Classifying Rocks and the Rock Cycle

The first rocks that formed were therefore igneous rocks. Igneous rocks are formed from molten material that has cooled and solidified. This molten material can either solidify at the surface of the Earth or beneath the surface, which gives rise to two types of igneous rock - extrusive and intrusive. This can be a little confusing at times but if you just remember that you go out through an exit so extrusive rocks have come out of a volcano.

When these first igneous rocks had formed, together with an atmosphere, weathering and erosion could take place. Weathering is the breaking down of rocks in situ (where they were formed) into smaller particles. This can be physical e.g. freeze-thaw weathering or it can be chemical e.g. attack by acid in rain and the atmosphere. As the rocks are weathered, wind and rain move the smaller particles downhill (erosion), eventually ending up in streams, then rivers. Through a variety of different processes, they are deposited at the edges of rivers, in lakes or the sea. Over millions of years, they are laid down in layers of sediment, compacted by the weight of overlying material and cemented by compounds deposited by circulating groundwater. The weathered materials have become a different type of rock called sedimentary.

Earth processes then caused these sedimentary rocks to be subjected to high temperatures and pressures. This changed them into new rocks. The process of change is called metamorphosis and so they are called metamorphic rocks. The important thing to realise is that, during this time of change, the sedimentary rocks had never been melted. If they had, and then solidified at a later date, they would have returned to being igneous rocks. Metamorphic rocks are usually hard and resistant to weathering, however, over long periods of time they do weather and are recycled as sedimentary rocks. If they are deeply buried in the Earth, they can melt and be recycled as igneous rocks.

The relationship between the three groups of rock is called the rock cycle and is a slow process that operates all the time. All rocks undergo weathering and can be changed into sedimentary rocks. All rocks can be subjected to high heat and pressure to become metamorphic rocks. All rocks can be melted to reform eventually as igneous rocks. When looking at a rock, you can only ever know its most recent history - the chemicals that make it up will probably have been through the rock cycle in different ways, many times over.