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What is the earth made of ?

How do we know what is inside the earth?
Astronauts have traveled about 228,000 miles (380,000 kilometers) above the earth's surface, but no one has ever been able to go very far beneath it.

The deepest mine that humans have ever dug is less than 2.4 miles (4 kilometers) deep, and the longest drill on an oil rig reaches less than 4.8 miles (8 kilometers).
But the center of our planet lies much deeper, about 4,000 miles (6,400 kilometers) below the surface of the earth!

The outer layer of the earth is called the crust. 
If the earth were an apple, the crust would be its skin.
The crust that lies under the oceans is mostly made of a rock called basalt. It is about 5 miles (8 kilometers) thick. The large areas of the earth's crust which are not covered by oceans are mostly made of a rock called granite. These areas are what we call land. They are the continents and islands of the planet.

The continental crust is usually about 19 miles (32 kilometers) thick. In some places, where there are high mountain ranges, it can be more than 25 miles (40 kilometers) thick. Underneath its crust, the earth is made up of three layers of hot rocks and metals. These layers are called the mantle, the outer core and the inner core.

The layer of rock below the crust is the mantle. This mantle is 1,800 miles (2,900 kilometers) thick. At its deepest point, the mantle has a temperature as high as 7950 °F (4,400 °C)—hot enough to melt iron! Some of the rocks here are so hot that they are liquid, or molten. Under the mantle lies the outer core. This is made of molten rocks. The outer core has a temperature ranging from 7950 °F (4,400 °C) to about 11,000 °F (6,100 °C). It is about 1,400 miles (2,250 kilometers) thick.

The center of the earth is called the inner core. It is about 800 miles (1,300 kilometers) thick. Scientists believe that it is ball-shaped and made of two types of metal, iron and nickel. Here, the temperature is very high—about 12,230 °F (7000 °C) —but the metals are solid. This is because of the enormous weight caused by the other layers pressing down on top of them.

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