- published: 23 Apr 2014
- views: 83802
An intrusion is liquid rock that forms under Earth's surface. Magma from under the surface is slowly pushed 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.
Intrusions are one of the two ways igneous rock can form; the other is extrusive, that is, a volcano eruption or similar event. Technically speaking, an intrusion is any formation of intrusive igneous rock; rock formed from magma that cools and solidifies within the crust of a planet. In contrast, an extrusion consists of extrusive rock; rock formed above the surface of the crust.
Intrusions vary widely, from mountain range sized batholiths to thin vein-like fracture fillings of aplite or pegmatite. 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.
See them hide under my bed
Hear them laugh and speak and plot!
I don't want to disappoint them,
I don't want to grief their joke, they seem so busy...
Little monsters under my bed
But my bed is so empty
You know
I should keep on hiding the truth
I should keep on letting them play
Heeey! See them hide here under my bed!
I don't want to disappoint them
They are so wicked numb!
Little monsters under my bed
But me bed is so empty
You have to know