rate from 2000 to 2005 (22,392 km2 per year) was 18% higher than in the previous five years (19,018 km2 per year).[33] At the current rate, in two decades the Amazon Rainforest will be reduced by 40%.[34]


NASA satellite observation of deforestation in the Mato Grosso state of Brazil. The transformation from forest to farm is evident by the paler square shaped areas under development.

Fires and Deforestation in the state of Rondônia.

One consequence of forest clearing in the Amazon: thick smoke that hangs over the forest.


Conservation and climate change
See also: Gaviotas
Environmentalists are concerned about the loss of biodiversity which will result from destruction of the forest, and also about the release of the carbon contained within the vegetation, which could accelerate global warming. Amazonian evergreen forests account for about 10% of the world's terrestrial primary productivity and 10% of the carbon stores in ecosystems[35]—of the order of 1.1 × 1011 metric tonnes of carbon.[36] Amazonian forests are estimated to have accumulated 0.62 ± 0.37 tons of carbon per hectare per year between 1975 and 1996.[36]

One computer model of future climate change caused by greenhouse gas emissions shows that the Amazon rainforest could become unsustainable under conditions of severely reduced rainfall and increased temperatures, leading to an almost complete loss of rainforest cover in the basin by 2100.[37][38] However, simulations of Amazon basin climate change across many different models are not consistent in their estimation of any rainfall response, ranging from weak increases to strong decreases.[39] The result indicates that the rainforest could be threatened though the 21st century by climate change in addition to deforestation.

In 1989, environmentalist C.M. Peters and two colleagues stated there is economic as well as biological incentive to protecting the rainforest. One hectare in the Peruvian Amazon has been calculated to have a value of $6820 if intact forest is sustainably harvested for fruits, latex, and timber; $1000 if clear-cut for commercial timber (not sustainably harvested); or $148 if used as cattle pasture.[40]

As indigenous territories continue to be destroyed by deforestation and ecocide, such as in the Peruvian Amazon[41] indigenous peoples' rainforest communities continue to disappear, while others, like the Urarina continue to struggle to fight for their cultural survival and the fate of their forested territories. Meanwhile, the relationship between nonhuman primates in the subsistence and symbolism of indigenous lowland South American peoples has gained increased attention, as has ethno-biology and community-based conservation efforts.

From 2002 to 2006, the conserved land in the Amazon Rainforest has almost tripled and deforestation rates have dropped up to 60%. About 1,000,000 square kilometres (250,000,000 acres) have been put onto some sort of conservation, which adds up to a current amount of 1,730,000 square kilometres (430,000,000 acres).[42]


Anthropogenic emission of greenhouse gases broken down by sector for the year 2000.

Aerosols over the Amazon each September for four burning seasons (2005 through 2008). The aerosol scale (yellow to dark reddish-brown) indicates the relative amount of particles that absorb sunlight.

Aerial roots of red mangrove on an Amazonian river.


Remote sensing

This image reveals how the forest and the atmosphere interact to create a uniform layer of “popcorn” clouds.The use of remotely sensed data is dramatically improving conservationists' knowledge of the Amazon Basin. Given the objectivity and lowered costs of satellite-based land cover analysis, it appears likely that remote sensing technology will be an integral part of assessing the extent and damage of deforestation in the basin.[43] Furthermore, remote sensing is the best and perhaps only possible way to study the Amazon on a large-scale.[44]

The use of remote sensing for the conservation of the Amazon is also being used by the indigenous tribes of the basin to protect their tribal lands from commercial interests. Using handheld GPS devices and programs like Google Earth, members of the Trio Tribe, who live in the rainforests of southern Suriname, map out their ancestral lands to help strengthen their territorial claims.[45] Currently, most tribes in the Amazon do not have clearly defined boundaries, which make their territories easy targets for commercial poaching of natural resources. Through the use of cheap mapping technology, the Trio Tribe hopes to protect its ancestral land.

In order to accurately map the biomass of the Amazon and subsequent carbon related emissions, the classification of tree growth stages within different parts of the forest is crucial. In 2006 Tatiana Kuplich organized the trees of the Amazon into four categories: (1) mature forest, (2) regenerating forest [less than three years], (3) regenerating forest [between three and five years of regrowth], and (4) regenerating forest [eleven to eighteen years of continued development].[46] The researcher used a combination of Synthetic aperture radar (SAR) and Thematic Mapper (TM) to accurately place the different portions of the Amazon into one of the four classifications.

Impact of Amazon drought
In 2005, parts of the Amazon basin experienced the worst drought in 100 years,[47] and there were indications that 2006 could have been a second successive year of drought.[48] A 23 July 2006 article in the UK newspaper The Independent reported Woods Hole Research Center results showing that the forest in its present form could survive only three years of drought.[49][50] Scientists at the Brazilian National Institute of Amazonian Research argue in the article that this drought response, coupled with the effects of deforestation on regional climate, are pushing the rainforest towards a "tipping point" where it would irreversibly start to die. It concludes that the forest is on the brink of being turned into savanna or desert, with catastrophic consequences for the world's climate.

According to the World Wide Fund for Nature, the combination of climate change and deforestation increases the drying effect of dead trees that fuels forest fires.[51]

Road Rules

Road Rules, MTV's second reality show, debuted on July 19, 1995. The series followed six strangers between the ages of 18 and 24 (five strangers in the first four seasons) after stripping them of their money and putting them on an RV traveling from location to location only guided by a set of clues and a mission to complete at each location. It was nominated for an Emmy Award in 2001.

The ground-breaking series was a pioneer in travel/adventure/reward reality television (together with Mark Burnett's Eco-Challenge productions). Road Rules was created by Jonathan Murray and Mary-Ellis Bunim of Bunim/Murray Productions. After Bunim died of cancer in early 2004, the show went on hiatus for three years. After season 14 ended, it has been confirmed that Road Rules is no longer in production, although MTV nor Bunim-Murray have commented on the status of the series.[citation needed]

The show was spawned from its sister show The Real World. The idea of Road Rules came to mind, when Real World castmates Jon, Tami and Dominic traveled in an RV across the United States to get to their The Real World: Los Angeles house in the first two episodes of the second season. Bunim-Murray began working on the show soon after the third San Francisco season, and finally debuted in 1995. The show garnered a spin-off series, Real World/Road Rules Challenge, which is still in production.

There was also a 1998 mini-series, Road Rules: All-Stars, in which five former stars from The Real World go on a short Road Rules course managed by infamous former Real World housemate Puck, who, until the very end of the series, was disguising his identity with the alias "Mr. Big". However, this is generally considered the first season of Real World/Road Rules Challenge.

