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)
How the founding father of geology made his first breakthrough.
Inspired by the layered cliffs near his farm, the founding father of geology, James Hutton, was the man behind a fundamental discovery. He revealed that sedimentary rocks were formed from layers of sand and mud laid down on the seabed.
How did Lord Kelvin estimate the age of the Earth?
Lord Kelvin, the eminent 19th and early 20th century scientist, was determined to work out the age of the Earth. A simple experiment with molten rock gave him figures for his calculations. (This experiment should only be carried out under controlled conditions and with professional supervision.)
Iain Stewart drops in on the Old Man of Stoer.
Iain Stewart takes a close look at the Old Man of Stoer, one of the highest sea stacks in Britain. His vertigo inducing visit has an important point: in sedimentary rocks, the oldest layers will be at the bottom and youngest at the top. They're laid down as sand and mud and gradually build up over millions of years.
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.
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.
In geology, 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 found 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.