Batholith

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Batholith

A batholith (from Greek bathos, depth + lithos, rock) is a large emplacement of igneous intrusive (also called plutonic) rock that forms from cooled magma

Magma

Magma is molten Rock that is found beneath the surface of the Earth, and may also exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and gas bubbles....
deep in the Earth's crust

Crust (geology)

In geology, a crust is the outermost solid shell of a planet or moon, which is chemically distinct from the underlying mantle . Crusts of Earth , our Moon, Mercury , Venus, and Mars have been generated largely by igneous processes, and these crusts are richer in incompatible elements than their respective mantle s....
. Batholiths are almost always made mostly of felsic

Felsic

Felsic is a term used in geology to refer to silicate minerals, magma, and rock which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium....
or intermediate rock-types, such as granite

Granite

Granite is a common and widely occurring type of Intrusion , felsic, igneous rock rock . Granite has a medium to coarse texture, occasionally with some individual crystals larger than the groundmass forming a rock known as Porphyry ....
, quartz monzonite

Quartz monzonite

Quartz monzonite is an Intrusion igneous rock that has an approximately equal proportion of orthoclase and plagioclase feldspars. The plagioclase is typically intermediate to sodic in composition, andesine to oligoclase....
, or diorite

Diorite

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A batholith (from Greek bathos, depth + lithos, rock) is a large emplacement of igneous intrusive (also called plutonic) rock that forms from cooled magma

Magma

Crust (geology)

Felsic

Granite

Quartz monzonite

Diorite

Diorite is a grey to dark grey intermediate Intrusion igneous rock composed principally of plagioclase feldspar , biotite, hornblende, and/or pyroxene....
(see also granite dome
Granite dome

A granite dome is a Dome of granite, formed by Exfoliation ....
).

Although they may appear uniform, batholiths are in fact structures with complex histories and compositions. They are composed of multiple masses, or plutons, bodies of igneous rock of irregular dimensions (typically at least several kilometers) that can be distinguished from adjacent igneous rock by some combination of criteria including age, composition, texture, or mappable structures. Individual plutons are crystallized from magma that traveled toward the surface from a zone of partial melting near the base of the Earth's crust.

Traditionally, these plutons have been considered to form by ascent of relatively buoyant

Buoyancy

In physics, buoyancy is the upward force that keeps things afloat. The net upward buoyancy force is equal to the magnitude of the weight of fluid displaced by the body....
magma in large masses called plutonic diapir
Diapir

A diapir is a type of intrusion in which a more mobile and ductily-deformable material is forced into brittle overlying rocks. Depending on the tectonic environment, diapirs can range from idealized mushroom-shaped Rayleigh-Taylor instability-type structures in regions with low tectonic stress such as in the Gulf of Mexico to narrow dike...
s. Because the diapirs are liquefied and very hot, they tend to rise through the surrounding country rock

Country rock (geology)

Country rock is a Geology term meaning the rock native to an area. It is similar and in many cases interchangeable with the terms basement and wall rocks....
, pushing it aside and partially melting it. Most diapirs do not reach the surface to form volcano

Volcano

A volcano is an opening, or rupture, in a planet's surface or Crust , which allows hot, molten rock, ash, and gases to escape from below the surface....
es, but instead slow down, cool and usually solidify 5 to 30 kilometers underground as plutons (hence the use of the word pluton; in reference to the Roman

Ancient Rome

Ancient Rome was a civilization that grew out of a small agricultural community founded on the Italian Peninsula as early as the 10th century BC....
god of the underworld Pluto). It has also been proposed that plutons commonly are formed not by diapiric ascent of large magma diapirs, but rather by aggregation of smaller volumes of magma that ascended as dikes.

A batholith is formed when many plutons converge together to form a huge expanse of granitic rock. Some batholiths are mammoth, paralleling past and present subduction zone

Subduction

In geology, subduction is the process that takes place at convergent boundary by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge....
s and other heat sources for hundreds of kilometers in continental crust

Continental crust

The continental crust is the layer of igneous, sedimentary, and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as Continental shelf....
. One such batholith is the Sierra Nevada Batholith

Sierra Nevada Batholith

The Sierra Nevada Batholith is a large batholith which forms the core of the Sierra Nevada mountain range in California, USA, exposed at the surface as granite....
, which is a continuous granitic formation that forms much of the Sierra Nevada in California

California

California is a U.S. state on the West Coast of the United States of the United States, along the Pacific Ocean. It is bordered by Oregon to the north, Nevada to the east, Arizona to the southeast, and to the south the Mexico state of Baja California....
. An even larger batholith, found predominantly in the Coast Mountains

Coast Mountains

The Coast Mountains are a major mountain range of western North America, extending from southwestern Yukon through the Alaska Panhandle and virtually all of the British Columbia Coast....
of western Canada

Canada

Canada is a country occupying most of northern North America, extending from the Atlantic Ocean in the east to the Pacific Ocean in the west and northward into the Arctic Ocean....
, extends for 1,800 kilometers and reaches into southeastern Alaska

Alaska

Alaska is the largest U.S. state of the United States by area; it is situated in the northwest extremity of the North American continent, with Canada to the east, the Arctic Ocean to the north, and the Pacific Ocean to the west and south, with Russia further west across the Bering Strait....
, which is called the Coast Plutonic Complex.

There is also an important geographic usage of the term batholith. For a geographer, a batholith is an exposed area of mostly continuous plutonic rock that covers an area larger than 100 square kilometers. Areas smaller than 100 kilometers are called stocks. However, the majority of batholiths visible at the surface (via outcroppings) have areas far greater than 100 square kilometers. These areas are exposed to the surface through the process of erosion

Erosion

For morphological image processing operations, see Erosion 'For use of in dermatopathology, see Erosion Erosion is the removal of solids in the natural environment....
accelerated by continental uplift

Orogeny

Orogeny refers to natural mountain building, and may be studied as a tectonic structural event, as a geographical event, and a chronological event: orogenic events cause distinctive structural phenomena and related tectonic activity, affect certain regions of rocks and crust, and happen within a specific period of time....
acting over many tens of millions to hundreds of millions of years. This process has removed several tens of kilometers of overlying rock in many areas, exposing the once deeply buried batholiths.

Batholiths exposed at the surface are also subjected to huge pressure differences between their former homes deep in the earth and their new homes at or near the surface. As a result, their crystal structure

Crystal structure

In mineralogy and crystallography, a crystal structure is a unique arrangement of atoms in a crystal. A crystal structure is composed of a motif, a set of atoms arranged in a particular way, and a lattice....
expands slightly and over time. This manifests itself by a form of mass wasting

Mass wasting

Mass wasting, also known as slope movement or mass movement, is the geomorphology process by which soil, regolith, and rock move downslope under the force of gravity....
called exfoliation

Exfoliation (geology)

Exfoliation joints or sheet joints are surface-parallel fracture systems in rock often leading to erosion of concentric slabs....
. This form of erosion causes convex and relatively thin sheets of rock to slough off the exposed surfaces of batholiths (a process accelerated by frost wedging). The result is fairly clean and rounded rock faces. A well-known example of the result of this process is Half Dome

Half Dome

Half Dome is a granite dome in Yosemite National Park, located at the eastern end of Yosemite Valley ? possibly Yosemite's most familiar sight. The granite crest rises more than above the valley floor....
, which is located in Yosemite Valley

Yosemite Valley

Yosemite Valley is a world-famous scenic location in the Sierra Nevada mountains of California. It is the centerpiece of Yosemite National Park, attracting visitors from all parts of the globe....
.

Batholith

See also

External links