We mainly see rocks in cliffs and road cuttings, but rocks also lie below the vegetation and soil in every landscape. Some of these rocks are useful as building stones, others are the source of valuable and precious metals. Rocks can seem to be a permanent feature of our landscape, but in fact they are being created, destroyed and recreated all the time. Over millions of years, volcanoes erupt, mountains are built, these are eroded by water and ice, and the rock fragments are laid down in rivers and on the seabed, only to be crumpled up to form mountains and eroded again, in a continuous cycle of processes that goes on to the present day.

The different rocks making up the surface of the earth form in different ways, and the processes involved leave their mark on the rocks they produce.

New rocks are formed where ‘magma’ or molten rock flows out onto the surface of the earth, like lava flows on the volcanoes of Hawaii, but they can also be formed by the weathering and erosion of existing rocks. The earth is a dynamic planet and the rocks are continually being recycled. There are 3 basic types of rock which are produced by three different processes acting to form rocks on the earth.

‘Igneous rocks’ are formed from molten rock that becomes solid when it cools, either on a volcano or deep in the ground in the earth’s crust:

They usually contain crystals. The number and size of crystals depends on how long they took to grow. Rocks which cool slowly, deep underground, grow big crystals, like granite. Rocks which cool very quickly at the surface like lavas, have minute crystals and can even be glassy.

‘Sedimentary rocks’ are made up of grains which have been eroded from other rocks, like igneous rocks. The gains are small rock fragments or individual mineral grains, and are often rounded because they have been transported by water or wind. The grains are laid down as sediments, in layers, like sand on the beach or mud in a river. Over time they get buried, become compacted and cemented into solid rock. Sedimentary rocks can contain fossils of plants or animals which were living at the time the rock was being deposited, or in some cases they are made completely of the fossil skeletons of plants and animals, forming a rock like limestone.

‘Metamorphic rocks’ are existing rocks which have been ‘changed’ or ‘metamorphosed’ by high temperature and pressure, usually after being buried deep within the earth. These rocks are made up of crystals, and are often banded, contain veins and can be flaky or sugary. Metamorphic rocks make up some of our most useful and beautiful building materials, like slate and marble.

Slate is a metamorphic rock with very tiny crystals. It was originally laid down as a soft mud, but it has been recrystallized into a hard, water resistant rock that can be split into thin sheets, making excellent roofing tiles. Marble is formed from limestone, but unlike slate it is not flaky. Marble doesn’t break into sheets like slate, therefore it is a good material for statues, as smooth surfaces can be carved in any direction.

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Content last updated: 25/09/2003

Dr. Janet Sumner

Dr Janet Sumner is a research fellow at the Open University. Her research is about active volcanoes Stromboli and Etna in Italy. She studies basaltic fire-fountains, their deposits and the hazards they represent. She is currently involved in computational fluid dynamic modelling and laboratory modelling of fire-fountains (using golden syrup).

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