An intrusion is liquid rock that becomes solid under the Earth's surface. Magma from under the surface is slowly pushed up into any cracks or spaces it finds. Sometimes it pushes existing rock out of the way, a process that may take millions of years.
As the rock slowly cools into a solid, the different parts of the magma crystallize into minerals. Many mountain ranges, such as the Sierra Nevada in California, are formed mostly by intrusive rock, such as large granite (or related rock) formations.
Intrusions are one of the two ways igneous rock can form. The other is in a volcanic eruption or similar event. Technically speaking, an intrusion is a formation of igneous rock formed from magma that cools and solidifies within the crust of a planet. In contrast, an extrusion consists of rock formed above the surface of the crust.
Intrusions vary widely, from mountain range sized batholiths to thin vein-like fracture fillings. When exposed by erosion, these cores called batholiths may occupy huge areas of Earth's surface. Large bodies of magma that solidify underground before they reach the surface of the crust are called plutons.
Intrusive structures are often classified according to whether or not they are parallel to the bedding planes of the earlier 'country' rock. A dyke is a relatively narrow intrusion which moves up, crossing older rock strata. A sill is a relatively thin intrusion, which makes a table along bedding planes. It moves along between existing strata.
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