ROCKS

What are rocks?

Rocks are made of grains that fit together. Each grain in the rock is made from a mineral, which is a chemical compound. The grains in a rock can be different colours, shapes and sizes.

Granite has interlocking grains. Sandstone has rounded grains.

Some types of rock have interlocking grains that fit tightly together. Granite is a rock with interlocking grains. Other types of rock have rounded grains. Sandstone is a rock with rounded grains.

Porous rocks

Rocks with rounded grains are more likely to absorb water than rocks with interlocking grains. This is because the water can get into the gaps between the grains. Rocks that absorb water are called porous.Rocks with rounded grains are usually softer and more crumbly than rocks with interlocking grains. So porous rocks tend to be softer than non-porous rocks.

Rocks with interlocking grains are more likely to be hard and non-porous

Rocks with rounded grains are more likely to be crumbly and porous

TYPES OF ROCKS

IGNEOUS ROCKS

There are three main types of rock: sedimentary, igneous and metamorphic.

Formation

The inside of the Earth is very hot - hot enough to melt rocks. Molten (liquid) rock forms when rocks melt. The molten rock is called magma. When the magma cools and solidifies, a type of rock called igneous rock forms.

What are they like?

Igneous rocks contain randomly arranged interlocking crystals. The size of the crystals depends on how quickly the molten magma solidified. The more slowly the magma cools, the bigger the crystals.

You may have done an experiment at school with a substance called salol. If molten salol cools slowly, you get big crystals. If it cools quickly, you get small crystals.

Obsidian and basalt

If the magma cools quickly, small crystals form in the rock. This can happen if the magma erupts from a volcano. Obsidian and basalt are examples of this type of rock. They are called extrusive igneous rocks because they form from eruptions of magma.

Granite and gabbro

Granite has large crystals.

If the magma cools slowly, large crystals form in the rock. This can happen if the magma cools deep underground. Granite and gabbro are examples of this type of rock. They are intrusive igneous rocks because they form from magma underground.

Unlike sedimentary rocks, igneous rocks do not contain any fossils. This is because any fossils in the original rock

will have melted when the magma formed.

SEDIMENTARY ROCKS

Formation of sedimentary rocks

A river carries, or transports, pieces of broken rock as it flows along. When the river reaches a lake or the sea, its load of transported rocks settles to the bottom. We say that the rocks are deposited. The deposited rocks build up in layers, called sediments. This process is called sedimentation.

The weight of the sediments on top squashes the sediments at the bottom. This is called compaction. The water is squeezed out from between the pieces of rock and crystals of different salts form.

The crystals form a sort of glue that sticks or cements the pieces of rock together. This process is called cementation.

These processes eventually make a type of rock called sedimentary rock. It may take millions of years for sedimentary rocks to form.

These are the different processes in order:

sedimentation → compaction → cementation

The river transports bits of rock, and deposits them on the bottom of the river bed.

Deposited rocks build up in layers, and the weight of the top layers compresses the bottom layers

The compression squeezes out water, leaving salt crystals that cement the rocks together.

What are they like?

Sedimentary rocks contain rounded grains in layers. Examples of sedimentary rock are chalk, limestone, sandstone, shale

The oldest layers are at the bottom and the youngest layers are at the top. Sedimentary rocks may contain fossils of animals and plants trapped in the sediments as the rock was formed.

METAMORPHIC ROCKS

Metamorphic rocks are the third type we'll look at. They formed from other rocks that are changed because of heat or pressure.

Formation

Earth movements can cause rocks to be deeply buried or squeezed. As a result, the rocks are heated and put under great pressure. They do not melt, but the minerals they contain are changed chemically, forming metamorphic rocks. Sometimes, metamorphic rocks are formed when rocks are close to some molten magma, and so get heated up. Metamorphic

rocks may form from rocks heated by magma

Remember that metamorphic rocks are not made from melting rock. (Rocks that do melt form igneous rocks instead.)

What are metamorphic rocks like?

When a metamorphic rock is formed under pressure, its crystals become arranged in layers. Slate, which is formed from shale, is like this. Slate is useful for making roof tiles because its layers can be split into separate flat sheets. Marble bedrock on the coast.

Marble is another example of a metamorphic rock. It is formed from limestone.

Metamorphic rocks sometimes contain fossils if they were formed from a sedimentary rock, but the fossils are usually squashed out of shape.

Metamorphic rocks can be formed from any other type of rock - sedimentary or igneous. Remember these two examples of common metamorhpic rocks and where they come from:

slate is formed from shale marble is formed from limestone

WEATHERING

Weathering is the wearing away of rocks. There are three types of weathering:

1) physical weathering 2) chemical weathering 3) biological weathering

1) Physical weathering

Physical weathering is caused by physical changes such as changes in temperature, freezing and thawing, and the effects of wind, rain and waves.

Temperature changes : When a rock gets hot it expands a little, and when a rock gets cold it contracts a little. If a rock is heated and cooled many times, cracks form and pieces of rock fall away. This type of physical weathering happens a lot in deserts, because it is very hot during the day but very cold at night.

Wind, rain and waves : Wind, rain and waves can all cause weathering. The wind can blow tiny grains of sand against a rock. These wear the rock away and weather it. Rain and waves can also wear away rock over long periods of time.

