WBBSE Class 7 Geography Chapter 4 Landforms Introduction
Not all parts of the Earth’s surface are the same. Some places are high, some places are undulating and some places are low-lying plains. This diversity of land on the Earth’s surface is called landform. The asthenosphere is the viscous or semi-liquid layer that has a depth of about 700 km below the Earth’s crust. The crust is divided into several pieces which are floating and moving within the asthenosphere. These pieces are called plates.
All landforms have been created by two main types of forces. Forces that act from within the Earth’s surface are called endogenetic forces and the ones acting from outside the Earth’s surface are called exogenetic forces. A mountain is a landform formed by rocks, usually more than 900 metres high and extends over a wide area. When the land gets uplifted due to compressional forces acting on it, a fold mountain is formed.
Tectonic movements may sometimes lead to the formation of cracks along the Earth’s surface. The middle segment of these cracks may either uplift or the segments of both sides may subduce so that the middle segment forms a block mountain. Lava, ashes and pieces of rocks that gush out during a volcanic eruption get accumulated all around a volcano, thereby forming a conical highland called a volcanic mountain. The topmost part of a mountain which has a needle-like shape is called a mountain peak.
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The lowland or depression between two mountains or mountain ranges is called a valley. A series of mountains, or more or less parallel lines of mountains extending over a large area is collectively called a mountain range.
A high elevated place on the Earth’s surface, where several mountain ranges meet or, from where several mountain ranges radiate in different directions, is known as a knot. A landform that suddenly rises over its surroundings with steep slopes on all sides, generally over 300m is called a plateau. It is also called a tableland.
Intermontane Plateaus are the highest and the most expansive type of plateaus. Vast stretches in South Africa, Western Australia, Antarctica and Greenland are occupied by Continental Plateaus.
Lava gushing during volcanic eruptions and spreading like sheets through large stretches of land can form expansive lava plateaus or Volcanic Plateaus. Plateaus are dissected by intervening rivers with the dissected pieces standing apart, interspersed by river valleys are called Dissected Plateaus.
A plain is a low, flat stretch of land. Lava plains form when lava accumulates over vast, flat, low-lying stretches. Deposits of alluvium accumulating in the valleys and beds of rivers, seas and lakes for many years form alluvial plains. A flat plain formed by the deposition of wind-blown fine sand particles is known as a loess plain.