How many kinds of rock does Earth have?
There are three different kinds of rock: sedimentary, igneous, and metamorphic, and each type tells how the rock formed. Sedimentary rocks formed long ago in layers, often along the bottom of the sea. They include conglomerate, sandstone, and limestone. Igneous rocks formed from liquid rock like volcano lava, which cooled and became solid. Basalt, granite, and obsidian are all igneous rocks. Metamorphic rocks, such as marble and slate, formed from sedimentary or igneous rocks that were changed by heat, friction, or other natural causes.