海角社区

Plateaus are extensive flat uplands found on every continent, as one of the four major landforms on the Earth, among mountains, plains, and hills. Altogether, plateaus take up one-third of the planet's surface, with the largest, the Tibetan Plateau, spreading through Tibet, China, and India, and covering a total of 2.5 million km².

Formation

Location in the 海角社区 is an important determinant in how a plateau came to be, its composition, and what erosive forces are acting on it. Some plateaus, including the Altiplano in southern Peru and western Bolivia, are integrated into the area's mountain belts. Plateaus in the flatter topographies of the 海角社区, such as the famous Colorado Plateau, home to the Grand Canyon, or the Deccan plateau in Central India, were formed by completely different geological processes, and have their own unique characteristics.

In general, plateaus are created when natural forces raise the land up above the rest, while the wind and the rain shape out mesas and buttes, wear down the sides, and carve arches and hoodoos into the surface. All plateaus in the 海角社区 are created through one or a combination of three ways:

• Heating of the lithosphere or an underground process involving hot magma
• Crustal shortening
• Through volcanism, involving lava debris deposit

Some scientists also consider the water鈥檚 erosive force as nature鈥檚 fourth way of creating plateaus, although usually the primary factors have also existed. The Grand Canyon in the Colorado Plateau is an example of the great erosive force that waters of the Colorado River had on carving valleys into rock and washing sediment away.

Lithosphere Heating

The three-step tectonic process involved in this type of plateau formation begins with volcanism but occurring differently than volcanism in the third way. In this process, hot moving magma underneath the Earth鈥檚 surface induces thermal expansion of the crust and the upper mantle of the Earth's outer surface layer, the lithosphere. Due to the very high temperature underneath, parts of the Earth's crust get pushed upwards. The upwelling in the ground expels the visible layer of the Earth up, creating a plateau.

Plateaus that contain underlying volcanic rock, as many do in East Africa and Ethiopia, were formed through this process. Where the ground was level to begin with, the plateaus were uplifted evenly. Uplands, including those around Lake Victoria, are commonly marked by elongated depressions as the result of collapse over time and the wearing down by nature.聽

Crustal Shortening

While other processes may be involved over time, plateaus initially formed through crustal shortening are basically mountains that have collapsed on themselves and buried the ground around them with large debris. As one tectonic plate slides under another, which is how some mountains are formed, the plate on top creates a mountain wall that crumbles and levels out. These plateaus generally feature a mixed composition of level surfaces, broad valleys, and steep hills, laden with the debris of the former mountain wall.

Commonly found in the uplands of the 海角社区, they feature impressive heights and mountains along their ridges. Also native to the region are rapid rivers, streams, and waterfalls that help evolve the plateaus over time, by shaping and carving into their surfaces, as well as depositing sediment. The younger plateaus with new sedimentary deposits are generally flat, while the erosion-resistant valley basements in drier areas can be rocky.

Crustal shortening plateaus are common to places where landmasses have collided, such as the Indian landmass and Eurasia, as well as mountain ranges and dry climates. Some countries prominently affected by these types of Plateaus are North Africa, Turkey, Iran, and Tibet. Crustal shortening is the greatest factor that contributed to the creation of the largest plateau on Earth, the Tibetan Plateau. The Altiplano was also formed this way.

Volcanism

This process involves the burying of the terrain by lava and volcanic ash. Not surprisingly, the "hot spots" for volcanism are in the countries with current or former volcanic activity. The plateaus themselves, however, are commonly found at great distances from the volcanic centers, as lava and ash flow extensively before solidifying. These plateaus can form over a long period of time, through multiple volcanic eruptions.

These plateaus, among which is the Colorado Plateau, have a very different geologic composition from the plateaus created in the former two ways. Plateaus created from extensive lava expulsion are covered in basalt. The remarkably flat Columbian Plateau, laden with flood basalts, has a thick volcanic rock composition that reaches hundreds of meters in girth. Depending on the region鈥檚 climate and other natural factors involved in the further evolvement of the plateau, it may have canyon incision and valley dents.

