Chemistry

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.

Some types of rock have interlocking grains that fit tightly together. e.g 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 rounded grains are more likely to be crumbly and porous

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

sedimentary, igneous and metamorphic.

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.

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.

Sedimentary rocks contain rounded grains in layers. Examples of sedimentary rock are:

• chalk
• limestone
• sandstone
• shale

Sedimentary rocks like sandstone have layers

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.

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.

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.

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.

They formed from other rocks that are changed because of heat or pressure.

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.)

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 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

Rocks gradually wear away. This is called weathering. There are three types of weathering:

• physical weathering

• chemical weathering

• biological weathering

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.

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.

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.

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.

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.

The solar system contains planets, their satellites, asteroids and comets. The planets orbit the Sun and take different amounts of time to do this.

We have night and day on Earth because the Earth spins on its axis. The tilt of the Earth's axis causes the seasons to change as we orbit the Sun in one Earth year.

The universe contains over 100 billion galaxies. A galaxy is a group of billions of stars. Our own galaxy is called the Milky Way, and it contains about 300 billion stars (300,000,000,000) and one of these is our Sun.

Planets and other objects go round the Sun, and these make up the solar system, with the Sun at the centre. The solar system contains different types of objects including:

• a star - the Sun

• planets, which go around the Sun

• satellites, which go around planets

• smaller objects such as asteroids and comets

There are eight planets in the solar system. Starting with Mercury, which is the closest to the Sun, the planets are:

• Mercury

• Venus

• Earth

• Mars

• Jupiter

• Saturn

• Uranus

• Neptune

All objects are attracted towards each other by a force called gravity. Gravity only becomes noticeable when there is a really massive object like a star, planet or moon.

Gravity and the solar system

Gravitational forces between the Sun and planets keep the planets in orbitaround the Sun. Without these forces, the planets would fly off into deep space

Their orbits are slightly squashed circles called ellipses. Diagrams often show the orbits as very squashed, but this is just to get a sense of perspective and to fit the diagrams onto the page.

The planets furthest out are also the coldest because they receive the least heat energy from the Sun.

Orbit times and speeds

Planets that are further from the Sun:

• move more slowly

• take more time to complete an orbit

For example, Mercury, the closest planet to the Sun, takes just 88 Earth days to complete an orbit. But Neptune, the furthest out, takes 164 Earth years to complete an orbit.

The planets spin as they orbit the Sun. It takes the Earth 24 hours to make one complete turn on its axis, so an Earth day is 24 hours long.

Different planets take different amounts of time to make one complete turn, so they have different lengths of day.

Day and night

The Sun lights up one half of the Earth, and the other half is in shadow. As the Earth spins we move from shadow to light and back to shadow and so on. It is daytime in the UK when our part of the planet is lit by the Sun. And it is night-time in the UK when our part of the planet is facing away from the Sun.

Path of the Sun

During the day, the Sun appears to move through the sky. Remember that this happens because the Earth is spinning on its axis. In the UK if we look south and follow the path of Sun in the sky during the day, 

The sun rises in the east and sets in the west

The Sun appears to move from east to west. That's because the Earth is spinning towards to the east, so we see the Sun first appear there at the start of the day. The Sun 'rises' in the east and 'sets' in the west.

One way to remember which way the Earth turns is to remember "w.e. spin", which means the Earth spins from west to east.

During the night, we cannot see the Sun. But the Earth is still spinning on its axis. This means that the stars appear to move from east to west in the sky, just as the Sun does in the day.

A planet's year is the time it takes to make one complete orbit around the Sun. The Earth goes once round the Sun in one Earth year. That's 365 Earth days.

The Earth's axis is the imaginary line through the centre of the Earth between the South and North poles. This axis istilted slightly compared to the way the Earth orbits the Sun.

We get different seasons (winter, spring, summer and autumn) because the Earth is tilted. 

• When the northern hemisphere is tilted towards the Sun it is summer in the UK.

• When the northern hemisphere is tilted away from the Sun it is winter in the UK.

Because of the tilt of the Earth's axis the Sun moves higher in the sky in summer, when we tilt towards it, than in winter.

The Moon orbits the Earth, so it is called a satellite of Earth. The Moon is anatural satellite. No-one built it or launched it into space.

Gravitational forces between the Moon and the Earth keep the Moon in orbit. The Moon does not produce light. But it does reflect light from the Sun, which is how we are able to see it from Earth. (Only stars produce light and they are called luminous for that reason.)

It takes the Moon 28 days to make a complete orbit of the Earth. As it orbits, we see the Moon lit from different angles. This is why we see phases of the Moon.

Sometimes the Moon looks like a full circle. That is called a 'full Moon'. At other times we see a crescent shaped Moon, because we can only see the edge of the part that is lit by the Sun.

Remember that we can only see the part of the Moon that reflects the Sun's light. How much of that we can see depends upon the position of the Moon in its orbit.