Fold Mountains

Formation Form along both destructive and collision plate boundaries, in other words where two plates are pushing towards each other. The best examples are the Himalayas, the Rockies, the Andes and the Alps, all of which are huge fold mountain ranges caused by the collision of two plates.

Theory The general theory is that as two plates, with land masses on them, move towards each other they push layers of accumulated sediment in the sea between them up into folds. Thus most fold mountains will continue to grow, as the plates constantly move towards each other.

Fold Mountains: Constructive Boundaries

Formation As already seen, at a destructive plate boundary the oceanic plate is subducted beneath the continental one. The molten material then rises to the surface to form volcanoes, either in an island arc (e.g. the West Indies) or on the continental land mass (e.g. the volcanoes of the Andes). In both cases Fold Mountains can be formed.

Formation When the Nazca plate dives under the South American one, their motion forward also has been pushing sediment together. This, over millions of years, has been pushed up into huge fold mountains: The Andes. Within them there are also volcanoes as the mountains are above the subduction zone.

Formation If an island arc has been formed, the same idea occurs. Over millions of years the movement of the two plates together will push the island arc nearer to the continent. As this occurs the sediments on the seabed are folded up to become huge mountains.

Fold Mountains: Collision Margins

Formation These occur less frequently, but two excellent examples are the Himalayas, where the Indian plate is moving North and East towards the stationary European plate, and the Alps, formed by the collision between the African and Eurasian plates. In these examples both plates are Continental ones, and so can neither sink nor be destroyed. The material between them is therefore forced upwards to form the mountains.

Formation For the Himalayas the material that now forms the mountains was originally on the bottom of the non-existent Tethy's Sea. As the Indian plate pushed towards the Eurasian one, the sediments were folded up to form the Himalayas, leaving the only trace of the sea to be the fossilised shells that you can find high up in the mountains.