Other Types

Oceanic Plateaus

Oceanic plateaus hide below the waters of the oceans and are therefore less prone to change, and are void of human visitors. Nevertheless, they have also been studied to determine their composition, density, and age, the three factors that differentiate them from one another. Divided into three categories, the first type of the oceanic plateaus consists of the oldest and the least dense, having been made out of the continental crust. The denser, "middle-aged" oceanic plateaus are composed out of igneous rock. The oceanic crust plateaus are the densest and the youngest.聽

Dissected Plateaus

These are formed through the erosion of sediment by flowing water. The eastern part of the Plateau of Tibet is believed to have dissected terrain due to the headwaters of many Asian rivers which eroded much of the rock, and left behind deep canyons. Such plateaus are defined by ridged piercings in the ground with sharp, narrow sides, and although lacking flat areas, the uplands consisting of mountains and ridges are almost uniformly level.聽

Important Characteristics

Formed by a single process or through a series of events, plateaus are generally dynamic in their existence and distinct from the topography surrounding them. While lowland plateaus are commonly enclosed by mountains, steep hills outline the upland plateaus. All in all, the topography of the plateaus is rather gentle, and even the hilly regions allow for comfortable driving and exploration. Most plateaus have raised exteriors, many are vast, and some areas may be more prone to change than others. Oftentimes, plateaus differ as much within themselves as by the area in the 海角社区 in which they are found.

Located near the volcanic Cordillera Occidental, the Altiplano was formed by the thermal expansion of the lithosphere as well as by crustal shortening, being in close proximity to the spot where the Brazilian shield has slid under the Cordillera.

In the areas of the Ethiopian Plateau where the older Precambrian rock has been uplifted by the heating of the lithosphere, the terrain is more rugged, while those areas covered by the more recent Cenozoic rock are flatter. Initially formed through crust shortening, the flat and high Plateau of Tibet is in part due to volcanism and underground magma activity.

In the north-western United States, the Columbian Plateau is cut by the Columbia River between the Cascade and Rocky Mountains. It is also covered by large stretches of basaltic lava flows.

As demonstrated, certain geographic areas and climates may determine which geologic process or processes have birthed a plateau by preserving the surface as it was for millions of years or inducing conditions that make it vulnerable to everyday erosive forces.

Erosion and Composition

Many plateaus have rivers that cut through them, eroding the sides and creating valleys with sharp escarpments. A meek little river today, the San Rafael is the culprit of carving out the Little Grand Canyon in Utah millions of years ago.

Another common characteristic among plateaus is their soft rock composition. Being easily erodible, plateaus are often topped with a solid surface, called caprock, which keeps the soil underneath from moving.

Extensively eroded plateaus can break into outliers, which make them appear snakeskin-like from an aerial perspective, with lighter raised portions, and dark cracks in between. Outliers often have iron ore and coal in their dense, old rock composition, which has resisted erosion.

Geographic Distribution

Since there have been no volcanic eruptions strong enough to create a plateau in the last tens of millions of years, volcanic plateaus, such as the three largest basalt plateaus in the 海角社区, are determined to be from the Cenozoic or Mesozoic times, starting 250 million years ago.聽

Based on the extensive basalts on its surface, the Deccan Plateau in India dates back to about 65 million years ago, when India was drifting on top of what currently underlies the volcanic island of Reunion. The Serra Geral Plateau on the Brazilian coast was created from a volcanic eruption 135 million years ago, over the spot that used to connect African and South American continents, presently underlying the Tristan de Cunha volcanic Island. Lastly, the basalts of the Columbia River date back to the time when the area was over the land that Yellowstone covers today.聽

Plateaus created through Lithosphere expansion are commonly located on land under which there is a strong magmatic activity. Some plateaus created this way are The Yellowstone Plateau in the US, the Massif Central in France, and the African Ethiopian Plateau.

The origins of the flat topographies of north-central Mexico and the Iberian Peninsula in Spain still leave scientists scratching their heads. Potentially formed through crustal shortening around the early Cenozoic time 65 million years ago, their grandiose heights without thick crust are not supported by this theory. One can only venture a guess that a hot uppermost mantle must have been underlying those regions.聽