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Foundations of Social Studies GEOGRAPHY LANDFORMS A landform is any natural formation of rock or dirt. Landforms can be as large as a mountain range, or as small as a hill; as “uninteresting” as a plain, or as fascinating as an arch. Geographers and geologists study how landforms are created, how they evolve and change, and where they are located. A term which is synonymous with the term landforms is topography. For example, we might say that the topography of an area is rough, if it is mountainous or hilly. We might say the topography is smooth, if it is a plain or if we are talking about a valley which has been carved by a glacier. The walls of many canyons have steep topography. If we are talking about a very large area, we would not be surprised if it was described as having varied topography because it had a wide variety of landforms. Below is a list of some important basic types of landforms but there are many, many more. arch butte dune mountain sinkhole archipelago canyon gulf peninsula strait atoll cape isthmus plain valley basin cliff mesa plateau volcano bay delta moraine promontory The Creation of Landforms The great variety of landforms on earth are the result of two basic forces: internal forces – which are those that come from deep inside the earth and push the landforms up, and external forces – which are those present on the surface of the earth or in the earth’s atmosphere and which wear the landforms down. Internal Processes The internal forces are usually called tectonic processes. The surface, or crust, of the earth is made of about a dozen rigid slabs, called plates. These are much larger than the continents as huge portions of the plates are under the oceans but still part of the same plate as the neighboring continent. Through a process called plate tectonics, these giants slabs move slowly across the surface of earth. As they do so, they grind past each other, push on top of each other and, in some places, are torn apart. As the plates move, grind, split, crash, and crumple they create some of the world’s largest landforms – mountain chain, plateau lands, and islands – by pushing each other up. When it occurs it feels jarring to humans and are called earthquakes, these changes in the plates happen very slowly, taking millions of years. How are these giant tectonic plates able to move slowly across the earth? Underneath the rigid layer of crust on which we live are deeper layers which are not solid. The earth’s crustal plates are able to move slowly across these inner layers which consist of molten rock called magma. Sometimes this magma is able to force its way up to the surface and form a volcano. This is Copyright © 2013, Davis School District especially common along the edges of some of the plates. Volcanism is another internal process which shapes the landforms we see on the surface of the earth. External Processes The surface of the earth is subject to forces which change its shape. The most important forces are weathering and erosion. Weathering occurs when rock surfaces decompose and begin to break up. Erosion refers to the actual movement of the broken particles away from their source. These two processes occur at the same time and result in the changing shape of land, that is, the creation of landforms. Weathering is caused by water on the surface of rocks, by plants whose roots break up rocks, and even by animals. There are also many ways for erosion to take place once weathering has begun to break a rock down. Rainfall, rivers, and glaciers, can all move those portions of rock which have been weathered and carve the surface of the earth. Coastlines – along lakes, seas, and oceans – are subject to erosion by the pounding of waves. Wind is another important erosional force. Different types of rocks result in different types of landforms. This is because different minerals, of which the rocks are made, erode in different ways. For example, if the surface area of a region is made of limestone it is likely that there will be many caves because limestone erodes easily in water. If a region is made of sandstone - large dunes, gravel deposits and steep cliffs may be expected. Man can also effect the creation of landforms. As pollution and chemicals in the air mixes with precipitation, the result is acid rain which can erode minerals more effectively and kill plant life. The loss of plant life leads to increased erosion. Overgrazing of animals and deforestation can also lead to increased erosion. Finally, man has the technological and mechanical ability – with the use of machinery – to physically alter the landforms of the earth by digging, blasting, and moving huge portions of the surface. Physiographic Regions Geographers divide the surface of the earth into huge regions based on the predominant type of landform found in the region. These are called physiographic regions because they are based on physical features. They are also sometimes called landform regions. The United States is usually divided into 25 separate and distinct regions based on physical geography. The largest region in the eastern US is the Coastal Plain. Two large regions make up most of the central United States. They are the Central Lowland and the Great Plains. In the western United States the largest single region is the Great Basin, but the Rocky Mountains cover nearly as large a territory. In summary, the earth’s surface can be divided into regions based on the topography, or landforms, which are found there. The topography is created by internal forces which push the land up and by external forces which wear it down into different shapes. These shapes, called landforms, are based on the type of rock of which they are made and the type of erosion which is wearing it down. Regions based on physical features are sometimes called physiographic regions. Copyright © 2013, Davis School District