Geography

Extent

The Americas make up most of the land in Earth's western hemisphere. The northernmost point of the Americas is Kaffeklubben Island, which is the most northerly point of land on Earth. The southernmost point is the islands of Southern Thule, although they are sometimes considered part of Antarctica. The mainland of the Americas is the world's longest north-to-south landmass. The distance between its two polar extremities, the Boothia Peninsula in northern Canada and Cape Froward in Chilean Patagonia, is roughly 14,000 km (8,700 mi). The mainland's most westerly point is the end of the Seward Peninsula in Alaska; Attu Island, further off the Alaskan coast to the west, is considered the westernmost point of the Americas. Ponta do Seixas in northeastern Brazil forms the easternmost extremity of the mainland, while Nordostrundingen, in Greenland, is the most easterly point of the continental shelf.

Geology

South America broke off from the west of the supercontinent Gondwana around 135 million years ago, forming its own continent. Around 15 million years ago, the collision of the Caribbean Plate and the Pacific Plate resulted in the emergence of a series of volcanoes along the border that created a number of islands. The gaps in the archipelago of Central America filled in with material eroded off North America and South America, plus new land created by continued volcanism. By three million years ago, the continents of North America and South America were linked by the Isthmus of Panama, thereby forming the single landmass of the Americas. The Great American Interchange resulted in many species being spread across the Americas, such as the cougar, porcupine, opossums, armadillos and hummingbirds.

Topography

The geography of the western Americas is dominated by the American cordillera, with the Andes running along the west coast of South America and the Rocky Mountains and other North American Cordillera ranges running along the western side of North America. The 2,300-kilometer-long (1,400 mi) Appalachian Mountains run along the east coast of North America from Alabama to Newfoundland. North of the Appalachians, the Arctic Cordillera runs along the eastern coast of Canada.

The largest mountain ranges are the Andes and Rocky Mountains. The Sierra Nevada and the Cascade Range reach similar altitudes as the Rocky Mountains, but are significantly smaller. In North America, the greatest number of fourteeners are in the United States, and more specifically in the U.S. state of Colorado. The highest peaks of the Americas are located in the Andes, with Aconcagua of Argentina being the highest; in North America Denali (Mount McKinley) in the U.S. state of Alaska is the tallest.

Between its coastal mountain ranges, North America has vast flat areas. The Interior Plains spread over much of the continent, with low relief. The Canadian Shield covers almost 5 million km2 of North America and is generally quite flat. Similarly, the north-east of South America is covered by the flat Amazon Basin. The Brazilian Highlands on the east coast are fairly smooth but show some variations in landform, while farther south the Gran Chaco and Pampas are broad lowlands.

Climate

The climate of the Americas varies significantly from region to region. Tropical rainforest climate occurs in the latitudes of the Amazon, American cloud forests, southeastern Florida and Darien Gap. In the Rocky Mountains and Andes, dry and continental climates are observed. Often the higher altitudes of these mountains are snow-capped.

Southeastern North America is well known for its occurrence of tornadoes and hurricanes, of which the vast majority of tornadoes occur in the United States' Tornado Alley, as well as in the southerly Dixie Alley in the North American late-winter and early spring seasons. Often parts of the Caribbean are exposed to the violent effects of hurricanes. These weather systems are formed by the collision of dry, cool air from Canada and wet, warm air from the Atlantic.

Hydrology

With coastal mountains and interior plains, the Americas have several large river basins that drain the continents. The largest river basin in North America is that of the Mississippi, covering the second largest watershed on the planet. The Mississippi-Missouri river system drains most of 31 states of the U.S., most of the Great Plains, and large areas between the Rocky and Appalachian mountains. This river is the fourth longest in the world and tenth most powerful in the world.

In North America, to the east of the Appalachian Mountains, there are no major rivers but rather a series of rivers and streams that flow east with their terminus in the Atlantic Ocean, such as the Hudson River, Saint John River, and Savannah River. A similar instance arises with central Canadian rivers that drain into Hudson Bay; the largest being the Churchill River. On the west coast of North America, the main rivers are the Colorado River, Columbia River, Yukon River, Fraser River, and Sacramento River.

The Colorado River drains much of the Southern Rockies and parts of the Great Basin and Range Province. The river flows approximately 1,450 miles (2,330 km) into the Gulf of California, during which over time it has carved out natural phenomena such as the Grand Canyon and created phenomena such as the Salton Sea. The Columbia is a large river, 1,243 miles (2,000 km) long, in central western North America and is the most powerful river on the West Coast of the Americas. In the far northwest of North America, the Yukon drains much of the Alaskan peninsula and flows 1,980 miles (3,190 km) from parts of Yukon and the Northwest Territory to the Pacific. Draining to the Arctic Ocean of Canada, the Mackenzie River drains waters from the Arctic Great Lakes of Arctic Canada, as opposed to the Saint-Lawrence River that drains the Great Lakes of Southern Canada into the Atlantic Ocean. The Mackenzie River is the largest in Canada and drains 1,805,200 square kilometers (697,000 sq mi).

The largest river basin in South America is that of the Amazon, which has the highest volume flow of any river on Earth. The second largest watershed of South America is that of the Paraná River, which covers about 2.5 million km2.

Ecology

North America and South America began to develop a shared population of flora and fauna around 2.5 million years ago, when continental drift brought the two continents into contact via the Isthmus of Panama. Initially, the exchange of biota was roughly equal, with North American genera migrating into South America in about the same proportions as South American genera migrated into North America. This exchange is known as the Great American Interchange. The exchange became lopsided after roughly a million years, with the total spread of South American genera into North America far more limited in scope than the spread on North American genera into South America.