Geomorphology


Nature Gallery (Geography)
Geomorphology
Geomorphology is the study of the form and structure of the Earth. The size and shape of the Earth's surface, on land and beneath the sea, is constantly changing. This is due to landform evolution and the processes of weathering, which have altered the original geological shape of the rocks. Landform evolution and weathering are controlled by many different processes, the most important being climate and Earth movements, or plate tectonics. Plate tectonics alter the geomorphology of the landscape more dramatically than any other natural process. For instance, they may create the steep slopes associated with mountain chains and volcanoes, whose continual uplift or outpouring of lava contributes more to topography than climate. Continual fault movements in earthquake-prone areas will also tend to dominate the landscape more than the modifying effects of erosion. Landscapes experiencing different types of climate are subject to very different types of weathering. In cold, mountain areas, glaciation is the dominant shaping force at work; water is the main weathering agent in warm, humid coastal and river regions; and wind erosion is responsible for shaping many of the landforms in dry deserts.

Effects of Cold Climates
At present, glacial conditions control the shape of the land in places such as Antarctica and Greenland, because the power of erosion is greater than the effects of wind or rain. Many glaciers also transport huge quantities of sediment, called till, which is deposited in ridges and piles known as moraines. These landforms may occur along the sides or bases of valley glaciers, or as ridges of debris, called terminal moraine, where the glacier snout melts and retreats. Melting glaciers produce large quantities of meltwater, causing springtime floods downstream. This water also carries silt, sand, and clay that is deposited across the front of the glaciated area as an outwash fan. Evidence of past glaciation can be seen in the U-shaped valleys of the Sierra Nevada in California, and on Norway's west coast, where these wide, flat-bottomed features have subsequently been flooded by the sea to form fjords. During the last Ice Age, from 2 million to 11,000 years ago, ice sheets covered the whole of northern Europe and North America. Ice sheets can spread over mountain ranges, lowlands, and oceans. They may contain significant rock debris, and are heavy enough to depress the Earth's crust beneath them. Upon melting, the crust rebounds, causing uplift and the formation of features such as raised beaches. Today, the Malaspina Glacier ice sheet in North America is 80 kilometres (50 miles) long and more than 300 metres (1,000 feet) thick. Even more impressive is the great Antarctica ice sheet, which covers an area of 12,393,000 square kilometres (4,784,964 square miles). In areas of cold climate away from the action of glaciers, the ground may stay frozen almost all year round. This condition, known as permafrost, causes distinctive polygonal and circular cracks in the overlying soil, which result from shrinkage of the ground when it freezes. During warm periods, water runs into the cracks, only to freeze again when the deep ice expands, forcing the cracks further apart. Effects of Humid Climates
In areas where there are slopes, humid climates will create rain that feeds the streams and rivers responsible for erosion of the land. If the land is continually uplifted by Earth movements, the rivers will cut deeply into the underlying rock, for example in the Samaria Gorge in Crete. This pattern of incision is typical of immature landforms. When uplift decreases, or when the slopes are worn down, the landforms show less evidence of incision and are considered "mature".

Rivers create landforms through erosion and deposition. Most erosion takes place upstream, while deposition generally occurs downstream. River erosion is caused by water dissolving rocks, and by the abrasion caused when cobbles, pebbles, and sand grains rub along the base of the river valley and against each other. These moving, eroding fragments are known as the river's load. The amount of load depends on the strength of the river currents.

Downstream from hills and mountains, river sedimentation creates a flat area of land called a floodplain. As the name suggests, this landform originates from the periodic flooding of the adjacent river, a process that results in widespread deposition of sediment. Flooding may occur each year, during springtime or monsoonal floods, and plays an important role in renewing the sediment and water that help soil formation. Floodplains are continually being shaped by the settling of new sediment, the growth of vegetation, and the erosion and deposition of sediment by rivers as they meander across the floodplain. When a meander in a river is abandoned by the river taking a shortcut across the river loop, an oxbow lake may be formed. This area of still water will also gradually fill with silt and clay. Floods are both destructive and constructive. They wash away roads, buildings, and soil from some areas, and deposit large volumes of sediment elsewhere on the floodplain or in the ocean. Floods are usually caused by rainstorms, especially cloudbursts, in upland areas, but snowmelt, earthquakes, dam bursts, and volcanic eruptions can also cause floods. Their unpredictable nature can cause huge damage to human life, as in Bangladesh, where floods occur every year. Coastal floods can also occur when high tides and onshore winds in storms or hurricanes cause lowland coasts to flood with seawater.

