Rocks and Minerals

Rocks are made of one or more minerals and are found everywhere on Earth — from mountain peaks to the ocean floor. Scientists classify rocks into three main types based on how they form: igneous, sedimentary, and metamorphic.

Igneous Rocks

Igneous rocks form when melted rock called magma (underground) or lava (above ground) cools and hardens. The word igneous comes from the Latin word for fire.

• Granite — forms underground from slowly cooling magma; has large crystals; used in buildings.
• Basalt — forms when lava cools quickly on the surface; has small crystals; found on ocean floors.
• Obsidian — forms from very fast cooling lava; looks like black glass.

Sedimentary Rocks

Sedimentary rocks form when layers of sand, mud, shells, and other materials are pressed together over a very long time. They often contain fossils.

• Sandstone — formed from grains of sand; often tan or red in color.
• Limestone — formed mainly from shells and coral; contains calcium carbonate.
• Shale — formed from mud and clay; splits into thin layers.

Metamorphic Rocks

Metamorphic rocks form when existing rocks are changed by intense heat and pressure deep underground. The word metamorphic means "changed in form."

• Marble — formed from limestone under heat and pressure; used in sculptures and buildings.
• Slate — formed from shale; splits into flat sheets; used on rooftops.
• Quartzite — formed from sandstone; very hard; used in construction.