What are rocks?
Rocks are naturally formed and are simply composed of crystals or particles of one or more minerals. Rocks are named according to the way in which they were formed and there are three types: igneous, sedimentary and metamorphic:
Igneous (from the Latin "ignis" - fire) -
Igneous rocks originates from molten matter deep inside the earth. Hot magma (liquid rock) comes up through cracks in the earth’s surface and fills other cracks or forms a layer on the surface. It then cools forming a very hard igneous rock. The Magma that reaches the surface of the earth is called lava. The most common form of lava is basalt. Glass is a man-made igneous rock.
Sedimentary (from the latin word meaning to settle)
Sedimentary rock is formed from broken bits and pieces of different rocks. The three most common forms of sedimentary rock by far are limestone, sandstone and shale. Sedimentary rocks are very important as three quarters of the mass of the continents are covered with a thin layer of them, and concentrated within them are important resources such as coal, oil, groundwater and iron ore.
Coal: formed from the conpression of plant remains such as moss, leaves, tree trunks etc.
Oil: formed from the compression of the remains of tiny plants and animals that lived in ancient seas. This is why oil is known as a fossil fuel.
See the Energy page for more information.
Metamorphic (from two greek words meaning change and form)
Metamorphic rocks formed from igneous or sedimentary rocks that have been changed, or metamorphosed, into a new rock. They form as a result of high pressures or temperatures. Sandstone gets changed into quartzite and limestone gets changed into marble. These new rocks are not as hard as igneous rock but can be very heavy. Slate, schist, gneiss and quartzite are other examples.
The oldest rocks in the world occur in Canada's northwest territories and are about 4 billion years old. However, these rocks are metamorphic and thus do not contain fossils or records of ancient life. So, the search for earliest life has now expanded to the oldest sedimentary rocks in the world, the 3.5 billion year old rock of North Western Australia and South West Africa. See the Origin of Life on Earth page for more information. Greenland also has 3.8 billion year old rocks at Isua. The oldest rocks in Ireland date back almost 2 billion years with the very oldest to be found on Inishtrahull - an island off the coast of northern Donegal.
The three rock types are transformed into one another by processes of weathering and erosion, melting and exposure to heat and pressure.
See the Rock Cycle page for more information.
Rocks high on Mount Everest contain fossils of sea creatures. Therefore, the world's highest mountains must have been formed on the seabed (and then thrust upwards by movements in the Earth's crust.
Measurement of rock age
Rock ages are measured by the decay of radioactive minerals within the rocks. Radioactivity (the generation of heat because of unstable atoms) was discovered in France in 1896.
Radiometric dating allows us to define the age of rocks. We know the rate of radioactive decay or change. By comparing the amount of radioactive elements with the amount of radioactive decay products we can determine how much time has elapsed since the rocks formed. In brief because we know that Uranium has a half life of 4.5bn years and that it eventually breaks down into lead, then we can determine age by measuring the ratio of Uranium to lead.
For more information, see exhibition panels "Rocks for Life" in What on Earth Exhbition pages.
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