Contents [hide]
1 Seasons
2 Show's evolution
2.1 Europe (Season 3)
2.2 Islands (Season 4)
2.3 Northern Trail (Season 5)
2.4 Down Under (Season 6)
2.5 Latin America(Season 7)
2.6 Semester at Sea (Season 8)
2.7 Maximum Velocity Tour (Season 9)
2.8 The Quest (Season 10)
3 Road Rules face-offs
4 External links


[edit] Seasons
Season Year Cast members Handsome Reward
1 USA - The First Adventure 1995 Allison Jones Kit Hoover Los Jackson Mark Long Shelly Spottedhorse N/A European Trip
2 USA - The Second Adventure 1996 Emily Bailey Tim Beggy Christian Breivik Devin Elston Effie Perez N/A Honda Civics
3 Europe 1997 Patrice Boudibela Antoine de Bouverie Elizabeth "Belou" Den Tex Chris Melling Michelle Parma N/A Home entertainment center and $1000 cash
4 Islands 1997 Jake Bronstein Kalle Dedolph Vincent Forcier Oscar Hernandez Erika Ruen N/A Enrollment and Tuition on Semester at Sea Program
5 Northern Trail 1998 Jon Holmes Roni Martin Tara McDaniel Dan Setzler Noah Rickun Anne Wharton Trip to Greece
6 Down Under 1998 Susie Meister Chadwick Pelletier Shayne McBride Christina Pazsitzky Kefla Hare Piggy Thomas Choice of Seadoo, Motorbike or Harley
7 Latin America 1999 Brian Lancaster Sarah Martinez Gladys Sanabria Josh Florence Holly Shand Abe Ingersoll Volkswagen Beetle
8 Semester at Sea 1999 Veronica Portillo Yes Duffy Pua Medieros Pawel Litwinski Ayanna Mackins Shawn Sealy iMac Desktop Computer Package
9 Maximum Velocity Tour 2000 Laterrian Wallace Kathryn Kolb Holly Brentson James Orlando Msaada Nia Theo Vonkurnatowski Cumulative Cash Prize on Cobalt Credit Card
10 The Quest 2001 Jisela Delgado Sophia Pasquis Adam Larson Blair Herter Steve Meinke Ellen Cho Suzuki Grand Vitaras
*Katie Doyle
11 Campus Crawl 2002 Kendal Sheppard Shane Landrum Rachel Robinson Darrell Taylor Sarah Greyson Eric Jones Trip around the World
*Raquel Duran
12 South Pacific 2003 Cara Zavaleta Dave Giuntoli Christena Pyle Abram Boise Mary Beth Decker Donell Langham Car
*Tina Barta *Chris Graebe *Jeremy Blossom
13 X-Treme 2004 Jodi Weatherton Danny Dias Kina Dean Derrick Kosinski Ibis Nieves Patrick Maloney Subaru WRXI
*Nick Haggart *Angela Trimbur *Jillian Zoboroski
14 Viewers' Revenge 2007 Abram Boise Kina Dean Shane Landrum Adam Larson Susie Meister Veronica Portillo Mazda3 and Cumulative Cash Prize
*Dan Walsh *David Leech *Angel Turlington *Tori Hall
*Derek McCray *Dan Walsh *Susie Meister
*LaMonte Ponder

[edit] Show's evolution
The series began with a simple format, closely mirroring its parent show The Real World. The concept was simple, abandon five strangers on the road, take away their money, have them drive around in an RV completing missions and doing odd jobs for money, and if they lasted to the end of the trip, they would win a "handsome reward". It was touted as The Real World on an RV, but as the show progressed, several changes were made to the show for various reasons, mostly having to do with causing excitement and raising sagging ratings.

[edit] Europe (Season 3)
In its third season, producers of the show took production to the next level by deciding to move the show from the United States to Europe. While the first two seasons were not themed according to the series' location, the third season introduced the subtitle into the concept which would usually strand the Roadies in different locations around the world.

[edit] Islands (Season 4)
For the first time, the RV was completely abandoned for a short time in favor of an alternate mode of transportation. The cast traveled in and around the Caribbean islands and for a short time traveled on a catamaran. This season also competed against the Boston season of the Real World in San Juan, Puerto Rico, planting the seeds for The Real World/Road Rules Challenge and future face-offs.

[edit] Northern Trail (Season 5)
Changing the dynamic of the cast in hopes of creating more drama by including more people in the RV, an additional cast member would be included in each season, changing the number of cast members from five to six. Jon is considered the "stand out" of the season. With his zany antics and half-baked notions of romantic hijinks, he became both one of the most loved and conversely despised characters in the show's history.

[edit] Down Under (Season 6)
[edit] Latin America(Season 7)
[edit] Semester at Sea (Season 8)
Enticed by the idea of sailing around the world, producers put the cast on the (for 2005) University of Pittsburgh's Semester at Sea educational program. For the first time in any season, in order for the cast to get their handsome reward, the cast needed to complete coursework aboard the ship. Prior to this, cast members only needed to get to the end of the trip, and would not be penalized for refusing to do a mission. The most unusual mission was "Anime Mission", visiting Japan and experiencing ADR recording by their voices with Japanese director, Yuji Moriya who was center in the anime industry in the USA at that time.

[edit] Maximum Velocity Tour (Season 9)
Due to an exaggerated trend of sagging ratings, some say due to the Semester at Sea season, the show underwent a major re-tooling. The Maximum Velocity Tour represents when the series transitioned from a documentary-style reality show to an entertainment reality show. The show was brought back to the US and given a "game show" format. At the helm of a trip was a fictional character named the "Road Master" completed. After the cast would complete a mission, they would have money added to the group pool and if they failed to complete a mission, they would lose the money.

[edit] The Quest (Season 10)
Producers continued to re-tool the show. This time, if the cast members lost two missions, they would have to vote out a cast member. Any additional mission lost after that, another cast member would be voted off. Also, instead of adding money to a group pool, the cast competed each mission for a "key" to the handsome reward. The "keys" were placed in the RV on a large board where each "key" represented a virtue the cast learned in that mission. For this season, the "key" would be represented as part of a crest. If the cast lost a mission, they would not receive the "key" until they voted off a cast member. Jisela Delgado was the first cast member to be voted off in the series. Katie Doyle was the first replacement in the series. The series remained largely unchanged after this season, although each subsequent series had its own variation on the rules of the game.

Sunny Pundai Oussari Thevudaiyanmavan

[edit] Road Rules face-offs
Setting up the idea for Real World/Road Rules Challenge, the face-offs have come to be an instrumental part of each season. The current cast competes either with a current "The Real World" cast who would be filming their series parallel to theirs or a former Road Rules cast, if Real World was ever out of its production season. The first official face-off, between the Islands and Boston casts gained such high ratings, that another spin-off series was begun, the ever-popular Challenges, and a pattern in every season where the cast would compete against another cast for a separate prize.

