S3.11.1 Chapter 1

Three kinds of rock

Why do some rocks sparkle and others crumble?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A All rocks are made the same way. They only differ in colour.
You picked A. Read on. At the end, the chapter comes back to your guess.
B Rocks form in different ways, and that changes their grains and how hard they are.
You picked B. Read on. At the end, the chapter comes back to your guess.
C Soft rocks are young rocks that have not finished growing.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Describe a rock by its grains, colour and hardness
Test the hardness of a rock by scratching it with a nail and a coin
Sort rocks into igneous, sedimentary and metamorphic by how they formed

Pick up a stone and look at it closely. Does it sparkle? Is it rough or smooth? Can you scratch it? Rocks are not all the same, and the differences tell us how each one was made.

A rock is made of small pieces. Each piece is a grain. A grain may be a crystal. A crystal is a small solid with flat sides that grew in one place. Or a grain may be a bit of older rock. You can describe any rock by three things: its grains, its colour and how hard it is.

Draw it first

Draw four rocks, close up

1 Draw a square. Fill it with a few large shapes that touch. These are the big crystals of granite.
2 Draw another square with many small shapes that touch. These are the tiny crystals of basalt.
3 Draw a third square with round grains. Leave a little space between them. This is sandstone, and the space holds the cement.
4 Draw a fourth square with wavy bands that run the same way. This is gneiss.

Granite has big crystals that lock together. Basalt has crystals so small that you need a lens to see them. Sandstone is round grains stuck together. Gneiss shows wavy bands.

Colour helps, but it can fool you. Granite can be pink, grey or white. Two rocks that look the same colour can be different kinds.

The hardness of a rock tells how easily it is scratched. A harder thing scratches a softer thing, and the softer thing cannot scratch it back. A soft rock crumbles. A hard rock does not.

A fingernail is about 2.5 on the scale. A copper coin is about 3.5. A steel nail is about 5.5.

Use the three things in order. Try the fingernail first. If it scratches the rock, the rock is softer than 2.5. If not, try the coin, then the nail. Some coins are steel with a thin coat of copper, and they scratch harder rock. Use a coin of real copper, or copper wire.

The coin scratches this rock and the fingernail does not. So its hardness lies between 2.5 and 3.5.

Try it

Scratch a rock

You need: a rock with a flat side, such as sandstone or chalk, your fingernail, a coin of real copper, or a piece of copper wire, a steel nail, a cloth.

Be safe: Never hit a rock. Chips can fly into your eyes. Push the nail along the rock with its point away from you. Wash your hands afterwards. Never go to cliffs, quarries or riverbanks alone to collect rocks.

Hold the rock firmly on a table. Rub your fingernail along the flat side with a firm push. Wipe the spot with the cloth and look.
Do the same with the copper coin. Always push the tool away from your body.
Do the same with the nail. Hold the nail so that its point faces away from you.
Write down which tool made a scratch and which did not.

What you will see: A soft rock is scratched by the fingernail. A harder one is not scratched by the fingernail, but the coin or the nail scratches it. The first tool that makes a scratch tells you the rock is softer than that tool.

If you cannot try it, this is what happens.

Now we can sort rocks. Scientists sort them by how they formed. Igneous rock formed when melted rock cooled and turned hard. Cooling slowly gives big crystals, like granite. Cooling fast gives small crystals, like basalt.

Sedimentary rock formed from layers of small bits, such as sand and mud. Over a very long time the layers were pressed and stuck together. Sandstone is one. Shale is made from pressed mud, and limestone mostly from the shells of small sea animals.

Metamorphic rock is rock that heat and pressure changed. It did not melt. Limestone becomes marble this way, and shale becomes slate. Gneiss is metamorphic too. Heat and pressure lined up its grains in bands.

Heat and pressure change limestone into marble. The rock never melts.

See it three ways

As a picture

In words

Igneous rock cooled from melted rock, such as granite. Sedimentary rock was pressed from layers of bits, such as sandstone. Metamorphic rock was changed by heat and pressure, such as marble.

A wrong idea many people have

You can tell the kind of rock by its colour.

Why it feels right: Many rocks you see first, such as black basalt or white marble, seem to keep one colour.

The test that breaks it: Look at granite in different places. It can be pink, grey or white. Marble can be white, pink or green. Rocks of one kind come in many colours.

What is true: Grains tell you much more than colour. Look for locked crystals, round grains or bands, and test the hardness. Marble and basalt both have locked crystals. But a nail scratches marble, and it does not scratch basalt.

Key words

grain: one small piece that a rock is made of.
crystal: a small solid with flat sides that grew in one place.
hardness: how hard it is to scratch a rock.
igneous: rock that formed when melted rock cooled.
sedimentary: rock made of small bits pressed and stuck together.
metamorphic: rock changed by heat and pressure, without melting.

Back to your guess

The answer is B: Rocks form in different ways, and that changes their grains and how hard they are.

Rocks are made in three main ways. How a rock formed decides its grains and its hardness.

Keep it

Copy this into your exercise book.

Igneous rock cooled from melted rock. Sedimentary rock is bits pressed together. Metamorphic rock was changed by heat and pressure. A harder thing scratches a softer one.

Draw: Draw three squares: locked crystals, round grains with cement, and wavy bands. Name the kind of rock under each.