Copyright © 2007 Dorling Kindersley
The Earth is covered in a layer of solid rock called the crust. Rocks are either SEDIMENTARY , IGNEOUS, or METAMORPHIC. Almost all rocks made of minerals, but different rocks contain different mixtures of minerals. Granite, for example, consists of quartz, feldspar, and mica. A rock can be identified by its overall colour, the minerals it contains, the size of the mineral grains, and its texture (mixture of grain sizes).
The solid rock that makes up the Earth’s crust is called bedrock. It can be seen on coasts and in mountains, where it is being worn away by erosion. Erosion breaks the bedrock into small pieces, forming soil and sediments (such as mud, sand, and gravel), which cover up the bedrock in most places. The sediments may later turn into sedimentary rocks.
Sedimentary rocks are made of particles of sediments such as sand and clay, or the skeletons and shells of sea creatures. When layers of loose sediment are buried and pressed down under more layers, the particles slowly cement together and lithify (form rock). Chemical sedimentary rocks, such as flint, form when minerals dissolved by water are deposited again.
Igneous rocks are created when magma (molten rock under the Earth’s crust) cools and becomes solid. Magma loses heat when it moves upwards at weak spots, such as cracks, in the crust. Extrusive igneous rocks form when magma reaches the surface and cools quickly. Fast cooling produces fine-grained rocks. Intrusive igneous rocks form when magma cools slowly underground. This allows the minerals to grow into coarse grains.
Metamorphic rocks are formed when the minerals in rocks are changed underground by heat and pressure. Contact metamorphic rocks are produced when rocks are heated by magma rising through the crust. Rocks that are folded or crushed by immense pressure deep in the crust are called regional metamorphic rocks. The properties (characteristics) of a metamorphic rock depend on its parent rock (the original rock type) and how it was formed.