A short guide to rocks

Tweet

A rock is simply an accumulation of mineral particles. These can be loose sand (to form sandstone), sticky clay (as in mudstone), the fossilised remains of living creatures (as in limestone), or crystals that become welded together (as in rocks created by volcanoes). Rocks are generally classified into three groups: Igneous rocks are formed by volcanic activity as molten magma or lava (magma forms underground and lava is when it reaches the surface) cools and crystals form. Sedimentary rocks are created as a result of the weathering of older rocks. The latter are broken down by wind and water and later re-form from the sediments that are produced by weathering. Metamorphic rocks are formed when extreme heat or pressure changes the character of already existing rocks (usually sedimentary ones but not always so). Metamorphism usually takes place deep underground and the rocks may only reach the surface millions of years after their creation. There are many ways in which rocks can form in all three groups, hence there is a huge range of rock types. The first rocks formed when planet Earth cooled down from being a mass of molten material some 4000 million years ago. The first sedimentary rocks only came about after the planet acquired an atmosphere some 2000 million years ago. The processes of rock creation are continuous, with sediments gradually turning into sedimentary rocks and volcanism creating potential new igneous rocks with every fresh eruption.