Rocks look still, but they change, very slowly. There are three main kinds of rock. They are igneous, sedimentary and metamorphic. All rocks are made of minerals. Each kind has its own story, and we will look at how each one begins, one at a time.

Igneous rock starts as hot melted rock. When it cools, it gets hard and turns into solid rock. How fast it cools matters a lot. Rock that cools slowly deep underground grows big crystals. Lava that cools fast grows tiny ones. That is how the rock gets its look.

Granite is a rock with crystals big enough to see. Obsidian is a rock that is a kind of volcanic glass. So some rocks are like glass, and some are very different. Each one tells how it was made. The story is in the rock, and it is a good one to read.

Wind and rain slowly break big rocks into smaller pieces. Sedimentary rocks are made from bits of older rocks and from living things. They often form in layers, one on top of the next. Sandstone is made of sand grains stuck together.

Most limestone forms in the sea. Some limestone is made from the shells and skeletons of sea animals. Coal is a rock that comes from plants that lived long ago. So plants and sea life can end up as rock. It all takes a long, long time.

Metamorphic rocks are rocks changed by great heat or pressure. They start out as another kind of rock. Heat and pressure can change a soft rock called shale into slate. Gneiss is a metamorphic rock with stripes or bands. Rock can get a new life this way. It is a rock with a new life.

How hard is a mineral? The Mohs scale ranks how hard minerals are by what can scratch what. Talc is a very soft mineral. Diamond is the hardest natural substance known. That is a big range from soft to hard.

Over very long times, one kind of rock can turn into another kind. Melted rock cools into igneous rock. Bits and living things make sedimentary rock. Heat and pressure make metamorphic rock. The rocks you see today may change again.

Words to know

• igneous: cooled melted rock
• metamorphic: changed rock
• crystals: mineral shapes
• gneiss: striped rock