An igneous intrusion is a mass of rock consolidated by crystallization of molten matter (magma) at a certain depth below the Earth's surface.
These rocks form a group called plutonic, distinct from that of volcanic assemblies of igneous extrusions (rocks formed on the surface, such as lava). When they penetrate cooler embedded rocks, the intrusive rocks heat them up and transform them (metamorphism), while the edge of the magma, when cooling faster than the interior, has smaller crystals and may appear glassy.
The intrusions take advantage of the fractures produced by the tensions of the cortex. This tension is evident in the ocean ridges where the crust breaks.
Most of the intrusions described above have basaltic composition. Apart from the giant masses such as Bushveld, they are made up of small grains of no more than one or two millimeters.
Major intrusions cool slowly, allowing the formation of large crystals. They also tend to be light colored granitic rocks, contrasting with dark basaltic intrusions.
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