A Closer Look: Mountains

What generalizations can be made about mountain building?

The cause of all mountain building is related to the movement of tectonic plates. Convergent plate boundaries, in particular, are places where mountains form. At these boundaries, one plate meets another. There are two types of convergent plate boundaries and mountains form differently at each.

Mountain Building Related To Subduction Zones

Subduction zones are places where the edge of one plate is forced under the edge of another plate. At subduction zones, mountains can be formed in two ways:

1) Pieces of buoyant lithosphere (the crust fused to the upper part of the mantle) riding on top of the downgoing plate may eventually be brought to the convergent boundary. Examples of buoyant lithosphere with continental crust include small continental fragments and island arcs (an arc-shaped formation of volcanoes built up from the sea floor). Buoyant lithosphere with oceanic crust includes oceanic plateaus (broad regions of thick oceanic crust). Regardless of type, buoyant lithosphere cannot be subducted when it is of the same density or less dense than the material with which it is colliding. As it merges with the overriding slab, the buoyant lithosphere attaches, or “accretes,” itself to the slab’s edge. Over time, a type of fold-thrust mountain belt can be created in which folded mountains appear as rock is pushed upward. The Coast Range of California and the Sierra Nevadas are two parallel mountain ranges formed at least partly in this way.

2) When an oceanic plate subducts, it brings with it materials, like water, that can induce melting in the mantle. This melting can lead to volcanism and the creation of mountain ranges. Examples of mountain ranges created by volcanism at subduction zones include the Andes Mountains in South America and the Cascade Mountains in the western United States.

Mountain Building Related To Continental Collision

In Session 5, we focused on mountain building that occurs where the once oceanic lithosphere between two continents completely subducts and the continental crust riding atop each plate collides. Continental collision is a special case of convergence. One continent may slide a short distance under the other, but continental crust never subducts. The two continents essentially weld together. Intense compression gradually squeezes rock upwards (and downwards) deforming and thickening the crust to create mountains. This is how the Appalachian Mountains formed and how the Himalaya Mountains are forming today. In the Appalachians, geologists can trace back hundreds of millions of years to identify three distinct collisions that each helped to create the mountains we see today. As a result, the Appalachians are an ideal tectonic setting to study metamorphic rocks.

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