Explain to your partner how igneous rocks are formed.

8H Rock factory

A volcano is a giant rock factory.

Magma (molten rock) from below the ground is pushed to the surface and cools.

The rock that forms is called igneous rock.

Investigation of the Formation of Crystals in Rocks

Learning Objectives:• To know that igneous rocks crystallise

from magma. • To be able to explain that the rate of

cooling and crystallisation determines the size of the crystals in igneous rock.

• To be able to conclude where certain igneous rocks would have formed around a volcano.

Investigation of crystallisation

Experiment 1• Warm a glass slide to around 50 C

and use a glass rod to spread some molten salol onto the slide. Then leave it to cool slowly on a heat-proof mat (see top diagram on the right).

Experiment 2• Prepare a glass slide as before and

then place it onto a chilled aluminium tray. Place the tray and slide into a freezer, or on to a large beaker containing ice (see bottom diagram on the right).

Practical Activity

Safety First: Wear safety

goggles Stand up Tuck your chair &

bags under your desk

Tie back long hair

Prepare a TV News Report

• You are going to be interviewed on TV about the formation of crystals in igneous rocks and how they are affected by temperature.

• You need to describe your demonstration to show why crystal sizes in igneous rocks vary. Briefly state your results and explain how these led you to reach your conclusions.

Example:

Scientists have made a major break-through in their understanding of the structure of the Earth. Careful observation and recording of data has enabled a leading team of scientists to describe how granite was formed and why different pieces of granite have different sized crystals in them. We interviewed one of the scientists who said …

magma

intrusive igneous

rock

extrusive igneous rock

Formation of igneous rocks

Igneous rocks are formed when magma cools and solidifies.

How are igneous rocks formed?

Deep in the ground is molten rock called magma. Sometimes, magma bursts through the surface causing volcanic eruptions.

When magma cools above the surface, extrusive igneous rocks are formed.

When magma cools below the surface, intrusive igneous rocks are formed.

How is the size of crystals in igneous rocks determined by the rate at which magma cools and solidifies?

Magma above the Earth’s surface cools quickly. Only small crystals are formed as the magma solidifies.

This is how extrusive igneous rocks like pumice and basalt are formed.

Size of crystals in igneous rocks

Magma below the Earth’s surface cools slowly. There is time for large crystals to grow as the magma solidifies.

This is how extrusive igneous rocks like granite are formed.

Explain how each rock was formed:

Rock A Rock B

In which state the particles in a rock could begin to move? Explain.

Investigation of the Formation of Crystals in Rocks

Learning Objectives:• To know that igneous rocks crystallise

from magma. • To be able to explain that the rate of

cooling and crystallisation determines the size of the crystals in igneous rock.

• To be able to conclude where certain igneous rocks would have formed around a volcano.