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. 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.
Types of mountains 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 = 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, 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 and shield volcanoes.
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 composite volcano or stratovolcano has a more steeply rising
cone, 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 = When plates collide or undergo
subduction, 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 = 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
= 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 Notes
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