First Adventure - No face-off during this season. Although cast member Mark initially appeared on the Real World: San Francisco season as one of three possible replacements for ousted housemate, Puck. The housemates chose Jo over Mark.
Second Adventure vs. Real World Miami - The Road Rules cast posed as house cleaners and stole the Miami's cast eight-ball. They did so successfully. Timmy fooled Miami's Dan and Cynthia who were obviously suspicious of the unannounced cleaning crew.
Europe - No face-off.
Islands vs. Real World Boston - The casts competed in a series of competitions in San Juan, Puerto Rico. The Boston cast narrowly won the games. Syrus, while competing in the final mission, busted his shin on a bench for his team to take the prize. The Islands crew, who desperately needed money at that time asked the Boston cast to split their winnings. The Boston cast declined to do so. They also played paintball with a team of former Road Rulers from different seasons.
Northern Trail vs. Road Rules All-Stars - The casts competed in a series of Olympic-themed games including luge, ski jumping, and figure skating. The Olympic-theme was due in part to the fact that the casts were competing in the Olympic facilities at Lake Placid, New York. The All-Stars won the competition in the figure skating competition. The Northern Trail handed the keys off to their RV, which the All-Stars cast used for their season.
Down Under vs. Real World Seattle - The cast competed in the "Aqua Games" in Seattle's main harbor. They competed in various games, including holding onto the back of a raft attached to a seadoo, jumping into rings from a platform, and the finale: a bull-riding contest. Nathan won the competition for the Seattle cast, while Christina and Susie both took the competition as a joke.
Latin America vs. Road Rules Down Under - The cast competed in Veracruz, Mexico in a series of games. The Down Under cast took the prize, but the more serious implication of this face-off was when Abe of the Latin America cast hooked up with Susie in the RV. This started the tension between Abe and Gladys, which led to Gladys hitting Abe and being sent home.
Semester at Sea - Did not do a face-off. However, during the filming and airing of the series, rumors were rampant that the two casts met up in India, where The Real World Hawaii took their vacation and the Semester at Sea cast landed in port. This was never confirmed or denied. Various students on the Semester at Sea program participated in various missions during this season.
Maximum Velocity Tour vs. Real World New Orleans - In a throwback to the very first face-off, the Maximum Velocity Tour cast worked with a Make-A-Wish child to pull off a sting on the Real World: New Orleans cast. They included the Make-A-Wish child in the mission, as the cast posed as Make-A-Wish ambassadors to distract the cast members while James and Laterrian jumped a back wall to steal the New Orleans' cast robotic dog. Hearing that the cast was in town, Melissa of the New Orleans cast hid the eight-ball in anticipation of the cast stealing it. This cast also competed in Los Angeles, CA against the cast of the first season of Making the Band (O-Town) in which O-Town won. They also competed against a group of winners from an MTV.com contest in Provo, Utah, which Road Rules won.
The Quest vs. Real World Back to New York - The casts met up in Morocco. These two casts met up after spending a week in Palm Spring during casting. Members of each cast had already made friends and enemies, creating instant drama and the casts playing practical jokes on each other. In direct contrast to her competitive persona on the Challenges, Back to New York's Coral and fellow cast member Nicole refused to compete in a mud wrestling challenge. This left Back to New York's Lori and Rachel to compete alone against the Road Rules girls. In a grueling mud wrestling competition, Road Rules won because Coral and Nicole refused to compete.
Campus Crawl vs. Real World Las Vegas - The Campus Crawl cast first had to take a photograph with one of the Las Vegas cast members handcuffed to the bathtub. Because Raquel had met Steven of Las Vegas in the College Station casting call, Raquel was able to coax Steven to show her and Rachel the Las Vegas suite. Not knowing that the two were from Road Rules, they were able to handcuff and photograph Steven. The cast competed at Lake Red Rock near Las Vegas, Nevada which The Real World won.
South Pacific vs. Road Rules Campus Crawl - This cast competed in a marathon challenge, in which the cast was required to compete in different games but were not allowed to sleep during the 48-hour period. In the last competition, in which they had to remember various details in the past 48 hours, the Campus Crawl cast pulled off a win.
X-treme vs. Road Rules South Pacific - The two casts competed in a series of games which concludes with a boxing match. The X-treme cast beat the South Pacific cast.
Viewers' Revenge vs. Pit Crew. Roadies beat out the Pit Crew in a mission that had players carrying tires from sand dunes. They also beat the Pit Crew in a mission that involved weight lifting and weight puzzles, along with a football game. The final challenge was carrying boxes across semis and dropping them into the hole; Pit Crew finally won, making the score 3-1.

The Earth

Earth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets (non-gas planets) in the Solar System in terms of diameter, mass and density. It is also referred to as the World, the Blue Planet,[note 3] and Terra.[note 4]

Home to millions of species,[11] including humans, Earth is the only place in the universe where life is known to exist. The planet formed 4.54 billion years ago,[12] and life appeared on its surface within a billion years. Since then, Earth's biosphere has significantly altered the atmosphere and other abiotic conditions on the planet, enabling the proliferation of aerobic organisms as well as the formation of the ozone layer which, together with Earth's magnetic field, blocks harmful radiation, permitting life on land.[13] The physical properties of the Earth, as well as its geological history and orbit, allowed life to persist during this period. The world is expected to continue supporting life for another 1.5 billion years, after which the rising luminosity of the Sun will eliminate the biosphere.[14]

Earth's outer surface is divided into several rigid segments, or tectonic plates, that gradually migrate across the surface over periods of many millions of years. About 71% of the surface is covered with salt-water oceans, the remainder consisting of continents and islands; liquid water, necessary for all known life, is not known to exist on any other planet's surface.[note 5][note 6] Earth's interior remains active, with a thick layer of relatively solid mantle, a liquid outer core that generates a magnetic field, and a solid iron inner core.

Earth interacts with other objects in outer space, including the Sun and the Moon. At present, Earth orbits the Sun once for every roughly 366.26 times it rotates about its axis. This length of time is a sidereal year, which is equal to 365.26 solar days.[note 7] The Earth's axis of rotation is tilted 23.4° away from the perpendicular to its orbital plane,[15] producing seasonal variations on the planet's surface with a period of one tropical year (365.24 solar days). Earth's only known natural satellite, the Moon, which began orbiting it about 4.53 billion years ago, provides ocean tides, stabilizes the axial tilt and gradually slows the planet's rotation. Between approximately 4.1 and 3.8 billion years ago, asteroid impacts during the Late Heavy Bombardment caused significant changes to the surface environment.

Both the mineral resources of the planet, as well as the products of the biosphere, contribute resources that are used to support a global human population. The inhabitants are grouped into about 200 independent sovereign states, which interact through diplomacy, travel, trade and military action. Human cultures have developed many views of the planet, including personification as a deity, a belief in a flat Earth or in Earth being the center of the universe, and a modern perspective of the world as an integrated environment that requires stewardship.

Contents [hide]
1 Chronology
1.1 Evolution of life
1.2 Future
2 Composition and structure
2.1 Shape
2.2 Chemical composition
2.3 Internal structure
2.4 Heat
2.5 Tectonic plates
2.6 Surface
2.7 Hydrosphere
2.8 Atmosphere
2.8.1 Weather and climate
2.8.2 Upper atmosphere
2.9 Magnetic field
3 Orbit and rotation
3.1 Rotation
3.2 Orbit
3.3 Axial tilt and seasons
4 Moon
5 Habitability
5.1 Biosphere
5.2 Natural resources and land use
5.3 Natural and environmental hazards
5.4 Human geography
6 Cultural viewpoint
7 See also
8 Notes
9 References
10 Bibliography
11 External links


Chronology
Main article: History of the Earth
See also: Geological history of Earth
Scientists have been able to reconstruct detailed information about the planet's past. The earliest dated solar system material is dated to 4.5672 ± 0.0006 billion years ago,[16] and by 4.54 billion years ago (within an uncertainty of 1%)[12] the Earth and the other planets in the Solar System formed out of the solar nebula—a disk-shaped mass of dust and gas left over from the formation of the Sun. This assembly of the Earth through accretion was largely completed within 10–20 million years.[17] Initially molten, the outer layer of the planet Earth cooled to form a solid crust when water began accumulating in the atmosphere. The Moon formed shortly thereafter, 4.53 billion years ago,[18] most likely as the result of a Mars-sized object (sometimes called Theia) with about 10% of the Earth's mass[19] impacting the Earth in a glancing blow.[20] Some of this object's mass would have merged with the Earth and a portion would have been ejected into space, but enough material would have been sent into orbit to form the Moon.

