With the aid of diagrams describe the characteristics and mode of formation of any two of the following (a) Yardang (b) Crater (c) Block mountain

Answer Details

Characteristics and mode of formation (any two)

(a) Yardang

Characteristics: A yardang is a long, narrow, streamlined ridge of hard rock separated by grooves or troughs, found in hot deserts. The ridges run parallel to the direction of the prevailing wind and have a steep, blunt front facing the wind and a tapering tail. They may be several metres high.

Mode of formation: Yardangs are formed by wind erosion (abrasion and deflation). In a desert where soft and hard rocks lie side by side in bands parallel to the wind, the wind, armed with sand, wears away the softer bands into grooves, leaving the harder bands standing up as ridges. Abrasion is strongest near the ground, undercutting the ridges. (Diagram: parallel ridges and grooves aligned with the wind arrow.)

(b) Crater

Characteristics: A crater is a roughly circular, funnel-shaped or basin-shaped depression at the top (summit) of a volcano, surrounding the vent. It has steep inner walls and may hold water to form a crater lake. A very large crater formed by collapse or violent explosion is called a caldera.

Mode of formation: A crater is formed during volcanic eruption. As lava, ash and gases are blown out through the vent, material piles up around the opening, and the funnel-shaped mouth left at the top of the cone is the crater. Collapse of the summit into the emptied magma chamber, or a violent explosion, may enlarge it into a caldera. (Diagram: a volcanic cone with a bowl-shaped crater and central vent.)

(c) Block mountain (horst)

Characteristics: A block mountain is an upstanding block of land with steep fault sides and a fairly level top, bounded by roughly parallel faults. Examples include the Ruwenzori and the Vosges.

Mode of formation: It is formed by faulting. When the crust is subjected to tension or compression, cracks (faults) develop, and a middle block is left standing higher either because it is uplifted or because the blocks on either side subside (sink). The raised block is the block mountain (horst); a sunken block between faults gives a rift valley. (Diagram: a central up-thrown block standing above two down-thrown blocks bounded by faults.)