The Rock Cycle
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What are rocks?
Some types of rock have interlocking grains that fit tightly together. Granite is a rock with interlocking grains. Other types of rock have rounded grains. Sandstone is a rock with rounded grains.
Rocks with rounded grains are more likely to absorb water than rocks with interlocking grains. This is because the water can get into the gaps between the grains. Rocks that absorb water are called porous.
Rocks with rounded grains are usually softer and more crumbly than rocks with interlocking grains. So porous rocks tend to be softer than non-porous rocks.
There are 3 main kinds of rock: Sedimentary, Igneous and Metamorphic
A river carries, or transports, pieces of broken rock as it flows along. When the river reaches a lake or the sea, its load of transported rocks settles to the bottom. We say that the rocks are deposited. The deposited rocks build up in layers, called sediments. This process is called sedimentation.
The weight of the sediments on top squashes the sediments at the bottom. This is called compaction. The water is squeezed out from between the pieces of rock and crystals of different salts form.
The crystals form a sort of glue that sticks or cements the pieces of rock together. This process is called cementation.
These processes eventually make a type of rock called sedimentary rock. It may take millions of years for sedimentary rocks to form.
Sedimentary rocks contain rounded grains in layers. Examples of sedimentary rock are
The oldest layers are at the bottom and the youngest layers are at the top. Sedimentary rocks may contain fossils of animals and plants trapped in the sediments as the rock was formed.
The river transports bits of rock, and deposits them on the bottom of the river bed.
Deposited rocks build up in layers, and the weight of the top layers compresses the bottom layers
The compression squeezes out water, leaving salt crystals that cement the rocks together.
If the magma cools quickly, small crystals form in the rock. This can happen if the magma erupts from a volcano. Obsidian and basalt are examples of this type of rock. They are called extrusive igneous rocks because they form from eruptions of magma.
If the magma cools slowly, large crystals form in the rock. This can happen if the magma cools deep underground. Granite and gabbro are examples of this type of rock. They are intrusive igneous rocks because they form from magma underground.
Unlike sedimentary rocks, igneous rocks do not contain any fossils. This is because any fossils in the original rock will have melted when the magma formed.
Earth movements can cause rocks to be deeply buried or squeezed. As a result, the rocks are heated and put under great pressure. They do not melt, but the minerals they contain are changed chemically, forming metamorphic rocks.
Sometimes, metamorphic rocks are formed when rocks are close to some molten magma, and so get heated up.
When a metamorphic rock is formed under pressure, its crystals become arranged in layers. Slate, which is formed from shale, is like this. Slate is useful for making roof tiles because its layers can be split into separate flat sheets.
Marble is another example of a metamorphic rock. It is formed from limestone.
Metamorphic rocks sometimes contain fossils if they were formed from a sedimentary rock, but the fossils are usually squashed out of shape.
Metamorphic rocks can be formed from any other type of rock - sedimentary or igneous. Remember these two examples of common metamorhpic rocks and where they come from:
slate is formed from shale and marble is formed from limestone
- When a rock gets hot it expands a little, and when a rock gets cold it contracts a little. If a rock is heated and cooled many times, cracks form and pieces of rock fall away. This type of physical weathering happens a lot in deserts, because it is very hot during the day but very cold at night.
- Wind, rain and waves can all cause weathering. The wind can blow tiny grains of sand against a rock. These wear the rock away and weather it. Rain and waves can also wear away rock over long periods of time.
- Water expands slightly when it freezes into ice. This is why water pipes sometimes burst in the winter. You might have seen a demonstration of this sort of thing at school - a jar filled to the brim with water eventually shatters after it is put into a freezer.The formation of ice can also break rocks. If water gets into a crack in a rock and then freezes, it expands and pushes the crack further apart. When the ice melts later, water can get further into the crack. When the rock freezes again, it expands and makes the crack even bigger.This process of freezing and thawing can continue until the crack becomes so big that a piece of rock falls off.
Water gets into a crack in a rock
The water freezes and expands, making the crack bigger
The water can get further into the crack, making it bigger and eventually will break of a piece of the rock.
Animals and plants can wear away rocks. This is called biological weathering. For example, burrowing animals such as rabbits can burrow into a crack in a rock, making it bigger and splitting the rock.
You may have seen weeds growing through cracks in the pavement. If you have gone for a walk in the countryside, you may even have seen bushes or trees growing from cracks in rocks or disused buildings. This is because plant roots can grow in cracks. As they grow bigger, the roots push open the cracks and make them wider and deeper. Eventually pieces of rock may fall away.
People can even cause biological weathering just by walking. Over time, paths in the countryside become damaged because of all the boots and shoes wearing them away.
The weathering of rocks by chemicals is called chemical weathering. Rainwater is naturally slightly acidic because carbon dioxide from the air dissolves in it. Minerals in rocks may react with the rainwater, causing the rock to be weathered.
Some types of rock are easily weathered by chemicals. For example, limestone andchalk are made of a mineral called calcium carbonate. When acidic rainwater falls on limestone or chalk, a chemical reaction happens. New soluble substances are formed in the reaction. These are washed away and the rock is weathered.
Chemical weathering can hollow out caves form and make cliffs fall away.
Some types of rock are not easily weathered by chemicals. For example,granite and gabbro are hard rocks that are weathered only slowly. Still some of their minerals do react with the acids in rainwater to form new, weaker substances that crumble and fall away.
When fossil fuels such as coal, oil and natural gas are burned, carbon dioxideand sulphur dioxide escape into the air. These dissolve in the water in the clouds and make the rainwater more acidic than normal. When this happens, we call the rain 'acid rain'.
Acid rain makes chemical weathering happen more quickly. Buildings and statues made from rock are damaged as a result. This is worse when the rock is limestone rather than granite. Acid rain also kills trees and fish.
The Rock Cycle
The Earth's rocks do not stay the same forever. They are continually changing because of processes such as weathering and large earth movements. The rocks are gradually recycled over millions of years. This is called the rock cycle.
For example sedimentary rocks can be changed into metamorphic rocks, and these can be weathered and the pieces transported away. These pieces could be deposited in lakes or seas and eventually form new sedimentary rock. Many routes through the rock cycle are possible.