Freeze-thaw: Water expands slightly when it freezes into ice. This is why water pipes sometimes burst in the winter. You might have seen a demonstration of this sort of thing at school - a jar filled to the brim with water eventually shatters after it is put into a freezer.

The formation of ice can also break rocks. If water gets into a crack in a rock and then freezes, it expands and pushes the crack further apart. When the ice melts later, water can get further into the crack. When the rock freezes again, it expands and makes the crack even bigger.

This process of freezing and thawing can continue until the crack becomes so big that a piece of rock falls off.

a)Water gets into a crack in a rock b)The water freezes and expands, making the crack bigger

c)The water can get further into d)This process of freezing and thawing can continue the crack, making it bigger. until the crack becomes so big that a piece of rock falls off.

2) Biological weathering

Animals and plants can wear away rocks. This is called biological

weathering. For example, burrowing animals such as rabbits can burrow into a crack in a rock, making it bigger and splitting the rock.

You may have seen weeds growing through cracks in the pavement. If

you have gone for a walk in the countryside, you may even have seen bushes or trees growing from cracks in rocks or disused buildings. This is because plant roots can grow in cracks. As they grow bigger, the roots push open the cracks and make them wider and deeper. Eventually pieces of rock may fall away.

People can even cause biological weathering just by walking. Over time, paths in the countryside become damaged because of all the boots and shoes wearing them away.

3) Chemical weatheringLimestone cavern in the Peak District

The weathering of rocks by chemicals is called chemical

weathering. Rainwater is naturally slightly acidic because

carbon dioxide from the air dissolves in it. Minerals in rocks

may react with the rainwater, causing the rock to be

weathered.

Some types of rock are easily weathered by chemicals. For example, limestone and chalk are made of a mineral called calcium carbonate. When acidic rainwater falls on limestone or chalk, a chemical reaction happens. New soluble substances are formed in the reaction. These are washed away and the rock is weathered. Chemical weathering can hollow out caves form and make cliffs fall away.

Gabbro, a hard wearing rock Some types of rock are not easily weathered by chemicals. For example, granite and gabbro are hard rocks that are

weathered only slowly. Still some of their minerals do react with the acids in rainwater to form new, weaker substances that crumble and fall away.

Acid rain: When fossil fuels such as coal, oil and natural gas are burned, carbon dioxide and sulphur dioxide escape into the air. These dissolve in the water in the clouds and make the rainwater more acidic than normal. When this happens, we call the rain 'acid rain'.

Acid rain makes chemical weathering happen more quickly. Buildings and statues made from rock are damaged as a result. This is worse when the rock is limestone rather than granite. Acid rain also kills trees and fish.

Erosion and transport

Erosion: Erosion is the movement of the broken pieces away from the site of weathering.

Basalt columns

For example, a basalt cliff may be weathered by freeze-thaw, a type of physical weathering. This means that pieces of the cliff may break away.

Erosion happens when these pieces of rock fall away down the cliff.

In the photograph you can see a basalt cliff. At the bottom there are heaps of rocks, caused by weathering then erosion.

Transport: Rivers and streams can move pieces of rock. This is called transport. Fast flowing rivers can transport large rocks, but slow moving rivers can only transport tiny pieces of rock.

As the pieces of rock are carried along by the water, they bash against each other and the river bed. They gradually wear away because of this. They become smaller and more rounded.

THE ROCK CYCLE

Sedimentary, for example chalk, limestone, sandstone and shale; Igneous, for example basalt and granite; Metamorphic, for example slate and marble

Continual change

The Earth's rocks do not stay the same forever. They are continually changing because of processes such as weathering and large earth movements. The rocks are gradually recycled over millions of years. This is called the rock cycle.

For example sedimentary rocks can be changed into metamorphic rocks, and these can be weathered and the pieces transported away. These pieces could be deposited in lakes or seas and eventually form new sedimentary rock. Many routes through the rock cycle are possible.

Sedimentation creates layers or rock particles Compaction and cementation presses the layers and sticks the particles together. This creates

sedimentary rock. Rocks underground that get heated and put under pressure are changed into metamorphic rock. Rocks underground that get heated so much they melt turn into magma. Magma is liquid rock.

Magma also comes from deeper inside the Earth, from an region called the mantle. Pressure can force magma out of the ground. This creates a volcano. When the magma cools it

turns into solid rock, called extrusive igneous rock. Magma that cools underground forms solid rock called intrusive igneous rock. Areas of rock can move slowly upwards, pushed up by pressure of the rocks forming underneath.

This is called uplift. Weathering breaks down rocks on the surface of the Earth. There are three types of weathering -

physical, chemical and biological. Wind and water move the broken rock particles away. This is called erosion. Rivers and streams transport rock particles to other places. Rock particles are deposited in lakes and seas, where they build up to form layers. This starts the

process of sedimentation which will create sedimentary rock.

BASALT LIMESTONE

GNEISS GRANITE

SANDSTONE MARBLE

PITCHSTONE CHALK

SLATE LAVA

CONGLOMERATE GARNET AMPHIBOLITE

GABBRO MARBLE

GRANITE GRITSTONE

MUDSTONE CLAY