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• The rocks on Earth are constantly changing due to many different processes.
• There are three main types of rock, with rocks changing between each type over millions of years.
• This rock recycling is a process called the rock cycle.

Watch this video to find out more about the rock cycle.

Rocks on Earth do not always stay the same.

Rocks are continually changing due to processes such as weathering, erosion and large earth movements. The rocks are gradually recycled over millions of years, changing between the different rock types.

This recycling of rocks is a process called the rock cycle.

The three rock types are:

• igneous rocks formed from the cooling of molten rock
• sedimentary rocks formed by small rock pieces being transported in rivers and laid down in layers
• metamorphic rocks formed from another rock under heat and pressure

Find out more about the different rock types in this guide.

Look at the diagram to get a better understanding of all the different processes of the rock cycle. These include weathering, erosion, transportation and sedimentation.

There are many different ways for rocks to be recycled.

For example:

sedimentary → metamorphic → sedimentary

1. Sedimentary rocks can change into metamorphic rocks due to heat and pressure from the movements of the Earth.
2. Those metamorphic rocks can be weathered, eroded, and the pieces transported away.
3. The pieces of rock could be deposited in a lake or sea, eventually forming new sedimentary rock.

Weathering is one of the many processes that occur in the rock cycle.

Weathering breaks down rocks on the surface of the Earth.

There are three types of weathering.

1. Biological weathering

This describes rocks being broken up by the roots of plants, or animals burrowing into them.

2. Chemical weathering

This describes rocks being broken up because substances in rainwater, rivers and seawater or the air, react with the minerals in the rocks.

3. Physical weathering

This describes rocks being broken up by changes in temperature, freezing and thawing of trapped water or the action of waves and rivers.

Another process of the rock cycle is erosion. Erosion is the process of moving the small pieces of rock formed by weathering. Erosion occurs from the action of water or wind.

Rock particles can be transported by rivers and form sedimentary rocks.

There are five stages involved in this:

transport → deposition → sedimentation → compaction → cementation

Developing ideas about the rock cycle

Over time there have been lots of different theories about the age of the Earth.

For thousands of years, most people believed that the Earth had always been in a steady state since its creation, and that rocks did not change. This was probably because the changes that occur to rocks are usually too slow to see.

In 1623

William Shakespeare wrote the play As You Like It. The character Rosalind says 'the poor world is almost six thousand years old', she was basing this on a calculation using events in the Bible.

At the time, six thousand years seemed like a really long time. Time periods like millions or billions of years were completely unbelievable to people.

Over one hundred years later

James Hutton observed the rocks where he lived in Scotland and argued that rocks and landscapes were continuously changing over incredibly long periods of time. He concluded that the world must be very old, with 'no vestige of a beginning—and no prospect of an end.'

Hutton used the presence of fossils found in sedimentary rocks as key evidence of his idea.

After another hundred years

Charles Lyell expanded on Hutton’s work.

Lyell specifically looked for fossilised remains of humans in sedimentary rocks. He didn’t find any human fossils in older layers of rocks and so concluded that the rocks were much older than humans.

Today:

Scientists today agree that the Earth is over 4.5 billion years old.

Rocks can be weathered by a physical process, like a change in temperature, freezing and thawing or the action of wind, waves or rain.

In this activity sugar cubes are used to represent big rocks. They are shaken in a container to model what might happen if they were picked up by waves and smashed against the sea bed over and over.

You will need:

• sugar cubes
• a container with a lid (for example a sandwich box)

Method

1. Gather your equipment. Make a note of the appearance of your sugar cubes. Consider the right kind of scientific terms you could use to describe their appearance, colour, size and shape.
2. Place the sugar cubes in the container and put the lid on. Shake the container for a few minutes.
3. Open the container and look at the sugar cubes. How have they changed? Make a note, again considering the words you use in your description.
4. Put the lid back on the container and shake it for a few minutes.
5. Open the container and look at the sugar cubes. How have they changed since you last looked at them?
6. Return the sugar to your sugar bowl and clear up the area you have been working in.

Overall, what is the effect of weathering on the sugar cubes? How could this best be described?