An
intrusion is an igneous rock body that forms from crystallized magma under Earth's surface.
Magma slowly pushes up from deep within the earth into any cracks or spaces it can find, sometimes pushing existing
country rock out of the way, a process that can 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, large
granite (or related rock) formations.