What is an intrusion?
Q: What is an intrusion?
A: An intrusion is magma (melted rock) which cools and becomes solid under the Earth's surface.
Q: How does an intrusion form?
A: Intrusions form when there are lines of weakness such as faults, joints, or bedding planes in the crust, allowing magma to enter these lines of weakness.
Q: What is the difference between intrusive and extrusive rocks?
A: Intrusive rocks are formed by magma cooling and becoming solid beneath the Earth's surface, whereas extrusive rocks are formed by magma cooling into rock above the surface of the crust. Both types of rocks are classed as igneous rocks.
Q: How long does it take for intrusive rocks to form?
A: The process of forming intrusive rocks may take millions of years. As the rock slowly cools into a solid, different parts of the magma crystallize into minerals.
Q: Are crystals usually larger in intrusive than extrusive rocks?
A: Yes, typically crystals are larger in intrusive than extrusive rocks.
Q: What type of formations do intrusions often create?
A: Intrusions often create huge granite (or related rock) formations such as mountain ranges like Sierra Nevada in California.
Q: What are some other terms used to describe intrusions?
A: Other terms used to describe intrusions include batholiths (large bodies of magma that solidify underground before they reach the surface), sills (intrusions which make a table along bedding planes), and dykes (intrusions which move up crossing older strata).
