What is the Earth made of? What is beneath its crust?

The Earth is made up of layers of rocky and metallic materials. Beneath the layers of lighter rock near the surface is heavy rock; and beneath the heavy rock there is rock containing metal as you go toward the center. The chief metals are iron and nickel. While we can not see into the interior of the Earth, we do have some ways of finding out about it. One way is by observing the effect of the Earth’s gravity upon the other planets, and then calculating what the Earth must weigh. When we do that, we find that it is far heavier than its surface rock would indicate. Therefore the Earth must have a very heavy inner core. This iron-nickel inner core is believed to be about 1,200 kilometers in radius.

The reason scientists think that iron and nickel make up most of this core is that many of the meteorites that fell on Earth are made chiefly of these two metals; and meteorites are believed to be fragments of planet-material. All planets contain the same materials in different proportions. Since we know that our lithosphere (upper rock layers that extend down to a depth of about 200 kilometers) does not contain high proportion of iron and nickel found in meteorites, we assume that these materials must be packed closely in the Earth’s inner core.

More reading:
Structure of the Earth (Wikipedia)

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