To know how igneous rocks are formed

Volcanos

What do you think the rocks surrounding this volcano will look like?

Look at the samples of igneous rocks What similarities do they have? What differences do they have?

Do they have crystals / layers / fossils / grains etc…

Gabbro

GraniteBasalt

Granite

Pumice

Basalt

Obsidian

Rhyolite

Andesite

All the rocks are formed from magma, so why do they have

different sized crystals?

What may affect how big the crystals form?

We live on the Earth’s crust, and underneath this is a layer called the mantle

The mantle and the crust can melt as the Earth is extremely hot - the molten rock is called magma

When magma cools it can form igneous rocks

Magma which reaches the Earth’s surface is called lava

Igneous rocks are formed when magma cools and solidifies

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

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

Their crystals vary in size depending upon how quickly the magma cools…

Igneous rock crystals vary in size, depending on how quickly the molten rock cools

Carry out the salol experiment – place the salol on a hot slide / cold slide and observe the difference in crystal size…

If salol cools slowly, the particles have longer to arrange themselves in a pattern. Big crystals form.

The same happens in intrusive rocks as its inside the earth and is still quite hot

Quick cooling makes small crystals – the particles have less time to make a pattern.

The same happens to magma when it cools quickly – normally outside of the volcano (lava)

magma cooling slowly

area where cooled magma has formed rock

The surrounding rocks insulate the magma. So it takes a long time to cool down.

Some magma cools under the Earth’s surface.

Granite is an example of a rock that was formed under the surface.

The crystals have time to grow big.

When magma leaves a volcano, we call it lava.

Lava cools quickly in cool air or under water.

14Elaborate

Rocks from quickly cooled lava have small crystals, just basalt.

Sometimes the lava cools so quickly that crystals don’t have time to form at all.

That explains bubbly pumice and glassy obsidian.

Complete the changing Earth worksheet

Complete the changing Earth worksheet

Lava quickly

igneous small

Volcano

Magma

slowly igneous

largeMantle