The Rock Cycle

For the moment, it doesn't matter what our rock is. It will be subject to weathering by the climate, freeze-thaw is an important one at the top of a mountain. As water gets into tiny cracks in our rock, at night it freezes. As it freezes, the water expands as it forms ice and pushes on the sides of the crack. After many hundreds or even thousands of these freeze-thaw cycles, the crack is enlarged to the point where a lump of the rock falls off. This lump rolls down the hill to form a scree slope or talus.

A combination of gravity and precipitation causes the lump of rock to slowly move down the slope further, eventually ending up in a mountain stream. As it is carried along by the stream, it is broken down into smaller and smaller pieces as the stream becomes a river. Where the river meets the sea (or a lake), the flow of the water slows down and the sediments are deposited on the sea (or lake) floor in layers. As more sediment is added at the top, the weight squeezes out the water from between the grains of the lowest layers of sediment as they are compressed. Chemicals precipitate (un-dissolve) around the grains, cementing them together to form sedimentary rock.

Imagine now that this sedimentary rock is being formed at a destructive plate boundary. Some of it will be pushed back upwards and folded, as a mountain range is formed. Weathering will begin and the sedimentary rock will be recycled.

Some of the sedimentary rock will be taken down with the plate that is being destroyed. Heat and pressure will change it into metamorphic rock, some of which will be uplifted and folded with the sedimentary rocks, also forming part of the mountain range. Weathering then begins and so the metamorphic rock is recycled as sedimentary rock. Some of the sedimentary rock can be taken deep into the mantle where it melts and is recycled as igneous rock.

Finally, all three types of rock can be taken down with the descending plate, to be recycled as either metamorphic rock or igneous rocks. So the next time you hold a piece of rock, just think about what a complex history it may have had!