How Do Mountains Form?

Fold, Block, and Volcanic

Folding Mountains

Mountains are built through a general process called "deformation" of the crust of the Earth. Deformation is another word for "folding". When two sections of the Earth's lithosphere collide, rather than being subducted, where one slab of lithosphere are forced down to deeper regions of the Earth. These slabs pile into each other, causing one or both slabs to fold up. This process lifts ,folds, and deforms the crust and produces a mountain range.


This process is shown in the figure below. The lithospheric slab on the right is subducted, while the force of the collision gradually causes the slab on the left to fold deeply.

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Fault-Block Mountains

The next way that mountains are formed is along fault lines. Blocks of Earth are uplifted and tilted over as two plates grind together. Mountains that form as blocks of rock move up or down along normal faults are called fault-block mountains.

Fault-block mountains are form when the lithosphere is stretched and pulled apart by forces in the Earth. The rocks at the earths crust are cool and rigid. When the lithosphere begins to stretch, the crust splits into large blocks. As the stretching continues, the blocks of rock move along the faults that separate them. This process is shown in the image below.

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Volcanic Arc

A volcanic arc is a chain of volcanoes formed above a subducting plate, positioned in an arc shape. Offshore volcanoes form islands, resulting in a volcanic island arc. Generally they result from the subduction of an oceanic tectonic plates under another tectonic plate, and often parallel an oceanic trench. The oceanic plate is saturated with water, and volatiles such as water drastically lower the melting point of the mantle.. As the oceanic plate is subducted, it is subjected to greater and greater pressures with increasing depth. This pressure squeezes water out of the plate and introduces it to the mantle. Here the mantle melts and forms magma at depth under the overriding plate. The magma ascends to form an arc of volcanoes parallel to the subduction zone. This process can be seen in the image below.


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Created by Monica Headen