Earth's Crustal Rocks Use the information below to identify rocks you have found. The Earth's crust is composed of 3 basic rock types. These rock types are classified by how the rock was originally formed. Under the right conditions any rock from one rock type can under go changes and become an entirely new rock type. This is called the rock cycle .
Igneous rocks are rocks that have cooled from molten rock.
Two common types of igneous rocks are basalt and granite. Igneous rocks are further broken down into 2 subcategories called intrusive rocks and extrusive rocks. Rocks that cool from lava rather rapidly generally have a very fine texture with small or no crystals visible. Basalt is an example of an extrusive igneous rock. Lava is magma that has reached the surface of the Earth.