Outgassing and volcanic activity produced the primordial atmosphere. Condensing water vapor, augmented by ice and liquid water delivered by asteroids and the larger proto-planets, comets, and trans-Neptunian objects produced the oceans.[21] The newly-formed Sun was only 70% of its present luminosity, yet evidence shows that the early oceans remained liquid—a contradiction dubbed the faint young Sun paradox. A combination of greenhouse gases and higher levels of solar activity served to raise the Earth's surface temperature, preventing the oceans from freezing over.[22]

Two major models have been proposed for the rate of continental growth:[23] steady growth to the present-day[24] and rapid growth early in Earth history.[25] Current research shows that the second option is most likely, with rapid initial growth of continental crust[26] followed by a long-term steady continental area.[27][28][29] On time scales lasting hundreds of millions of years, the surface continually reshaped itself as continents formed and broke up. The continents migrated across the surface, occasionally combining to form a supercontinent. Roughly 750 million years ago (Ma), one of the earliest known supercontinents, Rodinia, began to break apart. The continents later recombined to form Pannotia, 600–540 Ma, then finally Pangaea, which broke apart 180 Ma.[30]

Evolution of life
Main article: Evolutionary history of life
At present, Earth provides the only example of an environment that has given rise to the evolution of life.[31] Highly energetic chemistry is believed to have produced a self-replicating molecule around 4 billion years ago, and half a billion years later the last common ancestor of all life existed.[32] The development of photosynthesis allowed the Sun's energy to be harvested directly by life forms; the resultant oxygen accumulated in the atmosphere and formed in a layer of ozone (a form of molecular oxygen [O3]) in the upper atmosphere. The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes.[33] True multicellular organisms formed as cells within colonies became increasingly specialized. Aided by the absorption of harmful ultraviolet radiation by the ozone layer, life colonized the surface of Earth.[34]

Since the 1960s, it has been hypothesized that severe glacial action between 750 and 580 Ma, during the Neoproterozoic, covered much of the planet in a sheet of ice. This hypothesis has been termed "Snowball Earth", and is of particular interest because it preceded the Cambrian explosion, when multicellular life forms began to proliferate.[35]

Following the Cambrian explosion, about 535 Ma, there have been five mass extinctions.[36] The last extinction event was 65 Ma, when a meteorite collision probably triggered the extinction of the (non-avian) dinosaurs and other large reptiles, but spared small animals such as mammals, which then resembled shrews. Over the past 65 million years, mammalian life has diversified, and several million years ago, an African ape-like animal such as orrorin tugenensis gained the ability to stand upright.[37] This enabled tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain. The development of agriculture, and then civilization, allowed humans to influence the Earth in a short time span as no other life form had,[38] affecting both the nature and quantity of other life forms.

The present pattern of ice ages began about 40 Ma and then intensified during the Pleistocene about 3 Ma. The polar regions have since undergone repeated cycles of glaciation and thaw, repeating every 40–100,000 years. The last ice age ended 10,000 years ago.[39]

Future
Main article: Future of the Earth
See also: Risks to civilization, humans and planet Earth

The future of the planet is closely tied to that of the Sun. As a result of the steady accumulation of helium at the Sun's core, the star's total luminosity will slowly increase. The luminosity of the Sun will grow by 10% over the next 1.1 Gyr (1.1 billion years) and by 40% over the next 3.5 Gyr.[40] Climate models indicate that the rise in radiation reaching the Earth is likely to have dire consequences, including the possible loss of the planet's oceans.[41]

The Earth's increasing surface temperature will accelerate the inorganic CO2 cycle, reducing its concentration to lethal levels for plants (10 ppm for C4 photosynthesis) in 900 million years. The lack of vegetation will result in the loss of oxygen in the atmosphere, so animal life will become extinct within several million more years.[42] After another billion years all surface water will have disappeared[14] and the mean global temperature will reach 70 °C[42](158 °F). The Earth is expected to be effectively habitable for about another 500 million years,[43] although this may be extended up to 2.3 billion years if the nitrogen is removed from the atmosphere.[44] Even if the Sun were eternal and stable, the continued internal cooling of the Earth would result in a loss of much of its CO2 due to reduced volcanism,[45] and 35% of the water in the oceans would descend to the mantle due to reduced steam venting from mid-ocean ridges.[46]

The Sun, as part of its evolution, will become a red giant in about 5 Gyr. Models predict that the Sun will expand out to about 250 times its present radius, roughly 1 AU (150,000,000 km).[40][47] Earth's fate is less clear. As a red giant, the Sun will lose roughly 30% of its mass, so, without tidal effects, the Earth will move to an orbit 1.7 AU (250,000,000 km) from the Sun when the star reaches it maximum radius. Therefore, the planet is expected to escape envelopment by the expanded Sun's sparse outer atmosphere, though most, if not all, remaining life will be destroyed because of the Sun's increased luminosity.[40] However, a more recent simulation indicates that Earth's orbit will decay due to tidal effects and drag, causing it to enter the red giant Sun's atmosphere and be destroyed.[47]

Composition and structure
Main article: Earth science
Further information: Earth physical characteristics tables
Earth is a terrestrial planet, meaning that it is a rocky body, rather than a gas giant like Jupiter. It is the largest of the four solar terrestrial planets, both in terms of size and mass. Of these four planets, Earth also has the highest density, the highest surface gravity, the strongest magnetic field, and fastest rotation.[48] It also is the only terrestrial planet with active plate tectonics.[49]

Shape
Main article: Figure of the Earth

Size comparison of inner planets (left to right): Mercury, Venus, Earth and MarsThe shape of the Earth is very close to that of an oblate spheroid, a sphere squished along the orientation from pole to pole such that there is a bulge around the equator.[50] This bulge results from the rotation of the Earth, and causes the diameter at the equator to be 43 km larger than the pole to pole diameter.[51] The average diameter of the reference spheroid is about 12,742 km, which is approximately 40,000 km/π, as the meter was originally defined as 1/10,000,000 of the distance from the equator to the North Pole through Paris, France.[52]

Local topography deviates from this idealized spheroid, though on a global scale, these deviations are very small: Earth has a tolerance of about one part in about 584, or 0.17%, from the reference spheroid, which is less than the 0.22% tolerance allowed in billiard balls.[53] The largest local deviations in the rocky surface of the Earth are Mount Everest (8,848 m above local sea level) and the Mariana Trench (10,911 m below local sea level). Because of the equatorial bulge, the feature farthest from the center of the Earth is actually Mount Chimborazo in Ecuador.[54][55]

Chemical Composition of the Crust[56] Compound Formula Composition
Continental Oceanic
silica SiO2 60.2% 48.6%
alumina Al2O3 15.2% 16.5%
lime CaO 5.5% 12.3%
magnesia MgO 3.1% 6.8%
iron(II) oxide FeO 3.8% 6.2%
sodium oxide Na2O 3.0% 2.6%
potassium oxide K2O 2.8% 0.4%
iron(III) oxide Fe2O3 2.5% 2.3%
water H2O 1.4% 1.1%
carbon dioxide CO2 1.2% 1.4%
titanium dioxide TiO2 0.7% 1.4%
phosphorus pentoxide P2O5 0.2% 0.3%
Total 99.6% 99.9%
Chemical composition
See also: Abundance of elements on Earth
The mass of the Earth is approximately 5.98 × 1024 kg. It is composed mostly of iron (32.1%), oxygen (30.1%), silicon (15.1%), magnesium (13.9%), sulfur (2.9%), nickel (1.8%), calcium (1.5%), and aluminium (1.4%); with the remaining 1.2% consisting of trace amounts of other elements. Due to mass segregation, the core region is believed to be primarily composed of iron (88.8%), with smaller amounts of nickel (5.8%), sulfur (4.5%), and less than 1% trace elements.[57]

The geochemist F. W. Clarke calculated that a little more than 47% of the Earth's crust consists of oxygen. The more common rock constituents of the Earth's crust are nearly all oxides; chlorine, sulfur and fluorine are the only important exceptions to this and their total amount in any rock is usually much less than 1%. The principal oxides are silica, alumina, iron oxides, lime, magnesia, potash and soda. The silica functions principally as an acid, forming silicates, and all the commonest minerals of igneous rocks are of this nature. From a computation based on 1,672 analyses of all kinds of rocks, Clarke deduced that 99.22% were composed of 11 oxides (see the table at right.) All the other constituents occur only in very small quantities.[note 8]

