In this topic you will learn about different types of rocks.

Rocks are solids that make up Earth's crust. Rocks contain minerals. A mineral is a naturally occurring solid in Earth's crust with a definite structure and composition. Each mineral is made up of particular elements. It is not made of any matter that was once living, like decayed bone or shells.

A rock can be one mineral or a mixture of minerals. You can tell minerals apart by their properties. Properties include hardness, luster, and streak. A way to identify rocks is by identifying the minerals it contains. A rock gets its properties from the way it forms. Rocks form from several basic processes. They are grouped into three main groups.

A rock that forms when hot, liquid rock material cools and hardens into a solid is called an igneous rock. There are many kinds of igneous rock. Some igneous rocks form from magma, below Earth's surface. Magma may also reach all the way to Earth's surface before cooling and hardening. The rate of cooling determines the size of the rock crystals.

A rock that forms from pieces of other rocks that are squeezed or cemented together is called a sedimentary rock. Weathering breaks down rocks into bits and pieces of sediment. The sediment is carried away by the forces of erosion. Eventually the sediment is deposited. Layer upon layer of sediment builds up. The pressure, or weight increases. The upper layers press down the sediment in a bottom layer. This can cause very fine particles to become squeezed together and harden into a layer of solid rock.

A rock that forms from another kind of rock that is changed by heat, pressure, or by chemical reaction is called a metamorphic rock. Metamorphic rocks often form deep underground where temperatures are high, and the pressure is great from rocks above. Metamorphic rocks also form when rocks come in contact with hot magma or lava.

Rocks are continually changing from one kind into another in a never ending process called the rock cycle. The rock cycle has no beginning and no end. However, rocks did have to begin somewhere. There is a lot of evidence that Earth started out totally molten. There was no solid rock, only magma. Perhaps the very first rocks to form and begin the rock cycle were igneous rocks.