geomorphology

Typical features of coastal erosion: from the initial cracks in less resistant rock through to arches, stacks, and stumps that can occur as erosion progresses.

The crust of the Earth is made up of plates with different kinds of margins. In mid-ocean, there are constructive plate margins, where magma wells up from the Earth's interior, forming new crust. On continent–continent margins, mountain ranges are flung up by the collision of two continents. At an ocean–continent destructive margin, ocean crust is forced under the denser continental crust, forming an area of volcanic instability.

The shape of a dune indicates the prevailing wind pattern. Crescent-shaped dunes form in sandy desert with winds from a constant direction. Seif dunes form on bare rocks, parallel to the wind direction. Irregular star dunes are formed by variable winds.

A glacier picks up large boulders and rock debris from the valley and deposits them at the snout of the glacier when the ice melts. Some deposited material is carried great distances by the ice to form erratics.

There are two main types of volcano, but three distinctive cone shapes. Composite volcanoes emit a stiff, rapidly solidifying lava which forms high, steep-sided cones. Volcanoes that regularly throw out ash build up flatter domes known as cinder cones. The lava from a shield volcano is not ejected violently, flowing over the crater rim forming a broad low profile.

Branch of geology developed in the late 19th century, dealing with the morphology, or form, of the Earth's surface; nowadays it is also considered to be an integral part of physical geography. Geomorphological studies investigate the nature and origin of surface landforms, such as mountains, valleys, plains, and plateaux, and the processes that influence them. These processes include the effects of tectonic forces, weathering, running water, waves, glacial ice, and wind, which result in the erosion, mass movement (landslides, rockslides, mudslides), transportation, and deposition of rocks and soils. In addition to the natural processes that mould landforms, human activity can produce changes, either directly or indirectly, and cause the erosion, transportation, and deposition of rocks and soils, for example by poor land management practices and techniques in farming and forestry, and in the mining and construction industries.

Geomorphology deals with changes in landforms from the present to the geologic past, and in spatial scales ranging from microscale to mountains. For example, the formation of mountain ranges takes place over millions of years, as the Earth's crust cools and solidifies and the resulting layers, or plates, are folded, uplifted or deformed by the seismic activity of the underlying magma (see plate tectonics). The gouging out of river valleys by glacial erosion is a gradual process that takes place over thousands of years. Conversely, volcanic eruptions, by the ejection of rocks and gases and the rapid flow of molten lava down a mountainside, create rapid changes to landforms, as with the volcanic eruptions on the island of Montserrat in the West Indies. Similarly, the eruption of undersea volcanoes can result in the sudden birth of islands, while the consequent and rapidly moving tidal waves (tsunamis), can produce the unexpected inundation and destruction of low-lying coastal regions in their path.

Erosion and weathering Landforms are changed by the processes of weathering, erosion, transportation, and deposition. For example, the shape of a mountain range is largely the result of erosive processes that progressively remove material. These include weathering, mass movements, and soil-forming processes, such as the physical, chemical, or organic breakdown of rocks into small pieces by the action of wind, rain, temperature changes, plants, and bacteria. The form that erosion takes, and the degree to which it takes place, can vary with rock type. Unconsolidated sands and gravel are more easily eroded than solid granite, while rocks such as limestone are worn down more by chemical processes than by physical forces. The weathering process does not involve particle transportation except under the effects of gravity.

Transportation and deposition After materials have been broken down and loosened by weathering, they are transported by mass movement, wind action, or running water, and deposited in new locations: glaciers transport materials embedded in them, winds lift dust particles and carry them over great distances, precipitation falling on sloping land shifts soils downhill, water currents carry materials along a riverbed or out to sea. Through deposition, the particles accumulate elsewhere: rivers and glaciers carve valleys and deposit eroded material in plains and deltas, desert winds wear away rock and form huge sand dunes, waves erode rocky shorelines and create sandy beaches. River deltas such as those of the Nile and the Ganges, which are formed by the build-up of silt at the point where the river meets the sea, demonstrate the cumulative effects on landform of transportation and deposition.

Concepts and subdisciplines At the end of the 19th century, the American geologist William Morris Davis put forward the concept he called the geomorphic cycle. He believed that landforms progressed from a youthful stage of high rugged mountains to a more mature stage of rounded forms, eventually to be worn down in old age to almost level plains. This theory does not hold up under current insights, and any specific landscape can be regarded only as the balance between whatever forces of uplift and erosion are operating at a given time. Thus the current nature of a region does not allow us to reconstruct its past or predict its future; mountain building (orogeny) is a long-drawn-out, intermittent, and uneven process, and even if the progression from youth to old age were an uninterrupted sequence, this progression would produce different landforms, depending on climatic variables. The study of the different processes and variables involved has given rise to a number of subdisciplines such as evolutionary geomorphology, climatic geomorphology, structural geomorphology, tectonic geomorphology, process geomorphology, and applied geomorphology.

Applications Geomorphology has many practical applications. An understanding of landforms and the processes involved in their formation and development provides a basis for the mapping of soils and the location of resources such as minerals and fossil fuels. It can also help to minimize the environmental damage caused by mining and poor farming practices. Land clearance, combined with harmful farming techniques, can result in soil erosion, which in turn can rob fertile land of its productive topsoil and lead to desertification, as in the creation of the Dustbowl in North America in the 1930s. The use of windbreaks and correct planting and cultivation techniques can prevent this erosion, increase agricultural production, and at the same time help to conserve the existing landforms. Some past projects for the control of coastal erosion and flooding have led to an intensification of the problems rather than their alleviation. A greater knowledge of the mechanics of offshore waves and currents can lead to a better understanding of their long-term effects, and eventually to the development of more effective methods of control. The aim of much current research in geomorphology is to increase our understanding of the complexity and interrelatedness of environmental factors. This, in turn, will aid the protection of existing landforms, and increase our ability to predict the occurrences of natural hazards such as avalanches, river flooding, and coastal erosion, and assist in the control of their consequences.