Rocks and the Rock Cycle

Layers of rocks can be faulted, folded and tilted by large forces within the Earth’s crust. The study of these changes can give information about the strength and direction of the forces involved.

Sedimentary rocks – layers of rock fragments that were compressed and cemented together. They often contain fossils.
Igneous rocks – molten rock that cools and solidifies. They do not contain fossils.
Metamorphic rocks – existing rocks that are compressed and heated without melting. They have the same chemical composition as the rocks from which they were made.

Sedimentary Rocks

You should know some examples of sedimentary rocks and how they were formed, and understand why fossils may be found in them.

How are they formed?
Sedimentary rocks are formed when eroded fragments of old rocks and dead organisms settle (usually in seas or rivers) to form a sediment. Over millions of years, layers of sediment build up and are buried one on top of the other. They are compressed, and their weight squeezes out the water. Eventually the pieces of rock in the sediment become bonded together to form sedimentary rocks.

What are they like?
Sedimentary rocks have a layered appearance. They may contain fossil remains of animals and plants that were trapped as the rocks formed. These fossils can be used to date the rock.

Limestone, sandstone and mudstone are examples of sedimentary rocks.

Igneous Rocks

You should know some examples of igneous rocks and how they were formed, and understand why fossils are not found in them.

How are they formed?
Igneous rocks are formed when hot, molten rock (called magma) cools and solidifies. There are two main types of igneous rock, depending on where the magma cooled:

• Intrusive igneous rocks form when the magma comes from deep underground and is forced into the upper layers of the Earth’s crust, where it cools slowly.
• Extrusive igneous rocks form when the magma erupts from a volcano and cools quickly on the ground.

What are they like?
Igneous rocks contain randomly-arranged crystals. The crystals are large if the molten rock cooled slowly and small if it cooled quickly. Because igneous rocks are formed from molten rock, they do not contain fossils.

Granite is an example of an intrusive igneous rock with large crystals.
Basalt is an example of an extrusive igneous rock with small crystals.

Metamorphic Rocks

You should know some examples of metamorphic rocks and how they were formed, and understand how we can use their chemical composition as evidence of how they were formed.

How are they formed?
Metamorphic rocks are formed when existing igneous or sedimentary rocks are changed by pressure and heat, but without melting. This can happen by contact with cooling igneous rocks, or at the edges of tectonic plates, where very high temperatures and pressures occur as mountains are being formed.

What are they like?
Metamorphic rocks usually contain regularly-aligned crystals, and they may be shiny and hard. They sometimes flake into layers.

Marble, slate, schist and gneiss are examples of metamorphic rock.

Chemical composition
Metamorphic rocks may have the same chemical composition as other rocks, and this provides evidence for how they were formed. For example, marble is formed from limestone, and both are forms of calcium carbonate.

The Rock Cycle

The Earth’s crust is constantly changing, and the term ‘rock cycle’ refers to the constant recycling of material in the crust, summarised in the diagram below.

Evidence for movement

You need to be able to explain how the appearance of rocks provides evidence for movements in the Earth’s crust.

Sedimentary rocks contain evidence of how they were formed, including ripple marks from waves or water currents, and layers of different thickness and composition. Younger sedimentary rocks usually lie on top of older rocks, but this can change.

Rocks can be faulted or fractured. They can be folded and tilted, and they may even be turned upside down. Changes like these are evidence that the Earth’s crust is unstable and experiences very large forces.