The earth's crust is unstable, which is evident from the occurrence of earthquakes as well as of volcanic outbursts. Such events, even though, are seldom too frequent, they may produce new land from beneath the sea.
These activities are related to the internal earth movements. Earth movements are of all kinds and degrees, ranging from those that cause a fracture involving a displacement measurable in centimeters to those that give rise to mountain ranges, raised continents and depressed seas, and oceans. These earth movements are as a whole, known as "diastrophism".
The term 'diastrophism' has been defined as those crustal movements, gentle or severe, continuous or periodic, which cause the rocks to shift vertically, horizontally or in any other directions.
The diastrophic processes are broadly classified into two types as (1) Epeirogenic movement and (2) Orogenic movements. While the epeirogenic movements include the eartft movements, which are predominantly vertical, causing elevation or depression of extensive regions of the earth's crust without much deformation; organic movements include the horizontal earth movements acting more or less tangentially to the surface by which the rock are crumpled and folded along narrow belts resulting in the formation of great mountain ranges such as the Himalayas, Alps and Andes. It is, therefore, also known as 'mountain- building' movement.
This type of tectonic movement is also termed as 'Continent-building' movement as well as 'Oscillatory-movements', because of the probable change of the sign of movements when the elevation or uplifting is replaced by subsidence and the other way round.
They occur even in the present times and had been taking place in the past geological periods. Evidences of elevation and subsidence are best displayed on and near the coastline.
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