Original or Tectonic mountains are formed as a result of tectonic forces.The tectonic mountains may be categorized into:
- Fold mountains (the Himalayas, the Rockies, the Andes)
- Block mountains (Vosges mountains in France, the Black Forest in Germany, Vindhya and Satpura in India)
- Volcanic mountains (Cascade Range in the USA, Mount Kenya, Mount Kilimanjaro, Mount Fujiyama).
A fold is an undulating structure (wave-like) that forms when rocks or a part of the earth’s crust is folded (deformed by bending) under compressional stress. The folds are made up of multiple strata (rock layers).
The folds that are upwardly convex are called as anticlines. The core (centre) of an anticline fold consists of the older strata, and the strata are progressively younger outwards.
In contrast, the folds that are downwardly convex are called synclines. The core of a syncline fold consists of the younger strata, and the strata are progressively older outwards.
The term “block mountain” was introduced by W. M. Davis; other terms are “fault-block mountain” as given by D. W. Johnson (1903) or simply “fault blocks” of Strahler (1946), for the “initial land forms,” i.e., mountains, where geotectonically positive, produced by crustal fracturing. The simple symmetrically bounded positive fault block is known as a horst (q.v.), and thus we have “horst mountain” (Geikie, 1914), but the term block mountain may be applied also to tilted fault blocks and complex faulted uplands.
A volcanic mountain starts out as a simple crack in the Earth called a volcanic vent. Magma erupts out of the ground as lava flows, clouds of ash, and explosions of rock. This material falls back to Earth around the vent, and piles up around it. Over time (and sometimes quite quickly) a volcanic mountain builds up, with the familiar cone shape.