Mountain formation occurs due to a variety of geological processes associated with large-scale movements of Earth's crust (tectonic plates).
From the late 18th century until its replacement by plate tectonics in the 1960s, geosyncline was used to explain much mountain-building.
Types of mountains
There are five main types of mountains:
volcanic,
fold,
plateau,
fault-block, and
dome. A more detailed classification useful on a local scale predates
plate tectonics and adds to these categories.
Volcanic mountains
Movements of tectonic plates create
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 crust melts and drags water down with the subducting crust.
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
Stratovolcano and
Shield volcano.
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A shield volcano has a gently sloping cone because of 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.
Fold mountains
When plates collide or undergo
subduction (that is, ride one over another), the plates tend to buckle and fold, forming mountains. While volcanic arcs form at oceanic-continental plate boundaries, folding occurs at continental-continental plate boundaries. Most of the major continental mountain ranges are associated with thrusting and folding or
orogenesis. Examples are the
Balkan Mountains, the
Jura Mountains and the
Zagros mountains.
Block mountains
When a
fault block is raised or tilted, a block mountain can result.
Higher blocks are called
horsts, and troughs are called
. 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.
Another example is the Rila–Rhodope massif in Bulgaria, including the well defined horsts of Belasitsa (linear horst), Rila mountain (vaulted domed shaped horst) and Pirin—a horst forming a massive anticline situated between the complex graben valleys of the Struma and Mesta rivers.
Uplifted passive margins
Unlike orogenic mountains there is no widely accepted
geophysics model that explains elevated passive continental margins such as the Scandinavian Mountains, eastern
Greenland, the Brazilian Highlands, or Australia's Great Dividing Range.
Different elevated passive continental margins most likely share the same mechanism of uplift. This mechanism is possibly related to far-field stresses in Earth's
lithosphere. According to this view elevated passive margins can be likened to giant
anticline lithospheric folds, where folding is caused by horizontal compression acting on a thin to thick crust transition zone (as are all passive margins).
[Løseth and Hendriksen 2005]
Models
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 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
External links
