DEFINITION OF OROGENY

The building of mountains. Many early philosophers,

theologians, and scientists going back at least as far

as Francis Bacon were formulating theories about the forces

involved in uplifting and deforming mountains for the past

several or tens of centuries. By the middle 1800s the processes

involved in the formation of mountains became

known as orogeny. Early ideas suggested that mountains

were deformed and uplifted by magmatic intrusions, or

reflected an overall contraction of the Earth with mountain

belts representing cooling wrinkles as on a shriveled prune.

In a classical work in 1875, Eduard Suess published Die

Entstehung der Alpen in which he argued that mountain

belts on the planet did not follow any regular pattern that

would indicate global contraction, and he suggested that the

mountain belts represented contraction between rigid blocks

(now called cratons) and surrounding rocks on the margins

of these massifs. However, he still believed that the main

driving force was global contraction induced by cooling of

the Earth. In Suess’s last volume of Das Antlitz der Erde

(The Face of the Earth) (1909), he admitted that the

amount of shortening observed in mountain belts was

greater than could be explained by global cooling and contraction,

and he suggested that perhaps other translations

have occurred in response to tidal forces and the rotation of

the planet.

In 1915 and 1929 Alfred Wegener published Die Entstehung

der Kontinente (The Origin of the Continents) and Die

Entstehung der Kontinente und Ozeane (The Origin of the

Continents and Oceans). Wegener argued strongly for large

horizontal motions between cratons made of sial, and using

such data as the match of restored coastlines and paleontological

data, he founded the theory of continental drift. Several

geologists, including Alex du Toit, Reginald Daly, and

Arthur Holmes, documented geological ties between different

continents supporting the idea of continental drift. In the

1940s–1960s, geophysical exploration of the seafloor led to

the recognition of seafloor spreading and provided the data

that J. Tuzo Wilson needed to propose the modern theory of

plate tectonics in 1965.

With the development of the ideas of plate tectonics,

geologists now recognize that mountain belts are of three

basic types, including fold and thrust belts, volcanic mountain

ranges, and fault block ranges. Fold and thrust belts are

contractional mountain belts, formed where two tectonic

plates collided, forming great thrust faults, folds, metamorphic

rocks, and volcanic rocks. Detailed mapping of the

structure in the belt can enable geologists to reconstruct their

history and essentially pull them apart. It is found that many

of the rocks in fold and thrust belt types of mountain ranges

were deposited on the bottom of the ocean, continental rises,

slope, shelves, or on ocean margin deltas. When the two

plates collide, many of the sediments get scraped off and

deformed, forming the mountain belts. Thus, fold and thrust

mountain belts mark places where oceans have closed. Volcanic

mountain ranges include places such as Japan’s Mount

Fuji, and Mount St. Helens in the Cascades of the western

United States. These mountain ranges are not formed primarily

by deformation, but by volcanism associated with subduction

and plate tectonics. Fault-block mountains, such as the

Basin and Range Province of the western United States, are

formed by the extension or pulling apart of the continental

crust, forming elongate valleys separated by tilted faultbounded

mountain ranges.

See also CONTINENTAL DRIFT AND PLATE TECTONICS:

CONVERGENT PLATE MARGIN PROCESSES; PLATE TECTONICS.

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