Landforms

http://www.edu.pe.ca/southernkings/glacierjf.htm

Landforms are natural features of the landscape, natural physical features of the earth’s surface, for example, valleys, plateaus, mountains, plains, hills, loess, or glaciers.

Valleys

A valley is a hollow or surface depression of the earth bounded by hills or mountains, a natural trough in the earth’s surface, that slopes down to a stream, lake or the ocean, formed by water and/or ice erosion. Systems of valleys extend through plains, hills, and mountains. Rivers and streams flowing through valleys drain interior land regions to the ocean. At the bottom of many valleys is fertile soil, which makes excellent farmland. Most valleys on dry land are formed by running water of streams and rivers .The bottom of a valley is called its floor. Most floors slope downstream. Mountain valleys usually have narrow floors. The part of the floor along riverbanks is called flood plain. A valley’s sides are called valley walls or valley slopes. A valley wall is the side slope of a stream or glacial valley. Rain, frost, wind and the atmosphere are loosening materials which fall into the stream and are carried away.

The form of a valley depends upon the rate at which deepening and widening goes on. V-shaped valleys are caused by forces such as erosion and rivers. Valleys are not at all formed by rivers. Valleys that are not V-shaped were formerly occupied by glaciers and are characteristically U-shaped, formed by the huge bodies of ice that moved along; they carved the valleys as they passed, carrying away giant boulders and huge amounts of debris. Valleys are usually in a U-shaped form. Narrow deep valleys are sometimes called canyons. A valley has two characteristics, one is low land, another is being formed between hills or mountains. Valleys in low areas have an average slope; in the mountains, valleys are deep and narrow.

Erosion by rivers is a main valley-forming process; other processes, such as movement of the earth’s crust and glaciers, also have an important part in some cases. The rate at which a river deepens its valley depends on several factors.

One factor is how fast the water is going down a channel. The water will generally reach a maximum at the point where the slope is steep. One more factor is the resistance of material through where the river channel is cutting. At the same time that a channel cuts down a valley floor, erosion carries soil and sediment down the valley slopes toward the channel. A river can remove all the material supplied easily, from the slopes and from upstream. It can continue to cut even more deeply into the bed and increase the steepness of the sides. If material can be supplied to the channel faster than it can be carried away, then the excess material accumulates on the valley floor. Steep sided valleys are often found in young mountain areas where the land is still being lifted to create mountains. Steep sided valleys occur because the uplift tends to increase the channel slope, which in turn causes the river to cut more rapidly into its bed.

Plateaus

A plateau is a large highland area of fairly level land separated from surrounding land by steep slopes. Some plateaus, like the plateau of Tibet, lie between mountain ranges. Others are higher than surrounding land. Plateaus are widespread, and together with enclosed basins they cover about 45 percent of the Earth’s land surface. Some plateaus, such as the Deccan of India and the Columbia Plateau of the United States, are basaltic and were formed as the result of many lava flows covering hundreds of thousands of square kilometers that built up the land surface. Others are the result of upward folding; still others have been left elevated by the erosion of nearby lands. Plateaus, like all elevated regions, are subject to erosion, which removes great amounts of the upland surface. Low plateaus are often farming regions, while high plateaus are usually suitable for livestock grazing. Many of the world’s high plateaus are deserts. Other plateaus are the Colorado Plateau of the United States, the Bolivian plateau in South America, and the plateaus of Anatolia, Arabia, Iran, and the Tibet region of China and the Canadian Shield or Laurentian Plateau, a U-shaped region of ancient rock, the nucleus of North America, stretching north from the Great Lakes to the Arctic Ocean. Covering more than half of Canada, it also includes most of Greenland and extends into the United States as the Adirondack Mountains and the Superior Highlands. The first part of North America to be elevated above sea level, it has stayed almost wholly untouched by many encroachments of the sea upon the continent.

Mountains

Over long periods of time, mountains are created by tremendous forces in the earth with a steep top usually shaped up to a peak or ridge. Mountains occur more often in oceans than on land; some islands are the peaks of mountains coming out of the water. Mountains are formed by volcanism, erosion, and disturbances or uplift in the earth’s crust. Most geologists believe that the majority of mountains are formed by geological forces heat and pressure producing changes under the earth’s crust and movements in the earth’s crust. They call this movement plate tectonics. This theory sees the crust of the earth divided into a number of vast rigid plates that move about at the rate of a few centimeters a year. The uplift is caused by the collision of plates below the earth’s surface that triggers various geologic processes that produce this crustal uplift. Other processes are caused by horizontal compression that is the deformation of crustal strata which produces folds or wrinkles. The Himalayas, for example, were raised by the compression that accompanied collision of the Indian plate with the Eurasian plate. Another example is Europe’s Alps and Jura mountains which were also formed by horizontal compression, generated in their case by collision with the African plate and the Eurasian plate.

Some ranges of low mountains are raised by nontectonic processes, and are caused by sculpturing effects of differential erosion. Erosion occurs when wind, rain and ice are present. Mountains are impacted by erosion through the combined action of wind, rain and ice changing the shapes of the mountains.

Volcanism causes mountains to form. Examples of mountains formed by periodically dangerous volcanic action are Mount Ranier and Mount Saint Helens in the United States, Mount Erebus in Antarctica, Mount Vesuvius in Italy, and Mount Fuji in Japan. Many of these volcanic mountains have summit craters that still emit steam and debris; others that no longer show signs of volcanic activity may only be dormant, not extinct. Shield volcanoes found in Mauna Loa and Mauna Kea in Hawaii are less spectacular even when quite high.

