Look at a mountain and it seems to last forever; yet the rocks in it are always changing, only very slowly. The rock cycle is the never-ending change of rocks from one type into another over geologic time. Over very long times, one kind of rock can turn into another kind. To follow the cycle, it helps to know the three main kinds of rock: igneous, sedimentary and metamorphic. Rocks can be as small as pebbles or as big as mountains.

Rocks are made of minerals. A mineral is a natural solid with a set chemical makeup and an orderly crystal structure. Minerals are made by nature and are never alive. The Mohs scale ranks how hard minerals are by what can scratch what. Talc is a very soft mineral, quartz has a Mohs hardness of 7, and diamond is the hardest natural substance known.

Igneous rocks form when melted rock cools and hardens. Melted rock that erupts onto the surface is called lava. Igneous rocks that cool inside the Earth are called intrusive; those that cool at the surface are called extrusive. Rock that cools slowly deep underground grows big crystals; lava that cools fast grows tiny ones. Granite has crystals big enough to see. Obsidian is a volcanic glass, and pumice is a bubbly, holey volcanic rock.

Wind and rain slowly break big rocks into smaller pieces. Sedimentary rocks are made from bits of older rocks and from living things. Sedimentary rocks often form in layers. Sandstone is a rock made of sand grains stuck together.

Some sedimentary rocks form from the remains of large numbers of dead living things. Most limestone forms in the sea, and some limestone is made from the shells and skeletons of sea animals. Coal is a rock that comes from plants that lived long ago. In these rocks, life itself helped build the stone.

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. Marble is a rock made when limestone is changed by heat and pressure. Gneiss is a metamorphic rock with stripes or bands. The rocks are changed rather than made from scratch, so every metamorphic rock has an earlier life as some other rock.

Plates and uplift play a part too. Earth's outer layer is broken into huge slabs of rock called tectonic plates that slowly move. Mountain ranges can form where tectonic plates push together. Igneous rocks form where rock melts near plate boundaries and hot spots. Rock that cooled deep underground can end up on the surface after uplift and erosion. So a mountain top can be made of rock that once sat far below the ground. That is the rock cycle: over very long times, one kind of rock can turn into another kind.

Words to know

• metamorphic: changed by heat
• intrusive: cooled underground
• extrusive: cooled outside
• tectonic: of moving plates
• mineral: natural solid
• erosion: wearing away