Internal structure
Main article: Structure of the Earth
The interior of the Earth, like that of the other terrestrial planets, is divided into layers by their chemical or physical (rheological) properties. The outer layer of the Earth is a chemically distinct silicate solid crust, which is underlain by a highly viscous solid mantle. The crust is separated from the mantle by the Mohorovičić discontinuity, and the thickness of the crust varies: averaging 6 km under the oceans and 30–50 km on the continents. The crust and the cold, rigid, top of the upper mantle are collectively known as the lithosphere, and it is of the lithosphere that the tectonic plates are comprised. Beneath the lithosphere is the asthenosphere, a relatively low-viscosity layer on which the lithosphere rides. Important changes in crystal structure within the mantle occur at 410 and 660 kilometers below the surface, spanning a transition zone that separates the upper and lower mantle. Beneath the mantle, an extremely low viscosity liquid outer core lies above a solid inner core.[58] The inner core may rotate at a slightly higher angular velocity than the remainder of the planet, advancing by 0.1–0.5° per year.[59]

Geologic layers of the Earth[60]
Earth cutaway from core to exosphere. Not to scale. Depth[61]
km Component Layer Density
g/cm3
0–60 Lithosphere[note 9] —
0–35 ... Crust[note 10] 2.2–2.9
35–60 ... Upper mantle 3.4–4.4
35–2890 Mantle 3.4–5.6
100–700 ... Asthenosphere —
2890–5100 Outer core 9.9–12.2
5100–6378 Inner core 12.8–13.1

Heat
Earth's internal heat comes from a combination of residual heat from planetary accretion (about 20%) and heat produced through radioactive decay (80%).[62] The major heat-producing isotopes in the Earth are potassium-40, uranium-238, uranium-235, and thorium-232.[63] At the center of the planet, the temperature may be up to 7,000 K and the pressure could reach 360 GPa.[64] Because much of the heat is provided by radioactive decay, scientists believe that early in Earth history, before isotopes with short half-lives had been depleted, Earth's heat production would have been much higher. This extra heat production, twice present-day at approximately 3 billion years ago,[62] would have increased temperature gradients within the Earth, increasing the rates of mantle convection and plate tectonics, and allowing the production of igneous rocks such as komatiites that are not formed today.[65]

Present-day major heat-producing isotopes[66] Isotope Heat release
W/kg isotope Half-life

years Mean mantle concentration
kg isotope/kg mantle Heat release
W/kg mantle
238U 9.46 × 10-5 4.47 × 109 30.8 × 10-9 2.91 × 10-12
235U 5.69 × 10-4 7.04 × 108 0.22 × 10-9 1.25 × 10-13
232Th 2.64 × 10-5 1.40 × 1010 124 × 10-9 3.27 × 10-12
40K 2.92 × 10-5 1.25 × 109 36.9 × 10-9 1.08 × 10-12

Total heat loss from the earth is 4.2 × 1013 Watts.[67] A portion of the core's thermal energy is transported toward the crust by Mantle plumes; a form of convection consisting of upwellings of higher-temperature rock. These plumes can produce hotspots and flood basalts.[68] More of the heat in the Earth is lost through plate tectonics, by mantle upwelling associated with mid-ocean ridges. The final major mode of heat loss is through conduction through the lithosphere, majority of which occurs in the oceans due to the crust there being much thinner than that of the continents.[67]

Tectonic plates
Earth's main plates[69]
Plate name Area
106 km²
African Plate[note 11] 78.0
Antarctic Plate 60.9
Australian Plate 47.2
Eurasian Plate 67.8
North American Plate 75.9
South American Plate 43.6
Pacific Plate 103.3
Main article: Plate tectonics
The mechanically rigid outer layer of the Earth, the lithosphere, is broken into pieces called tectonic plates. These plates are rigid segments that move in relation to one another at one of three types of plate boundaries: Convergent boundaries, at which two plates come together, Divergent boundaries, at which two plates are pulled apart, and Transform boundaries, in which two plates slide past one another laterally. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation can occur along these plate boundaries.[70] The tectonic plates ride on top of the asthenosphere, the solid but less-viscous part of the upper mantle that can flow and move along with the plates,[71] and their motion is strongly coupled with patterns convection inside the Earth's mantle.

As the tectonic plates migrate across the planet, the ocean floor is subducted under the leading edges of the plates at convergent boundaries. At the same time, the upwelling of mantle material at divergent boundaries creates mid-ocean ridges. The combination of these processes continually recycles the oceanic crust back into the mantle. Because of this recycling, most of the ocean floor is less than 100 million years in age. The oldest oceanic crust is located in the Western Pacific, and has an estimated age of about 200 million years.[72][73] By comparison, the oldest dated continental crust is 4030 million years old.[74]

Other notable plates include the Indian Plate, the Arabian Plate, the Caribbean Plate, the Nazca Plate off the west coast of South America and the Scotia Plate in the southern Atlantic Ocean. The Australian Plate actually fused with Indian Plate between 50 and 55 million years ago. The fastest-moving plates are the oceanic plates, with the Cocos Plate advancing at a rate of 75 mm/yr[75] and the Pacific Plate moving 52–69 mm/yr. At the other extreme, the slowest-moving plate is the Eurasian Plate, progressing at a typical rate of about 21 mm/yr.[76]

Surface
Main articles: Landform and Extreme points of Earth
The Earth's terrain varies greatly from place to place. About 70.8%[77] of the surface is covered by water, with much of the continental shelf below sea level. The submerged surface has mountainous features, including a globe-spanning mid-ocean ridge system, as well as undersea volcanoes,[51] oceanic trenches, submarine canyons, oceanic plateaus and abyssal plains. The remaining 29.2% not covered by water consists of mountains, deserts, plains, plateaus, and other geomorphologies.

The planetary surface undergoes reshaping over geological time periods due to the effects of tectonics and erosion. The surface features built up or deformed through plate tectonics are subject to steady weathering from precipitation, thermal cycles, and chemical effects. Glaciation, coastal erosion, the build-up of coral reefs, and large meteorite impacts[78] also act to reshape the landscape.


Present day Earth altimetry and bathymetry. Data from the National Geophysical Data Center's TerrainBase Digital Terrain Model.The continental crust consists of lower density material such as the igneous rocks granite and andesite. Less common is basalt, a denser volcanic rock that is the primary constituent of the ocean floors.[79] Sedimentary rock is formed from the accumulation of sediment that becomes compacted together. Nearly 75% of the continental surfaces are covered by sedimentary rocks, although they form only about 5% of the crust.[80] The third form of rock material found on Earth is metamorphic rock, which is created from the transformation of pre-existing rock types through high pressures, high temperatures, or both. The most abundant silicate minerals on the Earth's surface include quartz, the feldspars, amphibole, mica, pyroxene and olivine.[81] Common carbonate minerals include calcite (found in limestone), aragonite and dolomite.[82]

The pedosphere is the outermost layer of the Earth that is composed of soil and subject to soil formation processes. It exists at the interface of the lithosphere, atmosphere, hydrosphere and biosphere. Currently the total arable land is 13.31% of the land surface, with only 4.71% supporting permanent crops.[7] Close to 40% of the Earth's land surface is presently used for cropland and pasture, or an estimated 1.3 × 107 km² of cropland and 3.4 × 107 km² of pastureland.[83]

The elevation of the land surface of the Earth varies from the low point of −418 m at the Dead Sea, to a 2005-estimated maximum altitude of 8,848 m at the top of Mount Everest. The mean height of land above sea level is 840 m.[84]