Plains

Plains are broad, nearly level stretches of land that have no great changes in elevation. Plains are generally lower than the land around them; they may be found along a coast or inland. Coastal plains generally rise from sea level until they meet higher landforms such as mountains or plateaus. Inland plains may be found at high altitudes.

Plant life on plains is controlled by the climate. Thick forests usually thrive on plains in humid climates, grasslands cover fairly dry plains such as the Great Plains in the United States. Plains are usually well populated because the soil and terrain are good for farming, and roads and railways are easily built between rural towns and cities.

A coastal plain is a stretch of lowland along a seacoast which slopes toward the sea. In most cases, such a plain may be an elevated part of the ocean floor. Solid materials are carried off by rivers or waves from other coastal plains; these materials are deposited along the shore extending the coast seaward. The Atlantic Coastal Plain is a good example of a fertile and well populated coastal plain. It lies along the eastern shore of North America from Nova Scotia to Florida. The sharp upward slope of the land along the plain’s inland edge is called the fall line. Coastal plains generally have a few poor harbors, but the mouth of rivers along the Atlantic seacoast have produced some fine harbors.

A flood plain is the floor of a river valley beyond the riverbed. A flood plain is formed of mud, sand, and silt that are left behind when the river overflows its banks. These materials are carried off by the river as it erodes the land upstream. A river in flood conditions can carry a large amount of eroded material, which the overflow waters deposit onto the flood plain.

Hills

Hills are elevations of the earth’s surface that have distinct summits, but are lower in elevation than mountains. Hills may be formed by a buildup of rock debris or sand deposited by glaciers and wind. Hills may be created by faults. Faults are a slight crack in the earth which can cause earthquakes. Hills are formed when these faults go slightly upward. The most famous hills in the world are the Loess hills. The Black Hills are also famous. You can find hills in low mountain valleys, valleys, plains, and even in your own backyard.

Hills are also formed by deep erosion of areas that were raised by disturbances in the earth’s crust. Erosion forms hills by carrying away all of the soil on a mountain, causing a hill to be left behind. Humans also make hills by digging soil up and dumping it in a giant pile. Volcanoes are also another way that hills are formed. Volcanoes form hills when they erupt. During the eruption, volcanic ash is spewed through the air; after the eruption, the lava or molten rock hardens and builds up a thick layer of lava rock. The ash falls on the hardened lava causing a layer of ash to form on the hill. When rain falls, this layer of ash mixes with the rainwater to form black colored water. This black water will freeze causing the lava rock to crack and crumble and eventually erode to form a hill.

Loess

Loess is a geologically recent deposit of silt or material which is usually yellowish or brown in color and consisting of tiny mineral particles brought by wind to the places where they now lie. It is a product of past glacial activity in an area. It is a sedimentary deposit of mineral particles which are finer than sand but coarser than dust or clay, deposited by the wind. Loess is a type of silt which forms fertile topsoil in some parts of the world. Loess deposits are usually a few meters thick. One of the key characteristics of these deposits is the ‘cat steps’. The soil has few clay particles to hold it together. It is composed mainly of quartz crystals which slide easily against each other, and is therefore very subject to erosion. Because of this, there are mini-earth slides, which form the steps.

Loess was formed during the time after the Ice Age when glaciers covered a great portion of the earth. When the climate warmed up, the warm temperatures melted the glaciers creating tremendous flows of water down into a valley or river, and exposing vast plains of mud. When these plains dried, strong winds blew the exposed sediments and swept the finer materials from the flood plains into huge clouds of dust, which were deposited into the bluffs, that is, bold steep banks. As silt accumulated, higher bluffs were formed. Often several loess deposits are stacked on top of each other, because each individual glacier produced new loess deposits. Topsoils made up of loess are found in the central and northwestern parts of United States, in central and eastern Europe, and in eastern China.

Glaciers

Ice sheets and glaciers form the largest component of perennial ice on this planet. Over 75% of the world’s fresh water is presently locked up in these frozen reservoirs. The Antarctic Ice Sheet is more complicated than the Greenland Ice Sheet. In the past three million years, there have been four glacial periods, each followed by an interglacial period. The most recent glacial period began about 75,000 years ago. The Antarctic Ice Sheets outlet glaciers include the steep and heavily creviced Beordmore Glacier, one of the worlds longest outlet.

A glacier is a huge mass of ice that flows slowly over land. They form in the cold polar regions and in high mountains. The low temperatures in these places enable large amounts of snow to build up and turn inti ice. Most glaciers range in thickness from about 91 to 3,000 meters. Glaciers are formed when more snow falls during the winter than melts and evaporates in summer. The extra snow gradually builds up in layers. Its increasing weight causes the snow crystals under the surface to become compact, grainlike pellets. At depths of 15 meters or more, the pellets are further compressed into thick crystals of ice. These crystals combine to form glacial ice. The ice eventually becomes so thick that it begins to move under the pressure of its own great weight.

Huge moving sheets of ice are wearing down parts of the earth’s surface. In Antarctica and Greenland, ice covers all but the tips of the highest mountains. These ice sheets are located in Greenland, Antarctica and other frozen parts of North and South Pole. The ice sheet that covers much of Greenland is growing smaller because of a gradual rise in temperature in the area since the early 1900s.