Science

Materials: Rocks

Today we are going to investigate rocks.
When dealing with any materials we often group them. It is no different with rocks, we will group them into three types:

Igneous
Sedimentary
Metamorphic

Each of these rock types have things in common and by looking at them you should be able to tell which is which.

Igneous
Igneous comes from a Greek word meaning fire. This gives us a clue about igneous rocks. All igneous rocks are formed from hot liquid rock called lava.
All igneous rocks are hard and are often made up of interlocking crystals.
The rate at which the rock cools determines the size of the crystals. If the rock has got large crystals the rock cooled slowly (probably underground). If the crystal size is very small the rock cooled very quickly.
Some igneous rocks are granite and basalt.

Sedimentary
Sediments are what we get when fine particles settle at the bottom of water.
This gives us a clue to what these rocks are made of and how they were formed. Can you guess?
Answer

Did you guess?
These rocks have been formed out of sand, mud, shale and the bodies of dead creatures.
They were formed at the bottom of large bodies of water; seas, oceans and lakes.

Sedimentary rocks are made from fine grains or pebbles and these grains can be rubbed away. These rocks often contain fossils.
These rocks are often softer than the igneous rocks.
Some sedimentary rocks are sandstone, shale, chalk and limestone.

Metamorphic
Metamorphic means "to change". This gives us a clue about how these rocks were formed.
Metamorphic rocks have been changed from exitising rocks (igneous and sedimentary).
These rocks are often banded, shiny and flaky.
Slate (yes that stuff they put on the roof) is a metamorphic rock. Slate is made from mudstone (sedimentary rock) that has been heated and squashed by pressure. Marble is what you get after limestone (sedimentary) has been heated and put under great pressure.

Remember that rocks take thousands of years to form and change.

Concrete and brick are man-made rock. They have been made from existing rocks for building materials. Later you will learn about the rock cycle. All this is a part of science called geology - "geo" meaning "Earth."

Time for a quick recap.

Here are some properties of some rocks. Can you tell if they are igneous, sedimentary or metamorphic?

1. A rock that is made from fine grains and contains fossils
Answer

1. Did you get it?
Sedimentary

2. A rock that contains interlocking very small crystals that can not be rubbed off?
Answer

2. It is of course an igneous rock.

3. A banded, shiny rock that contains fossils but can't be rubbed away.
Answer

3. Yes it's the one that's hard to say metamorphic (meta-mor-phic).

So you just need to know what the names mean and the properties of each. 
Here are the names again. I hope this will help you out:

 
Good luck!

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