Hydrosphere
Main article: Hydrosphere

Elevation histogram of the surface of the Earth. Approximately 71% of the Earth's surface is covered with water.The abundance of water on Earth's surface is a unique feature that distinguishes the "Blue Planet" from others in the Solar System. The Earth's hydrosphere consists chiefly of the oceans, but technically includes all water surfaces in the world, including inland seas, lakes, rivers, and underground waters down to a depth of 2,000 m. The deepest underwater location is Challenger Deep of the Mariana Trench in the Pacific Ocean with a depth of −10,911.4 m.[note 12][85] The average depth of the oceans is 3,800 m, more than four times the average height of the continents.[84]

The mass of the oceans is approximately 1.35 × 1018 metric tons, or about 1/4400 of the total mass of the Earth, and occupies a volume of 1.386 × 109 km3. If all of the land on Earth were spread evenly, water would rise to an altitude of more than 2.7 km.[note 13] About 97.5% of the water is saline, while the remaining 2.5% is fresh water. The majority of the fresh water, about 68.7%, is currently in the form of ice.[86]

About 3.5% of the total mass of the oceans consists of salt. Most of this salt was released from volcanic activity or extracted from cool, igneous rocks.[87] The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms.[88] Sea water has an important influence on the world's climate, with the oceans acting as a large heat reservoir.[89] Shifts in the oceanic temperature distribution can cause significant weather shifts, such as the El Niño-Southern Oscillation.[90]

Atmosphere
Main article: Earth's atmosphere
The atmospheric pressure on the surface of the Earth averages 101.325 kPa, with a scale height of about 8.5 km.[8] It is 78% nitrogen and 21% oxygen, with trace amounts of water vapor, carbon dioxide and other gaseous molecules. The height of the troposphere varies with latitude, ranging between 8 km at the poles to 17 km at the equator, with some variation due to weather and seasonal factors.[91]

Earth's biosphere has significantly altered its atmosphere. Oxygenic photosynthesis evolved 2.7 billion years ago, forming the primarily nitrogen-oxygen atmosphere that exists today. This change enabled the proliferation of aerobic organisms as well as the formation of the ozone layer which, together with Earth's magnetic field, blocks ultraviolet solar radiation, permitting life on land. Other atmospheric functions important to life on Earth include transporting water vapor, providing useful gases, causing small meteors to burn up before they strike the surface, and moderating temperature.[92] This last phenomenon is known as the greenhouse effect: trace molecules within the atmosphere serve to capture thermal energy emitted from the ground, thereby raising the average temperature. Carbon dioxide, water vapor, methane and ozone are the primary greenhouse gases in the Earth's atmosphere. Without this heat-retention effect, the average surface temperature would be −18 °C and life would likely not exist.[77]

Weather and climate
Main articles: Weather and Climate
The Earth's atmosphere has no definite boundary, slowly becoming thinner and fading into outer space. Three-quarters of the atmosphere's mass is contained within the first 11 km of the planet's surface. This lowest layer is called the troposphere. Energy from the Sun heats this layer, and the surface below, causing expansion of the air. This lower density air then rises, and is replaced by cooler, higher density air. The result is atmospheric circulation that drives the weather and climate through redistribution of heat energy.[93]

The primary atmospheric circulation bands consist of the trade winds in the equatorial region below 30° latitude and the westerlies in the mid-latitudes between 30° and 60°.[94] Ocean currents are also important factors in determining climate, particularly the thermohaline circulation that distributes heat energy from the equatorial oceans to the polar regions.[95]


Source regions of global air massesWater vapor generated through surface evaporation is transported by circulatory patterns in the atmosphere. When atmospheric conditions permit an uplift of warm, humid air, this water condenses and settles to the surface as precipitation.[93] Most of the water is then transported back to lower elevations by river systems, usually returning to the oceans or being deposited into lakes. This water cycle is a vital mechanism for supporting life on land, and is a primary factor in the erosion of surface features over geological periods. Precipitation patterns vary widely, ranging from several meters of water per year to less than a millimeter. Atmospheric circulation, topological features and temperature differences determine the average precipitation that falls in each region.[96]

The Earth can be sub-divided into specific latitudinal belts of approximately homogeneous climate. Ranging from the equator to the polar regions, these are the tropical (or equatorial), subtropical, temperate and polar climates.[97] Climate can also be classified based on the temperature and precipitation, with the climate regions characterized by fairly uniform air masses. The commonly used Köppen climate classification system (as modified by Wladimir Köppen's student Rudolph Geiger) has five broad groups (humid tropics, arid, humid middle latitudes, continental and cold polar), which are further divided into more specific subtypes.[94]

Upper atmosphere

This view from orbit shows the full Moon partially obscured by the Earth's atmosphere. NASA image.See also: Outer space
Above the troposphere, the atmosphere is usually divided into the stratosphere, mesosphere, and thermosphere.[92] Each of these layers has a different lapse rate, defining the rate of change in temperature with height. Beyond these, the exosphere thins out into the magnetosphere. This is where the Earth's magnetic fields interact with the solar wind.[98] An important part of the atmosphere for life on Earth is the ozone layer, a component of the stratosphere that partially shields the surface from ultraviolet light. The Kármán line, defined as 100 km above the Earth's surface, is a working definition for the boundary between atmosphere and space.[99]

Due to thermal energy, some of the molecules at the outer edge of the Earth's atmosphere have their velocity increased to the point where they can escape from the planet's gravity. This results in a slow but steady leakage of the atmosphere into space. Because unfixed hydrogen has a low molecular weight, it can achieve escape velocity more readily and it leaks into outer space at a greater rate than other gasses.[100] The leakage of hydrogen into space is a contributing factor in pushing the Earth from an initially reducing state to its current oxidizing one. Photosynthesis provided a source of free oxygen, but the loss of reducing agents such as hydrogen is believed to have been a necessary precondition for the widespread accumulation of oxygen in the atmosphere.[101] Hence the ability of hydrogen to escape from the Earth's atmosphere may have influenced the nature of life that developed on the planet.[102] In the current, oxygen-rich atmosphere most hydrogen is converted into water before it has an opportunity to escape. Instead, most of the hydrogen loss comes from the destruction of methane in the upper atmosphere.[103]

Magnetic field

The Earth's magnetic field, which approximates a dipole.Main article: Earth's magnetic field
The Earth's magnetic field is shaped roughly as a magnetic dipole, with the poles currently located proximate to the planet's geographic poles. According to dynamo theory, the field is generated within the molten outer core region where heat creates convection motions of conducting materials, generating electric currents. These in turn produce the Earth's magnetic field. The convection movements in the core are chaotic in nature, and periodically change alignment. This results in field reversals at irregular intervals averaging a few times every million years. The most recent reversal occurred approximately 700,000 years ago.[104][105]

The field forms the magnetosphere, which deflects particles in the solar wind. The sunward edge of the bow shock is located at about 13 times the radius of the Earth. The collision between the magnetic field and the solar wind forms the Van Allen radiation belts, a pair of concentric, torus-shaped regions of energetic charged particles. When the plasma enters the Earth's atmosphere at the magnetic poles, it forms the aurora.[106]


Orbit and rotation
Rotation
Main article: Earth's rotation

Earth's axial tilt (or obliquity) and its relation to the rotation axis and plane of orbit.Earth's rotation period relative to the Sun—its mean solar day—is 86,400 seconds of mean solar time. Each of these seconds is slightly longer than an SI second because Earth's solar day is now slightly longer than it was during the 19th century due to tidal acceleration.[107]

