Man is now preparing to explore the moon and other planets and they still don’t know exactly what his own earth is made of! A sort of rough answer to this question would be the earth is a big ball, or sphere, made mostly of rock.
Inside the earth, the rock is melted, but the outside cover is hard rock. Less than one-third of the earth’s surface island and more than two-thirds are water. Now let’s consider this in a little more detail.
The outside of the earth is a crust of rock about 10 to 30 miles thick. This crust is sometimes called the lithosphere. The high parts of this crust are the continents, and the low parts of it hold the waters of the oceans and the great inland seas and lakes.
All the water on the surface, including the oceans, lakes, rivers, and all the smaller streams, is called the hydrosphere. Men have been able to examine only the outermost part of the crust of rock that forms the outside of the earth, which is why it’s so hard to know what the earth is like on the inside.
In drilling wells and digging mines, it has been found that the deeper the hole is made, the higher the temperature becomes. At two miles below the surface of the earth, the temperature is high enough to boil water.
But scientists have also been able to find out about the inside of the earth from studies of earthquakes. They believe that the temperature does not increase as rapidly deep down as it does in the crust.
So they think that at the core or center of the earth the temperature may not be more than 10.000 degrees Fahrenheit. Of course, that’s plenty hot-since a temperature of 2.200 degrees would melt rocks!
The crust of the earth has two layers. The upper layer, which makes the continents, is of granite. Under the layer of granite is a thick layer of very hard rock called basalt.
Scientists believe that at the center of the earth is a huge ball of molten iron, with a diameter of about 4.000 miles. Between the central ball and the rocky crust is a shell about 2.000 miles thick called the mantle. The mantle is probably made of a kind of rock called olivine.
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