Rocks
Rocks can be grouped into three types: igneous, sedimentary and metamorphic.
Igneous rocks are formed from molten lava or magma. If the molten material cools deep
in the Earth it cools slowly and the crystals have plenty of time to grow. The crystals are
therefore large. An example of this type of rock is granite. If the molten material comes
out onto the surface of the Earth it cools quickly and the crystals do not have time to
grow. These igneous rocks have small crystals, an example is basalt.
Sedimentary rocks are formed from the weathered remains of other rocks. These rocks
are formed on sea beds, river beds, deserts etc. They usually contain fossils and are
deposited in layers. Examples include limestone, sandstone, shale etc.
Metamorphic rocks are formed from other rocks. Sometimes igneous or sedimentary
rocks are subject to huge pressures and temperatures, such as during mountain building.
The original rocks change into metamorphic rocks such as schist, slate and gneiss.