There are four types of coast: rocky cliffs, where erosion is important; sandy beaches and dunes, where sediment is transported; coastal plains, such as estuaries, lagoons, and salt marshes, where clay and silt may be trapped; and deltas, where rivers enter the sea and deposit great volumes of sediment. All of these areas are related. Fragments of rock washed down from the cliffs are stored on beaches before being swept offshore during storms, or onto marshes and into lagoons. Cliffs often show dramatic features of destruction such as caves, natural arches, and stacks. By contrast, beaches are often composed of small fragments of nearby rocks, which explains the black sand found on the beaches of the Canary Islands, Azores, or Hawaii. Beach sand may also come from the sea, when offshore sand or shell banks are eroded by storms and the sediment dumped near to the land.

Beaches are not permanent features, since the sand is often simply being stored before moving on. They also change in response to storms, when large banks of sand accumulate near to the land. Marine currents move beach sand on a more regular basis than storms. Currents that are parallel to the coast, known as longshore drift, create spits of sand across river estuaries, as at Spithead in the Solent Channel, England. As longshore drift continues, spits grow out across estuary mouths, where they may be deflected out to sea by discharging water. When a spit extends out to an island, a lagoon is formed behind it, and the spit is then called a tombolo: Chesil Beach, in England, is an example of a tombolo. Coasts are constantly changing due to erosion, sediment transport, and deposition, and are consequently very unstable places to live. They are also prone to storm damage when ocean waves and winds hit the land. Coastal erosion may cause the destruction of buildings, port facilities, waterways, and amenities. Sea walls and breakwaters are built to combat this destruction. Similarly, constructing sediment traps such as groynes, and dredging sediment in ports, are methods used to limit the problem of sediment accumulation. Effects of Arid Climates
Deserts are arid areas of land where more water is lost through evaporation than is gained from precipitation. As well as hot, sandy deserts such as the Sahara, there are rocky deserts, cold deserts, and sparsely vegetated deserts. All of these deserts are dry, and their landforms are shaped by wind erosion, wind transport, episodic flooding, and evaporation. In the Southern Hemisphere, deserts such as the Atacama Desert in South America, the Namib and Kalahari Deserts in southern Africa, and the entire continental interior of Western Australia are arid because they lie under the eastern side of persistent high-pressure areas. Northern Hemisphere deserts, such as Death Valley in Nevada and eastern California, which lies inland beyond the wall of the Sierra Nevada, are rain shadow deserts. In central China, the high, cold Gobi Desert lies north of the Himalayas, the world's highest mountains, which block moist air masses sweeping north from the Indian Ocean. In the driest desert areas, mean annual precipitation commonly ranges from a maximum of 450 mm (18 in) to 0 mm, with periods of as much as 30 months when no rain is recorded. In general, throughout desert areas mean annual temperatures range from 2 C to 18C (35 F to 65 deg F) for cold deserts and from 18 C to 27 C (65 F to 80 F) for hot deserts. Water comes either from very episodic precipitation, or from underground sources of groundwater. Extreme temperature variations of up to 30C between day and night are typical of desert climates. Hot daytime temperatures cause rocks to expand, and as the air cools dramatically at night, they contract again.

Over time, stresses build up in the continually expanding and contracting rock, until eventually the surface layer fractures or shatters. This is called exfoliation, as it is rather like a tree or animal shedding an outer layer of bark or skin. Wind-sculpted rocks may form peculiar pedestals and the straight-sided pebbles known as ventifacts and driekanters, all of which may have a desert varnish from the polishing effects of wind-blown sand. Wind-deposited sediments form linear or curved dunes, or end up in inland salt lakes, such as the Great Salt Lake in North America, or on coastal plains, for example in the Persian Gulf. Water erosion occurs during flash floods, when water may flow in sheets across rocky surfaces, or along storm channels known as wadis. The load carried by water as it flows down wadis may disperse onto further open plains where one type of alluvial fan will be generated: another type of fan occurs at the foot of gorges in rocky desert hills. Mesas and buttes are commonly found in arid or semi-arid areas, such as Arizona and Utah, and are also formed by erosion due to occasional heavy rains. Global climate change and inefficient control of soil erosion in agriculture can cause desertification, which is of major concern as it affects some of the poorest countries of the world, for example in the Sahel region of North Africa. Combating the spread of deserts depends on developing good irrigation and better methods of agriculture, both of which require energy and education. Changes in climate are known to have occurred both in the geological past, tens and hundreds of millions of years ago, and in the recent past, hundreds and thousands of years ago. Thus landforms seen in one climate belt at the present day may in fact have formed many years ago in a rather different climate. The glaciated valleys of Scotland and the Lake District in England are examples of where a much colder climate has left a geomorphology now in a warmer climate. Similarly, satellite pictures have revealed traces of ancient water systems in the rocks in desert regions of Africa.