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.


Flint is hard and breaks into sharp pieces. It forms from silica in seabed sediments and grows into nodules with an irregular shape. It is often found as bands within chalk.


Chalk is soft, white, and fine-grained limestone. It is made from the remains of microscopic sea creatures, which were deposited at the bottom of ancient seas.


Conglomerate is made up of rounded pebbles embedded fine-grained rock. It is formed when shingle is buried by other sediments.


Clay is a very fine-grained sedimentary rock. It is soft and crumbly when dry, and sticky when wet. Buried clay gradually turns to claystone and shale.


Sandstone is a medium-grained sedimentary rock made from sand grains. It is formed when sandy beaches or river beds are buried by other sediments.


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.


The Organ Pipes at Twyfelfontein in Namibia are made of basalt. This extrusive rock formation occurs when lava (volcanic magma) erupts and solidifies. The basalt cracks into flat-sided columns because it shrinks as it cools. Basalt is the Earth’s most common igneous rock.


Dark, shiny obsidian is also known as volcanic glass. It is formed when volcanic lava cools so quickly that crystal grains do not have time to form. Prehistoric people used obsidian to make sharp tools.


Granite’s colour varies with the minerals it contains. This pink granite shows grains of pink feldspar, white quartz, and black mica. It is formed by slow cooling of molten rock deep in the Earth.


Porphyrite (also called microdiorite) is a grey or dark grey intrusive igneous rock. It takes its name from its texture of large grains (called porphyrites) set in a background of small 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.


Migmatite is a mixture of dark-coloured schist or gneiss and a lighter coloured rock similar to granite. This piece was found in the Highlands of Scotland.


Slate is fine-grained and dark grey or green. It splits easily into flat sheets, and is used to make roof and floor tiles. It is formed from the sedimentary rocks mudstone or shale.


In a schist, lines of crystals can often be seen with the naked eye. This sample contains large crystals of garnet. Schists are mostly medium-grained and come from shales or granites.


Gneiss (pronounced nice) is a coarse-grained, grey, or pink regional metamorphic rock formed from limestone or granite. Light and dark layers of minerals can be seen rippling across the rock.

Copyright © 2007 Dorling Kindersley


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