Rocks and the rock cycle

Learning objectives

By the end of this topic you should be able to:

• describe how igneous, sedimentary and metamorphic rocks form
• explain why crystal size depends on how fast magma cools
• explain weathering, erosion and deposition
• describe the rock cycle
• link the properties of rocks to their uses
• discuss quarrying and building materials

Before you start

You will use these skills. If any feel shaky, review them first.

• solids, liquids and melting (see States of matter)
• acids and simple reactions
• reading tables and patterns

Key vocabulary

Igneous rock

Rock formed when magma or lava cools and becomes solid.

Sedimentary rock

Rock formed from layers of sediment that are compacted and cemented.

Metamorphic rock

Rock changed by heat and pressure without melting.

Weathering

The breaking down of rocks where they are.

Erosion

The carrying away of rock pieces by water, wind or ice.

Magma / lava

Molten rock underground / at the surface.

Understanding the ideas

1. What is it?

   Rocks are grouped by how they form. Igneous rocks form when molten rock cools; sedimentary rocks form from layers of sediment; metamorphic rocks form when other rocks are changed by heat and pressure.

2. Why does it happen?

   Weathering breaks rocks down, erosion carries the pieces away, and deposition drops them in layers that become new rock. Deep underground, heat and pressure change rocks, and some melt into magma. This never-ending set of changes is the rock cycle.

3. How do we know?

   Geologists identify rocks by looking at their grains or crystals, testing hardness, and seeing whether they fizz with acid. Fossils show which rocks formed from ancient sea beds.

4. Why does it matter?

   Rocks give us building stone, cement, salt, metals and soils. Knowing how rocks weather helps us protect old buildings and choose the right materials.

5. What does it connect to?

   Rocks link to states of matter and acids in chemistry, to fossils and evolution in biology, and to plate tectonics and landforms in Earth science.

Identifying the three rock types

• Igneous (granite, basalt): interlocking crystals, no layers or fossils. Large crystals mean slow cooling underground; small crystals mean fast cooling at the surface.
• Sedimentary (sandstone, limestone): rounded grains, often in layers, may contain fossils, and often soaks up water.
• Metamorphic (marble, slate): crystals or bands formed under heat and pressure, and usually hard.
• Acid test: limestone and marble fizz, because they contain calcium carbonate.

Rocks in the real world

Fossil sea shells are found in the mountains of northern Pakistan, showing that these rocks were once under the sea. The Khewra salt mine and marble from the northern areas are important resources. Old limestone buildings in polluted cities are slowly weathered by acid rain.

Worked examples

Assessment tips

Rock questions often use the rock cycle diagram and simple tests. Expect to:

• Describe how each rock type forms, using the correct processes.
• Explain crystal size, layers and fossils.
• Use properties to identify rocks and choose their uses.
• Plan simple experiments that model rock processes.

Common mistakes: mixing up weathering and erosion; saying metamorphic rocks form by melting; saying granite cooled quickly; and thinking rocks never change.

Check your understanding

Quick questions on the ideas above. Try each one before using a hint.

Practice questions

Criterion-linked questions

Criterion B: inquiring and designing

Criterion C: processing and evaluating

Criterion D: reflecting on the impacts of science

Challenge questions

Harder problems in unfamiliar contexts. Plan before you calculate.

Topic check

Five questions picked at random from the whole topic. Take a new set whenever you like.

Review your mistakes

Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.

Your progress

Tracked separately for each skill, on this device only.