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Intrusive Igneous Rock
Intrusive igneous rock is a rock
that is formed from magma.
It is a rock that formed underground, as the magma pushed, or intruded, into preexisting rock by moving upwards through cracks between grains.
Some of the magma reaches the surface as the volcano erupts and becomes extrusive igneous rock.
Some, however, never reaches the surface and forms intrusions underground.
Yosemity granite. By Larry via Flickr.com
Those intrusions have an array of shapes, such as vertical dikes, horizontal sills, blister-shaped laccoliths, or batholiths - systems of many plutons.
They can be very spectacular millions of years later when the rock has been uplifted and the intrusions uncovered by erosion.
The rocks that belong to this group are phaneritic or coarse grained. Because it is much hotter under the Earth's surface than above it, the minerals in those rocks get much more time to grow than get the minerals of rocks that form above the surface.
The rocks that belong to this group are granites, syenites, monzonite, diorite, gabbro, kimberlite, dunite, peridotite and others.
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