Rocks are formed from minerals and can be divided into three general types.
Sedimentary: A rock that is formed at or near the Earth's surface from either deposits formed by the products of erosion and weathering (such as sand and clay), or from organic debris (eg shells or plants), or from the solid salts left behind when water evaporates. Sedimentary rocks often have a layered appearance. Examples: Sandstone and limestone.
Metamorphic: Any rock that is altered by high temperatures and pressures, but has not melted. Metamorphic rocks often have a wavy, deformed appearance. Examples: Marble and schist.
Igneous: A rock that forms from molten magma. Igneous rocks are crystalline and can form either within the Earth's crust or, if they are erupted from a volcano, at the Earth's surface. Examples: granite and basalt.
Image: Folds in a metamorphic rock (credit: Herve Conge, ISM/SPL)
Kate Humble visits Dolaucothi mine in Wales.
Kate Humble visits Dolaucothi mine in Wales to see rocks that are rich in gold. Expert Alwyn Annals explains the history of the rocks that now form the mine. Mineral-rich hot water concentrated the gold in the rocks of Dolaucothi mine when they were crushed and folded by a collision between continents millions of years ago.
Anna Grayson follows a trail of geological clues.
Anna Grayson follows a trail of geological clues to find out how modern day England was separated from what is now France hundreds of thousands of years ago during an ice age. She speaks to geologist Phil Gibbard who helps her understand the region's geological history.
Ray Mears explains how the Malvern Hills provide pure water.
Ray Mears visits the Malvern Hills and explains their geological history and why the region has such pure springwater.
Anna Grayson traces the geological history of the Lizard in Cornwall.
Anna Grayson explains the geological history of a region of southern Cornwall called the Lizard. The rocks that form the Lizard are unique - it is possible to see rocks and minerals that formed deep inside the Earth.
David Attenborough explains how lava erupts and cools.
David Attenborough explains how basalt lava erupts and cools - sometimes creating beautiful formations such as Giant's Causeway in Northern Ireland and Fingal's Cave in Scotland.
In geology, a rock is a naturally occurring solid aggregate of one or more minerals or mineraloids. For example, the common rock granite is a combination of the quartz, feldspar and biotite minerals. The Earth's outer solid layer, the lithosphere, is made of rock.
Rocks have been used by mankind throughout history. From the Stone Age rocks have been used for tools. The minerals and metals we find in rocks have been essential to human civilization.
Three major groups of rocks are defined: igneous, sedimentary, and metamorphic. The scientific study of rocks is called petrology, which is an essential component of geology.