In between the mountain systems lie wide plains that contain deep, rich soil. Much of the soil was formed from material deposited during the most recent glacial period. This Ice Age reached its peak about 18, years ago. As glaciers retreated, streams of melted ice dropped sediment on the land, building layers of fertile soil in the plains region. North America contains a variety of natural wonders. Landforms and all types of vegetation can be found within its boundaries.
Yellowstone National Park, in the U. Despite its name, Greenland is mostly covered with ice. Its ice is a remnant of the great ice sheets that once blanketed much of the North American continent. Greenland is the only place besides Antarctica that still has an ice sheet. From the freezing Arctic to the tropical jungles of Central America , North America enjoys more climate variation than any other continent. Parts of the Canadian province of British Columbia and the U.
South America is the fourth-largest continent and extends from the sunny beaches of the Caribbean Sea to the frigid waters near the Antarctic Circle. These islands even host some Antarctic birds, such as penguins, albatrosses, and terns.
Early Spanish explorers visiting the islands for the first time saw small fires dotting the land. These fires, made by indigenous people, seemed to float on the water, which is probably how the islands got their name—Tierra del Fuego means "Land of Fire.
Many active volcanoes dot the range. These volcanic areas are fueled by heat generated as a large oceanic plate, called the Nazca Plate, grinds beneath the plate carrying South America. The central-southern area of South America has pampas , or plains.
These rich areas are ideal for agriculture. The growing of wheat is a major industry in the pampas. Grazing animals, such as cattle and sheep, are also raised in the pampas region.
In volume, the Amazon is the largest river in the world. More water flows from it than from the next six largest rivers combined. Water flows more than meters 3, feet —almost a mile. The falls are so high that most of the water evaporates into mist or is blown away by wind before it reaches the ground. South American rainforests contain an enormous wealth of animal and plant life. More than 15, species of plants and animals are found only in the Amazon River basin.
Many Amazonian plant species are sources of food and medicine for the rest of the world. Scientists are trying to find ways to preserve this precious and fragile environment as people move into the Amazon basin and clear land for settlements and agriculture. In total area, the continent of Europe is only slightly larger than the country of Canada. However, the population of Europe is more than twice that of South America.
Most European countries have access to the ocean. The continent is bordered by the Arctic Ocean in the north, the Atlantic Ocean in the west, the Caspian Sea in the southeast, and the Mediterranean and Black Seas in the south.
Early Europeans learned the river systems of the Volga, Danube, Don, Rhine, and Po, and could successfully travel the length and width of the small continent for trade , communication , or conquest. European explorers were responsible for colonizing land on every continent except Antarctica. This colonization process had a drastic impact on the economic and political development of those continents, as well as Europe.
In the east, the Ural Mountains separate Europe from Asia. The nations of Russia and Kazakhstan straddle both continents. To the south, the Alps form an arc stretching from Albania to Austria, then across Switzerland and northern Italy into France. A large area of gently rolling plains extends from northern France eastward to the Urals. A climate of warm summers, cold winters, and plentiful rain helps make much of this European farmland very productive.
Almost all of Europe sits on the massive Eurasian Plate. Africa Africa, the second-largest continent, covers an area more than three times that of the United States. From north to south, Africa stretches about 8, kilometers 5, miles. It is connected to Asia by the Isthmus of Suez in Egypt. A series of falls and rapids along the southern part of the river makes navigation difficult. The Nile has played an important role in the history of Africa.
In ancient Egyptian civilization , it was a source of life for food, water, and transportation. The top half of Africa is mostly dry, hot desert. The middle area has savannas, or flat, grassy plains. This region is home to wild animals such as lions, giraffes, elephants, hyenas, cheetahs, and wildebeests.
The central and southern areas of Africa are dominated by rainforests. Much of Africa is a high plateau surrounded by narrow strips of coastal lowlands.
Hilly uplands and mountains rise in some areas of the interior. Glaciers on Mount Kilimanjaro in Tanzania sit just kilometers from the tropical jungles below. Even though Kilimanjaro is not far from the Equator, snow covers its summit all year long. The rift valley actually starts in southwestern Asia. The Great Rift Valley is a site of major tectonic activity, where the continent of Africa is splitting into two.
Geologists have already named the two parts of the African Plate. The area of central-eastern Africa is important to scientists who study evolution and the earliest origins of humanity. This area is thought to be the place where hominids began to evolve. The entire continent of Africa sits on the African Plate. Asia Asia, the largest continent, stretches from the eastern Mediterranean Sea to the western Pacific Ocean.
There are more than 40 countries in Asia. Some are among the most-populated countries in the world, including China, India, and Indonesia. The continent of Asia includes many islands, some of them countries.
The plateaus in Central Asia are largely unsuitable for farming and are thinly populated. Skip to navigation Skip to content. This site is being redeveloped. For all the latest ABC Science content click here. Site Navigation Video Audio Photos.