Earth's rotation period relative to the fixed stars, called its stellar day by the International Earth Rotation and Reference Systems Service (IERS), is 86164.098903691 seconds of mean solar time (UT1), or 23h 56m 4.098903691s. [108][note 14] Earth's rotation period relative to the precessing or moving mean vernal equinox, misnamed its sidereal day, is 86164.09053083288 seconds of mean solar time (UT1) (23h 56m 4.09053083288s).[108] Thus the sidereal day is shorter than the stellar day by about 8.4 ms.[109] The length of the mean solar day in SI seconds is available from the IERS for the periods 1623–2005[110] and 1962–2005.[111]

Apart from meteors within the atmosphere and low-orbiting satellites, the main apparent motion of celestial bodies in the Earth's sky is to the west at a rate of 15°/h = 15'/min. This is equivalent to an apparent diameter of the Sun or Moon every two minutes; the apparent sizes of the Sun and the Moon are approximately the same.[112][113]

Orbit
Main article: Earth's orbit
Earth orbits the Sun at an average distance of about 150 million kilometers every 365.2564 mean solar days, or one sidereal year. From Earth, this gives an apparent movement of the Sun eastward with respect to the stars at a rate of about 1°/day, or a Sun or Moon diameter every 12 hours. Because of this motion, on average it takes 24 hours—a solar day—for Earth to complete a full rotation about its axis so that the Sun returns to the meridian. The orbital speed of the Earth averages about 30 km/s (108,000 km/h), which is fast enough to cover the planet's diameter (about 12,600 km) in seven minutes, and the distance to the Moon (384,000 km) in four hours.[8]

The Moon revolves with the Earth around a common barycenter every 27.32 days relative to the background stars. When combined with the Earth–Moon system's common revolution around the Sun, the period of the synodic month, from new moon to new moon, is 29.53 days. Viewed from the celestial north pole, the motion of Earth, the Moon and their axial rotations are all counter-clockwise. Viewed from a vantage point above the north poles of both the Sun and the Earth, the Earth appears to revolve in a counterclockwise direction about the Sun. The orbital and axial planes are not precisely aligned: Earth's axis is tilted some 23.5 degrees from the perpendicular to the Earth–Sun plane, and the Earth–Moon plane is tilted about 5 degrees against the Earth-Sun plane. Without this tilt, there would be an eclipse every two weeks, alternating between lunar eclipses and solar eclipses.[8][114]

The Hill sphere, or gravitational sphere of influence, of the Earth is about 1.5 Gm (or 1,500,000 kilometers) in radius.[115][note 15] This is maximum distance at which the Earth's gravitational influence is stronger than the more distant Sun and planets. Objects must orbit the Earth within this radius, or they can become unbound by the gravitational perturbation of the Sun.


Illustration of the Milky Way Galaxy, showing the location of the Sun.Earth, along with the Solar System, is situated in the Milky Way galaxy, orbiting about 28,000 light years from the center of the galaxy. It is currently about 20 light years above the galaxy's equatorial plane in the Orion spiral arm.[116]

Axial tilt and seasons
Main article: Axial tilt
Because of the axial tilt of the Earth, the amount of sunlight reaching any given point on the surface varies over the course of the year. This results in seasonal change in climate, with summer in the northern hemisphere occurring when the North Pole is pointing toward the Sun, and winter taking place when the pole is pointed away. During the summer, the day lasts longer and the Sun climbs higher in the sky. In winter, the climate becomes generally cooler and the days shorter. Above the Arctic Circle, an extreme case is reached where there is no daylight at all for part of the year—a polar night. In the southern hemisphere the situation is exactly reversed, with the South Pole oriented opposite the direction of the North Pole.


Earth and Moon from Mars, imaged by Mars Global Surveyor. From space, the Earth can be seen to go through phases similar to the phases of the Moon.By astronomical convention, the four seasons are determined by the solstices—the point in the orbit of maximum axial tilt toward or away from the Sun—and the equinoxes, when the direction of the tilt and the direction to the Sun are perpendicular. Winter solstice occurs on about December 21, summer solstice is near June 21, spring equinox is around March 20 and autumnal equinox is about September 23.[117]

The angle of the Earth's tilt is relatively stable over long periods of time. However, the tilt does undergo nutation; a slight, irregular motion with a main period of 18.6 years. The orientation (rather than the angle) of the Earth's axis also changes over time, precessing around in a complete circle over each 25,800 year cycle; this precession is the reason for the difference between a sidereal year and a tropical year. Both of these motions are caused by the varying attraction of the Sun and Moon on the Earth's equatorial bulge. From the perspective of the Earth, the poles also migrate a few meters across the surface. This polar motion has multiple, cyclical components, which collectively are termed quasiperiodic motion. In addition to an annual component to this motion, there is a 14-month cycle called the Chandler wobble. The rotational velocity of the Earth also varies in a phenomenon known as length of day variation.[118]

In modern times, Earth's perihelion occurs around January 3, and the aphelion around July 4. However, these dates change over time due to precession and other orbital factors, which follow cyclical patterns known as Milankovitch cycles. The changing Earth-Sun distance results in an increase of about 6.9%[119] in solar energy reaching the Earth at perihelion relative to aphelion. Since the southern hemisphere is tilted toward the Sun at about the same time that the Earth reaches the closest approach to the Sun, the southern hemisphere receives slightly more energy from the Sun than does the northern over the course of a year. However, this effect is much less significant than the total energy change due to the axial tilt, and most of the excess energy is absorbed by the higher proportion of water in the southern hemisphere.[120]

Moon
Characteristics Diameter 3,474.8 km
2,159.2 mi
Mass 7.349 × 1022 kg
8.1 × 1019 (short) tons
Semi-major axis 384,400 km
238,700 mi
Orbital period 27 d 7 h 43.7 m
Main article: Moon
The Moon is a relatively large, terrestrial, planet-like satellite, with a diameter about one-quarter of the Earth's. It is the largest moon in the Solar System relative to the size of its planet. (Charon is larger relative to the dwarf planet Pluto.) The natural satellites orbiting other planets are called "moons" after Earth's Moon.

The gravitational attraction between the Earth and Moon causes tides on Earth. The same effect on the Moon has led to its tidal locking: its rotation period is the same as the time it takes to orbit the Earth. As a result, it always presents the same face to the planet. As the Moon orbits Earth, different parts of its face are illuminated by the Sun, leading to the lunar phases; the dark part of the face is separated from the light part by the solar terminator.

Because of their tidal interaction, the Moon recedes from Earth at the rate of approximately 38 mm a year. Over millions of years, these tiny modifications—and the lengthening of Earth's day by about 23 µs a year—add up to significant changes.[121] During the Devonian period, for example, (approximately 410 million years ago) there were 400 days in a year, with each day lasting 21.8 hours.[122]

The Moon may have dramatically affected the development of life by moderating the planet's climate. Paleontological evidence and computer simulations show that Earth's axial tilt is stabilized by tidal interactions with the Moon.[123] Some theorists believe that without this stabilization against the torques applied by the Sun and planets to the Earth's equatorial bulge, the rotational axis might be chaotically unstable, exhibiting chaotic changes over millions of years, as appears to be the case for Mars.[124] If Earth's axis of rotation were to approach the plane of the ecliptic, extremely severe weather could result from the resulting extreme seasonal differences. One pole would be pointed directly toward the Sun during summer and directly away during winter. Planetary scientists who have studied the effect claim that this might kill all large animal and higher plant life.[125] However, this is a controversial subject, and further studies of Mars—which has a similar rotation period and axial tilt as Earth, but not its large Moon or liquid core—may settle the matter.

