Types of Rocks

Rocks are everywhere on Earth. Scientists sort them into three main types based on how they formed. Igneous rocks come from melted rock that cooled down. Sedimentary rocks are made from layers of sand, mud, or tiny shells. Metamorphic rocks are rocks that were changed by heat or pressure.

Igneous and Sedimentary Rocks

Igneous rocks form when hot melted rock called magma cools and hardens. Granite and basalt are common igneous rocks. Sedimentary rocks form when tiny pieces of rock, sand, or shells pile up in layers over time. The layers press together and harden. Sandstone and limestone are sedimentary rocks.

Metamorphic Rocks

Metamorphic rocks start as one type of rock and change into another. This happens deep underground where there is great heat and pressure. The heat and pressure change the rock's minerals and texture. Marble is a metamorphic rock that started as limestone. Slate started as a soft rock called shale.

Fun Facts

• The oldest rocks on Earth are about 4 billion years old.
• Pumice is an igneous rock that is so light it can float on water.
• The White Cliffs of Dover in England are made of chalk, a type of sedimentary rock.

Did You Know?

Rocks are always changing from one type to another in a process called the rock cycle. A rock you see today could become a completely different type of rock millions of years from now!