Mountain formation

Thrust and reverse fault movement are an important component of mountain formation.

Illustration of mountains that developed on a fold that thrusted.

Mountain formation refers to the geological processes that underlie the formation of mountains. These processes are associated with large-scale movements of the earth's crust (plate tectonics). Mountain formation is related to plate tectonics. Folding, faulting, volcanic activity, igneous intrusion and metamorphism are all parts of the orogenic process of mountain building. The understanding of specific landscape features in terms of the underlying tectonic processes is called tectonic geomorphology, and the study of geologically young or ongoing processes is called neotectonics.

to understand mountains we must first know more about their geomorphic history. In most mountain studies this is not done: instead the rock structures inside the mountains are described in detail, with the tacit assumption that whatever made the structures also made the mountains. This assumption is not warranted.

— Cliff Ollier and Colin Pain

Types of mountains

See also: List of mountain types

There are three main types of mountains: volcanic, fold, and block. A more detailed classification useful on a local scale predates plate tectonics and adds to the above categories.

Volcanic mountains

See also: Stratovolcano, Shield volcano, and Guyot

File:ISS-38 Kliuchevskoi Volcano on Kamchatka.jpg

Annotated view includes Ushkovsky, Tolbachik, Bezymianny, Zimina, and Udina stratovolcanoes of Kamchatka, Russia. Oblique view taken on November 16, 2013 from ISS.

Stratovolcanoes associated with a subduction zone (left) and a spreading ridge volcano (right). A hotspot volcano is center.

Movements of tectonic plates create volcanoes along the plate boundaries, which erupt and form mountains. A volcanic arc system is a series of volcanoes that form near a subduction zone where the crust of a sinking oceanic plate melts.

Most volcanoes occur in a band encircling the Pacific Ocean (the Pacific Ring of Fire), and in another that extends from the Mediterranean across Asia to join the Pacific band in the Indonesian Archipelago. The most important types of volcanic mountain are composite cones or stratovolcanoes (Vesuvius, Kilimanjaro and Mount Fuji are examples) and shield volcanoes (such as Mauna Loa on Hawaii, a hotspot volcano).

A shield volcano has a gently sloping cone due to the low viscosity of the emitted material, primarily basalt. Mauna Loa is the classic example, with a slope of 4°-6°. (The relation between slope and viscosity falls under the topic of angle of repose.) The composite volcano or stratovolcano has a more steeply rising cone (33°-40°), due to the higher viscosity of the emitted material, and eruptions are more violent and less frequent than for shield volcanoes. Besides the examples already mentioned are Mount Shasta, Mount Hood and Mount Rainier.

Fold mountains

See also: Fold (geology), Fold and thrust belt, and Fold mountain

Zard-Kuh, a fold mountain in the central Zagros range of Iran.

When plates collide or undergo subduction (that is – ride one over another), the plates tend to buckle and fold, forming mountains. Most of the major continental mountain ranges are associated with thrusting and folding or orogenesis. Examples are the Jura and the Zagros mountains.

Block mountains

Main article: Fault-block mountain

Fault-block mountain of tilted type.

Sierra Nevada Mountains (formed by delamination) as seen from the International Space Station.

When a fault block is raised or tilted, block mountains can result. Higher blocks are called horsts and troughs are called grabens. A spreading apart of the surface causes tensional forces. When the tensional forces are strong enough to cause a plate to split apart, it does so such that a center block drops down relative to its flanking blocks.

An example is the Sierra Nevada Range, where delamination created a block 650 km long and 80 km wide that consists of many individual portions tipped gently west, with east facing slips rising abruptly to produce the highest mountain front in the continental United States.

Models

See also: Extensional tectonics, Rift valley, Rift, Prediction of volcanic activity, and Geomorphology

Hotspot volcanoes

Hotspots are supplied by a magma source in the Earth's mantle called a mantle plume. Although originally attributed to a melting of subducted oceanic crust, recent evidence belies this connection. The mechanism for plume formation remains a research topic.

Fault blocks

Several movements of the earth's crust that lead to mountains are associated with faults. These movements actually are amenable to analysis that can predict, for example, the height of a raised block and the width of an intervening rift between blocks using the rheology of the layers and the forces of isostasy. Early bent plate models predicting fractures and fault movements have evolved into today's kinematic and flexural models.

See also

• Seamount
• Batholith
• Continental collision
• Tectonics
• Orogeny

External links

• NASA Goddard Planetary Geodynamics Laboratory: home page
• NASA Goddard Planetary Geodynamics Laboratory: Volcanology Research
• Plate tectonics pictures from UNR
• Rotating globe showing areas of earthquake activity