DEFINITION OF PLUTON

A general name for a large cooled igneous intrusive

body in the Earth. The classification of the specific type of

pluton is based on its geometry, size, and relations to the

older rocks surrounding the pluton, known as country rock.

Concordant plutons have boundaries parallel to layering in

the country rock, whereas discordant plutons have boundaries

that cut across layering in the country rock. Dikes are

tabular but discordant intrusions, and sills are tabular and

concordant intrusives. Volcanic necks are conduits connecting

a volcano with its underlying magma chamber. A famous

example of a volcanic neck is Devils Tower in Wyoming.

Some plutons are so large that they have special names.

Batholiths have a surface area of more than 60 square miles

(100 km2).

Geologists have long speculated on how such large volumes

of magma intrude the crust, and what relationships

these magmas have to the style of volcanic eruption. One

mechanism that may operate is assimilation, where the hot

magma melts surrounding rocks as it rises, causing it to

become part of the magma. In doing so, the magma becomes

cooler, and its composition changes to reflect the added melted

country rock. It is widely thought that magmas may rise

only a very limited distance by the process of assimilation.

Some magmas may forcefully push their way into the crust if

there are high pressures in the magma. One variation of this

forceful emplacement style is diapirism, where the weight of

surrounding rocks pushes down on the melt layer, which

squeezes its way up through cracks that can expand and

extend, forming volcanic vents at the surface. Stoping is a

mechanism whereby big blocks get thermally shattered and

drop off the top of the magma chamber and fall into the

chamber, much like a glass ceiling breaking and falling into

the space below. Many if not most plutons seem to be

emplaced into structures such as faults, utilizing the weakness

provided by the structure for the emplacement of the magma.

Some plutons are emplaced into active faults, intruding into

spaces created by gaps that open up between misaligned segments

of the moving fault zone.

See also PETROLOGY; STRUCTURAL GEOLOGY.

plutonic See PLUTON.

plutonism See PLUTON.

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