Viewed from Earth, the Moon is just far enough away to have very nearly the same apparent-sized disk as the Sun. The angular size (or solid angle) of these two bodies match because, although the Sun's diameter is about 400 times as large as the Moon's, it is also 400 times more distant.[113] This allows total and annular eclipses to occur on Earth.



A scale representation of the relative sizes of, and distance between, Earth and Moon.
The most widely accepted theory of the Moon's origin, the giant impact theory, states that it formed from the collision of a Mars-size protoplanet called Theia with the early Earth. This hypothesis explains (among other things) the Moon's relative lack of iron and volatile elements, and the fact that its composition is nearly identical to that of the Earth's crust.[126]

Earth has at least two co-orbital asteroids, 3753 Cruithne and 2002 AA29.[127]

Habitability
See also: Planetary habitability

A range of theoretical habitable zones with stars of different mass (our Solar System at center). Not to scale.A planet that can sustain life is termed habitable, even if life did not originate there. The Earth provides the (currently understood) requisite conditions of liquid water, an environment where complex organic molecules can assemble, and sufficient energy to sustain metabolism.[128] The distance of the Earth from the Sun, as well as its orbital eccentricity, rate of rotation, axial tilt, geological history, sustaining atmosphere and protective magnetic field all contribute to the conditions necessary to originate and sustain life on this planet.[129]

Biosphere
Main article: Biosphere
The planet's life forms are sometimes said to form a "biosphere". This biosphere is generally believed to have begun evolving about 3.5 billion years ago. Earth is the only place in the universe where life is known to exist. Some scientists believe that Earth-like biospheres might be rare.[130]

The biosphere is divided into a number of biomes, inhabited by broadly similar plants and animals. On land primarily latitude and height above the sea level separates biomes. Terrestrial biomes lying within the Arctic, Antarctic Circle or in high altitudes are relatively barren of plant and animal life, while the greatest latitudinal diversity of species is found at the Equator.[131]

Natural resources and land use
Main article: Natural resource
The Earth provides resources that are exploitable by humans for useful purposes. Some of these are non-renewable resources, such as mineral fuels, that are difficult to replenish on a short time scale.

Large deposits of fossil fuels are obtained from the Earth's crust, consisting of coal, petroleum, natural gas and methane clathrate. These deposits are used by humans both for energy production and as feedstock for chemical production. Mineral ore bodies have also been formed in Earth's crust through a process of Ore genesis, resulting from actions of erosion and plate tectonics.[132] These bodies form concentrated sources for many metals and other useful elements.

The Earth's biosphere produces many useful biological products for humans, including (but far from limited to) food, wood, pharmaceuticals, oxygen, and the recycling of many organic wastes. The land-based ecosystem depends upon topsoil and fresh water, and the oceanic ecosystem depends upon dissolved nutrients washed down from the land.[133] Humans also live on the land by using building materials to construct shelters. In 1993, human use of land is approximately:

Land use Percentage
Arable land 13.13%[7]
Permanent crops 4.71%[7]
Permanent pastures 26%
Forests and woodland 32%
Urban areas 1.5%
Other 30%

The estimated amount of irrigated land in 1993 was 2,481,250 km².[7]

Natural and environmental hazards
Large areas are subject to extreme weather such as tropical cyclones, hurricanes, or typhoons that dominate life in those areas. Many places are subject to earthquakes, landslides, tsunamis, volcanic eruptions, tornadoes, sinkholes, blizzards, floods, droughts, and other calamities and disasters.

Many localized areas are subject to human-made pollution of the air and water, acid rain and toxic substances, loss of vegetation (overgrazing, deforestation, desertification), loss of wildlife, species extinction, soil degradation, soil depletion, erosion, and introduction of invasive species.

A scientific consensus exists linking human activities to global warming due to industrial carbon dioxide emissions. This is predicted to produce changes such as the melting of glaciers and ice sheets, more extreme temperature ranges, significant changes in weather conditions and a global rise in average sea levels.[134]

Human geography
Main article: Human geography
See also: World

Cartography, the study and practice of map making, and vicariously geography, have historically been the disciplines devoted to depicting the Earth. Surveying, the determination of locations and distances, and to a lesser extent navigation, the determination of position and direction, have developed alongside cartography and geography, providing and suitably quantifying the requisite information.

Earth has approximately 6,740,000,000 human inhabitants as of November 2008.[135] Projections indicate that the world's human population will reach seven billion in 2013 and 9.2 billion in 2050.[136] Most of the growth is expected to take place in developing nations. Human population density varies widely around the world, but a majority live in Asia. By 2020, 60% of the world's population is expected to be living in urban, rather than rural, areas.[137]

It is estimated that only one eighth of the surface of the Earth is suitable for humans to live on—three-quarters is covered by oceans, and half of the land area is either desert (14%),[138] high mountains (27%),[139] or other less suitable terrain. The northernmost permanent settlement in the world is Alert, on Ellesmere Island in Nunavut, Canada.[140] (82°28′N) The southernmost is the Amundsen-Scott South Pole Station, in Antarctica, almost exactly at the South Pole. (90°S)


The Earth at night, a composite of DMSP/OLS ground illumination data on a simulated night-time image of the world. This image is not photographic and many features are brighter than they would appear to a direct observer.Independent sovereign nations claim the planet's entire land surface, with the exception of some parts of Antarctica. As of 2007 there are 201 sovereign states, including the 192 United Nations member states. In addition, there are 59 dependent territories, and a number of autonomous areas, territories under dispute and other entities.[7] Historically, Earth has never had a sovereign government with authority over the entire globe, although a number of nation-states have striven for world domination and failed.[141]

The United Nations is a worldwide intergovernmental organization that was created with the goal of intervening in the disputes between nations, thereby avoiding armed conflict.[142] It is not, however, a world government. While the U.N. provides a mechanism for international law and, when the consensus of the membership permits, armed intervention,[143] it serves primarily as a forum for international diplomacy.

The first human to orbit the Earth was Yuri Gagarin on April 12, 1961.[144] In total, about 400 people visited outer space and reached Earth orbit as of 2004, and, of these, twelve have walked on the Moon.[145][146][147] Normally the only humans in space are those on the International Space Station. The station's crew, currently six people, is usually replaced every six months.[148] Humans traveled the farthest from the planet in 1970, when Apollo 13 crew was 400,171 km away from Earth.[149][150]


Cultural viewpoint
Main article: Earth in culture

The first photograph ever taken by astronauts of an "Earthrise", from Apollo 8The name "Earth" was derived from the Anglo-Saxon word erda, which means ground or soil. It became eorthe in Old English, then erthe in Middle English.[151] The standard astronomical symbol of the Earth consists of a cross circumscribed by a circle.[152]

Earth has often been personified as a deity, in particular a goddess. In many cultures the mother goddess, also called the Mother Earth, is also portrayed as a fertility deity. Creation myths in many religions recall a story involving the creation of the Earth by a supernatural deity or deities. A variety of religious groups, often associated with fundamentalist branches of Protestantism[153] or Islam,[154] assert that their interpretations of these creation myths in sacred texts are literal truth and should be considered alongside or replace conventional scientific accounts of the formation of the Earth and the origin and development of life.[155] Such assertions are opposed by the scientific community[156][157] and other religious groups.[158][159][160] A prominent example is the creation-evolution controversy.

In the past there were varying levels of belief in a flat Earth,[161] but this was displaced by the concept of a spherical Earth due to observation and circumnavigation.[162] The human perspective regarding the Earth has changed following the advent of spaceflight, and the biosphere is now widely viewed from a globally integrated perspective.[163][164] This is reflected in a growing environmental movement that is concerned about humankind's effects on the planet