Email the editor Share this article Email a friend. Follow us. The most noteworthy landmass accretion is the deposition of alluvium, often containing precious metals, on riverbanks and in river deltas. How do continents grow by accretion?
Don Mac. Feb 23, Related questions How does plate tectonics relate to pangaea? How does temperature change as depth increases within the earth? Nature has already accomplished a widespread sampling through the erosion and deposition of sediments. Lowly muds, now turned into solid sedimentary rock, give a surprisingly good average composition for the exposed continental crust. These samples are, however, missing those elements that are soluble in water, such as sodium and calcium.
Among the insoluble materials that are transferred from the crust into sediments without distortion in their relative abundances are the 14 rare-earth elements, known to geochemists as REEs.
These elemental tags are uniquely useful in deciphering crustal composition because their atoms do not fit neatly into the crystal structure of most common minerals. They tend instead to be concentrated in the late-forming granitic products of a cooling magma that make up most of the continental crust. Because the REE patterns found in a variety of sediments are so similar, geochemists surmise that weathering, erosion and sedimentation must mix different igneous source rocks efficiently enough to create an overall sample of the continental crust.
All the members of the REE group establish a signature of upper crustal composition and preserve, in the shapes of the elemental abundance patterns, a record of the igneous events that may have influenced the makeup of the crust.
Using these geochemical tracers, geologists have, for example, determined that the composition of the upper part of the continental crust approximates that of granodiorite, an ordinary igneous rock that consists largely of light-colored quartz and feldspar, along with a peppering of various dark minerals.
Deep within the continental crust, below about 10 to 15 kilometers, rock of a more basaltic composition is probably common. The exact nature of this material remains controversial, and geologists are currently testing their ideas using measurements of the heat produced within the crust by the important radioactive elements uranium, thorium and 40 K, the radioactive isotope of potassium. But it seems reasonable that at least parts of this inaccessible and enigmatic region may consist of basalt trapped and underplated beneath the lower-density continents.
It is this physical property of granitic rock--low density--that explains why most of the continents are not submerged. Continental crust rises on average meters above sea level, and some 15 percent of the continental area extends over two kilometers in elevation. These great heights contrast markedly with the depths of ocean floors, which average about four kilometers below sea level--a direct consequence of their being lined by dense oceanic crust composed mostly of basalt and a thin veneer of sediment.
At the base of the crust lies the so-called Mohorovicic discontinuity a tongue-twisting name geologists invariably shorten to "Moho". This deep surface marks a radical change in composition to an extremely dense rock rich in the mineral olivine that everywhere underlies both oceans and continents.
Geophysical studies using seismic waves have traced the Moho worldwide. Such research has also indicated that the mantle below the continents may be permanently attached at the top. These relatively cool subcrustal "keels" can be as much as kilometers thick and appear to ride with the continents during their plate-tectonic wanderings.
Support for this notion comes from the analysis of tiny mineral inclusions found within diamonds, which are thought to originate deep in this subcrustal region. Measurements show that diamonds can be up to three billion years old and thus demonstrate the antiquity of the deep continental roots. It is curious to reflect that less than 50 years ago, there was no evidence that the rocks lining ocean basins differed in any fundamental way from those found on land.
The oceans were simply thought to be floored with foundered or sunken continents. This perception grew naturally enough from the concept that the continental crust was a world-encircling feature that had arisen as a kind of scum on an initially molten planet.
Although it now appears certain that Earth did in fact melt very early, it seems that a primary granitic crust, of the type presumed decades ago, never actually existed.
To answer this question, one needs to consider the earliest history of the solar system. In the region of the primordial solar nebula occupied by Earths orbit, gas was mostly swept away, and only rocky debris large enough to survive intense early solar activity accumulated.
These objects themselves must have grown by accretion, before finally falling together to form our planet, a process that required about 50 million to million years. Late in this stage of formation, a massive planetesimal, perhaps one the size of Mars, crashed into the nearly fully formed Earth. The rocky mantle of the impactor was ejected into orbit and became the moon while the metallic core of the body fell into Earth. As might be expected, this event proved catastrophic: it totally melted the newly formed planet.
As Earth later cooled and solidified, an early basaltic crust probably formed. It is likely that at this stage the surface of Earth resembled the current appearance of Venus; however, none of this primary crust has survived. Whether it sank into the mantle in a manner similar to that taking place on Earth or piled up in localized masses until it was thick enough to transform into a denser rock and sink remains uncertain.
In any event, there is no evidence of substantial granitic crust at this early stage. Telltale evidence of such a crust should have survived in the form of scattered grains of the mineral zircon, which forms within granite and is very resistant to erosion. Although a few ancient zircons dating from near this time have been found the oldest examples are from sedimentary rocks in Australia and are about 4.
More information about the early crust comes from the most ancient rocks to have survived intact. These rocks formed deep within the crust just less than four billion years ago and now outcrop at the surface in northwest Canada. This rock formation is called the Acasta Gneiss.
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