Orogeny facts for kids

Mount Cook, New Zealand. It lies in the Southern Alps, the mountain range which runs the length of the South Island. New Zealand sits on top of the junction of two oceanic plates.

Even when mountains are worn down, the evidence is there in the remaining rocks. Millook, Cornwall

Orogeny is the process of mountain-building. It takes place when two tectonic plates come together. Orogens develop while a continental plate is crumpled and thickened to form mountain ranges, and involve a great range of geological processes collectively called orogenesis. Examples:

Where a continent rides over an oceanic plate (non-collisional):
- Andes in South America
Where two continental plates converge (collide):
- Alps
- Himalayas

Another, less common cause is when a plate moves over a hot spot in the Earth's mantle. Examples:

Hawaiian chain of islands
Yellowstone National Park

There is a third process, called delamination. Here an unstable portion of cold root of the lithosphere drips down into the mantle, decreasing the density of the lithosphere and causing buoyant uplift. Example:

The Sierra Nevada in California.

Areas that are rifting apart, such as mid-ocean ridges and the East African Rift have mountains due to thermal buoyancy caused by the hot mantle underneath them, which pushes them up. This thermal buoyancy is known as dynamic topography. Examples:

Images for kids

Two processes that can contribute to the formation of orogens. Top: delamination of orogenic roots into the asthenosphere; Bottom: Subduction of lithospheric plate to mantle depths. The two processes lead to differently located metamorphic rocks (bubbles in diagram), providing evidence as to which process actually occurred at convergent plate margins.
Continental collision of two continental plates to form a collisional orogen. Typically, continental crust is subducted to lithospheric depths for blueschist to eclogite facies metamorphism, and then exhumed along the same subduction channel. (example: the Alps)
Sierra Nevada Mountains (a result of delamination) as seen from the International Space Station.
Mount Rundle, Banff, Alberta.
The Foreland Basin System
An example of thin-skinned deformation (thrust faulting) of the Sevier Orogeny in Montana. Note the white Madison Limestone repeated, with one example in the foreground (that pinches out with distance) and another to the upper right corner and top of the picture.
Map of world geologic provinces