Mountains, plateau and plains are major landforms on the continents. Landforms change not only from place to place but also with time. We are more aware of sudden changes brought about by volcanoes, earthquakes or floods, but gradual changes are more widespread and take place continuously. Landforms are caused by the action of two types of natural processes on the land. The External Processes in the atmosphere and the hydrosphere affect the land surface. The Internal Processes in the interior of the earth cause changes on the land surface.
The external processes act slowly, wearing down the highlands and depositing materials in the lowlands. The internal processes cause movements of the earth’s crust, creating mountains and plateau. The specific landforms at any place are an outcome of these two processes at a given time.
Weathering is the process of the rocks breaking up due to weather changes - temperature, moisture and precipitation. Weathering produces a layer of loose particles of rocks on the land surface. If this layer stays undisturbed over a long period, slow chemical and organic changes lead to soil formation. The soil, essential for plant growth, consists of mineral matte as sand and clay as well as organic matter like decayed leaves, flowers and dead tissues of organisms, minute bacteria and earthworms. Soil formation is mainly governed by climate parent rock, topography and the type of vegetation. Among these, the climate is the most important as it affects weathering of the rocks, the quantity of moisture in the soil and the vegetation.
Degradation is a process in which material is removed from the highlands by erosion of the land while aggradation is a process of deposition of such materials in the lowlands, leading to a gradual increase in the level. Both degradation and aggradation take place simultaneously over different areas.
Rivers, glaciers, winds and waves are the main agents of gradation. Winds may transport material even up a slope, but such action is limited to fine sand and dust particles. As such, wind action is quite common in deserts, where vegetation is rare. Glaciers are limited to Polar Regions and ice-capped mountains. The action of running water, the most widespread mode of gradation, is known as the normal process of gradation.