Question: Do you know what the difference is between intrusive and extrusive rocks ?

James Verdon answered on 25 Jun 2012:

Hi kylethebrainiac,
Intrusive and extrusive rocks are both types of igneous rocks, which mean they formed from molten magma which cooled and solidified. The difference is that extrusive rocks were erupted from a volcano onto the surface of the earth, while intrusive rocks got trapped in the crust, and never reach the surface, and instead cooled where they were.

This means that the difference between them is how fast they cooled. Because they are on the surface, exposed to the air and water, extrusive rocks cool quickly. This means that the crystals, which grow whenever igneous rocks cool, cannot grow very large. Usually, with extrusive rocks you can’t see the crystals unless you have a microscope. Examples of extrusive rocks are basalts and rhyolites.

Intrusive rocks are trapped within the crust when they cool. This means they cool much slower. This gives time for the crystals to grow to much larger sizes. Usually, with intrusive rocks you can see the crystals with your bare eyes. If you can, do a google images search for ‘Pegmatites’, and you should see some rocks with impressive crystals. Examples of intrusive rocks are gabbros and granites.

I hope that answers your question Kyle. Have a comment if you’d like to know more about any of the different rock types (igneous, metamorphic or sedimentary).