Wednesday, July 27, 2011

External forces that can modify landforms

Crustal movements lead to the formation of landforms like volcanoes and plateaux on the Earth's surface. Theses lanforms are constantly changing , with new landforms being created and others destroyed. These are a few reasons that lead to changing landforms:

Weathering is a process by which rocks are progressively broken down into fragments where they are. Weathering can be caused by changes in temperature, action of water and action of plants.

Large changes in temperature can cause weathering. For example in the desert, the hot sun heats up the surface of the rock during the day and causes the surface area to expand. However, the inside of the rock remains cool as rocks are poor conductors of heat. At night, the atmospheric air temperature drops. The surface layer cools and contracts fater than the inside of the rock. This constant expansion and contraction weakens the rock's surface layer. If water is present, the rock will crack and the surface layer will break off in layers or into small pieces.

Water is also an important agent in weathering. Rainwater combines with carbon dioxide in the air to form carbonic acid which dissolves certain types of rocks such as limestone(soft rock). When acidic rainwater seeps into cracks in the limstone, it dissolves parts of the rocks. Over time, the acid can wear down enough limestone rocks to form landforms such as caves.

ON a high mountain, water from melting snow or rain may enter cracks in rocks. At night, when the temperature falls below 0 degrees celsius, the water turns into ice. When the water freezes, it expands and causes the cracks in the rock to widen. During the day, when the temperature rises, the ice melts. This alternate freezing of water and melting of ice causes the cracks to become wider, eventually breaking the rocks apart.

Weathering can also be caused by plants. For example, teh wind may deposit seeds into the cracks of rocks. As the seeds grow into plants, the roots grow into the rocks and force the cracks in the rocks to penetrate concrete and cause cracks in pavements.

Unlike weathering which breaks down rocks into fragments where they are, erosion is the process of wearing down surface materials and moving them from one place to another. Erosion is caused mainly by moving water, waves and wind.

Water in streams and rivers carry a lot of sediments, or small paricles such as silt, fine sand and mud. As this water flows over rocks, it wears down the rocks in its path. The sediments in the water act like sandpaper, scraping and wearing away the rocks. Over time, the action of the water and the sediments it carries gradually erode the Earth's surface.

Waves are powerful agents of erosion. The constant breaking down of waves against the shore, togother with the grinding action of sand and stonescarries in the waves, erodes the sashore to produce sandy beaches, caves and cliffs.

Wind erosion is common in dry areas where there are few plants to hold the soil or othere loose weathered materials in its place. In deserts, strong wings can carry large amounts of sand and wear away the surfaces of rocks in their paths. The sand polishes and smoothens the surface of these rocks. As sand is heavy, it cannot be blown very high into the air. Hence, erosion tends to take place mainly at the base of rocks.

Sources: our textbook
Cherie and Nicole :)


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