DEFINITION OF IGNEOUS ROCKS

Rocks that have crystallized from a melt or

partially molten material (known as magma). Magma is a

molten rock within the Earth; if it makes its way to the surface,

it is known as lava. Different types of magma form in different

tectonic settings, and many processes act on the magma as

it crystallizes to produce a wide variety of igneous rocks.

Most magma solidifies below the surface, forming igneous

rocks (ignis is Latin for fire). Igneous rocks that form below

the surface are called intrusive (or plutonic) rocks, whereas

those that crystallize on the surface are called extrusive (or volcanic)

rocks. Rocks that crystallize at a very shallow depth are

called hypabyssal rocks. Intrusive igneous rocks crystallize

slowly, giving crystals an extended time to grow, thus forming

rocks with large mineral grains that are clearly distinguishable

with the naked eye. These rocks are called phanerites. In contrast,

magma that cools rapidly forms fine-grained rocks.

Aphanites are igneous rocks in which the component grains

can not be distinguished readily without a microscope and are

formed when magma from a volcano falls or flows across the

surface and cools quickly. Some igneous rocks, known as porphyries,

have two populations of grain size—a very large

group of crystals (called phenocrysts) mixed with a uniform

groundmass (or matrix), filling the space between the large

crystals. This indicates two stages of cooling, as when magma

has resided for a long time beneath a volcano, growing big

crystals. When the volcano erupts, it spews out a mixture of

the large crystals and liquid magma that then cools quickly.

Once magmas are formed from melting rocks in the

Earth, they intrude the crust, and may take several forms. A

pluton is a general name for a large cooled igneous intrusive

body in the Earth. 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, for example, 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 them to

become part of the magma. In doing this, 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 then 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, 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.

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