igneous rocks

Igneous rocks form when magma, or melted rock from inside the Earth, cools and hardens. They can form above ground when magma erupts from a volcano as lava. They can also form deep inside the Earth's crust and then be exposed at the surface by the overlying rocks wearing away. The crystals found in igneous rocks have angular shapes. They have not been worn like sedimentary rocks or squished like metamorphic rocks.

Definition: rock that forms when magma cools, hardens, and crystallizes
Where it forms: on the Earth's surface or inside the Earth
Two types: intrusive or extrusive
Common examples: pumice, basalt, granite
Where it's found: on every continent and on the ocean floor
Cool fact: Volcanoes and most mountains are made of igneous rocks.

Image credits: courtesy of AMNH; Rosamond Kinzler: AMNH.