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Q Why are rocks different colours?

A The colour of a rock depends on the minerals it contains. For example, biotite mica, hornblende and pyroxenes are dark brown, dark green or black, so a rock containing a high percentage of these minerals will appear dark or "melanocratic". "Leucocratic" rocks contain lighter-coloured minerals such as feldspar, quartz and muscovite mica. Other colours come from staining. For example, sandstones, made mainly of quartz sand grains, may be stained red by iron minerals or green by copper ones.

Q What percentage of the world's surface is water? And what percentage of this is saltwater, freshwater and frozen water?

A About 70 per cent of the Earth's surface is water. Of this 70 per cent, 98 per cent is saltwater, leaving 2 per cent as freshwater. Of the 2 per cent that is fresh, about 90 per cent is frozen. This frozen water is locked up in the Antarctic ice sheets and glaciers on the Alps.

Q What is peat?

A Peat is the name given to the layers of dead, rotting plants that accumulate in swampy places in cold and temperate regions. As these layers are buried by other material, they are compressed into solid material. Peat can be burned for fuel although it is less efficient than, say, coal because it represents a much less pure form of carbon.

Q What is the biggest island in the world?

A Greenland covers 2,175,000 sq km. Australia is bigger, but is usually regarded as a continental land mass.

Q If computers work at 66MHz, why don't we go deaf?

A Hertz are a measure of frequency, not just the pitch of a sound. When a computer operates at 66MHz, this just means that its processor checks all its other parts 66 million times per second.

Q Why do some spacecraft go round different planets to get to their destination faster?

A The technique used here is called a gravity assist, or sling shot. The gravitational field around the planets is used to change the spacecraft's velocity without using fuel. As the craft approaches the planet, it is pulled in towards the planet - speeding it up. This is exactly the same as spinning a conker round on a piece of string. As you shorten the piece of string, the conker goes round faster and faster. It's the same with the spacecraft, but because the spacecraft is not attached to the planet it can fly off, once it has swung past, with its extra speed.

Q Do people grow in space?

A In space the body does not experience the same downward pull of gravity. As a result the gaps between the vertebrae expand and astronauts do actually get slightly taller. Back on Earth, of course, gravity quickly squashes them back to normal height.

CHRISTOPHER RILEY

These questions and answers are provided by Science Line. You can use its Dial-A-Scientist service on 0345 600 444.

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