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Physics - Inside the Earth

We have seen our planet from space and we know much about its shape and size. We know the Earth is like a giant ball, slightly flattened at the poles and with a slight bulge at the equator. We know that a journey around the Earth would measure more than 40,000 kilometres.

We know, too, that from the Earth’s surface to its middle is more than 6000 kilometres-farther than from London to New York. Yet no one has drilled deeper than ten kilometres. Down there the rocks are hotter than boiling water (over 100 °C ). But no one has ever seen the rocks that are deeper than that.

S-Waves and P-Waves

Luckily, scientists have other ways of finding out about the Earth’s interior. They study the effects of earthquakes and man-made explosions. Both send shock waves through the planet. The so-called S-waves only pass through solid substances. P-waves pass through solids or liquids. Both waves move at different speeds through different substances.

Scientists measured how long it took different places on the Earth’s surface. This showed them that the Earth’s inside is made up of several layers.

The Layered Earth

The four main layers of the Earth are the crust, the mantle, the outer core and the inner core. The hard crust- the outer rocky layer is the thinnest layer. The crust is about 40 kilometres thick under the continents but far thinner below the oceans.

The light rocks making up the crust float on the heavier rocks of the mantle. This layer is about 2900 kilometres thick, and very hot. Parts of the upper mantle move like sticky tar. Below the mantle lies the outer core, an iron and nickel shell more than 2200 kilometres thick. Immense heat makes it liquid. Lastly comes the inner core- a ball of metal about 2440 kilometres across, and even hotter than the outer core. The layers pressing on it from above help to keep the inner core solid.

A Giant Magnet

The molten metal of the outer core flows very slowly. It probably takes ten years to flow about 100 kilometres. But scientists think that this is fast enough to make the core act like a dynamo, to produce electricity.

This makes the Earth behave as a giant magnet with its ends at the magnetic north and south poles, near the geographic poles. This is why compass needles point north-south.

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