5.2.1. Diastrophism

All processes that move, elevate or build up portions of the earth’s crust come under diastrophism. These forces operate slowly and their effects are visible only after thousands of years. The diastrophic forces include both the vertical and horizontal movements. They include:

(a) Orogenic processes involve mountain building through severe folding, faulting, thrusting, often as a result of plate tectonics. It includes forces of compression and tension which are tangential to the earth’s surface in contrast to radial forces under epeirogenesis. Under compression forces, sediments within geosynclines are buckled and deformed into long, linear mountain chains (Himalayas). Under the operation of

intense tensional forces, the rock strata are fractured. The line along which displacement of the fractured rock strata takes place is called the fault line (Narmada rift valley).

(b) Epeirogenic processes involve upliftment or depression of the Earth’s crust at a continental scale which moves the crustal rocks enmasse in a vertical or radial direction. It is a continental building process. Epeirogenic movement can be permanent or transient. The movement is caused by a set of forces acting along the Earth radius, such as those contributing to isostasy and faulting. For ex - Epeirogenic movement has caused the southern Rocky Mountain region to be uplifted from 1300 to 2000m in the past.

(c) Earthquake1 involves a shock or series of shocks due to sudden movement of crustal rocks within the crust or mantle. Earthquakes are generally associated with boundaries of tectonic plates. There are instances where earthquakes have occurred well inside the tectonic plate. The release of energy occurs along the fault. A fault is a sharp break in the crustal rocks. Tendency of rocks to move apart at some point of time overcomes the friction. This causes release of energy and the energy waves in all directions.

(d) Plate tectonics involves horizontal movements of crustal plates.