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Earth’s Layers

Have you ever dug a hole in your backyard, or in the playground? How deep were you able to dig? One foot? Two feet? Or perhaps you really worked hard and were able to dig a hole that was three feet deep.

What would you need if you wanted to dig a really deep hole? You could use a shovel, but eventually it would become too difficult. As the hole became deeper, you would need a way to remove the dirt as you loosened it, otherwise you would not be able to throw the dirt out of the hole. As your hole became deeper, you would quickly dig through the soft regolith, or dirt, and begin hitting your shovel against hard bedrock. In order to continue digging, you would need a jack hammer or a drill to break through.Suppose that you had all of this equipment available to you, how deep could you dig? The deepest mines on Earth are only about 2.5 miles (4km) deep. Some geologists have used massive drilling equipment to take narrow core samples from as deep as 7 miles (11km) beneath the surface of the Earth.

This seems like a very deep hole, but when compared to the overall radius of the Earth, it is just scratching the surface. The Earth is approximately 4,000 miles (6,400 kilometers) from surface to center.

If scientists have never studied any materials from a depth below 7 miles (11km), then how is it that we know what is in the center of the Earth? How can we know what the core of the Earth is made of, if we have never seen it?

The answer is actually quite simple. While it is true that we cannot study the Earth’s core using visible light, we can study it using other senses. The most important thing we use to sense the Earth’s core are seismic waves. Seismic waves are waves of energy caused either by earthquakes or by massive man-made explosions.

Seismic waves help scientists study the Earth's interior.

Scientists are able to measure these waves as they pass through the Earth. As these waves encounter different materials, they change in important ways, becoming longer, shorter, faster, or slower. Geologists study these changes in the waves, and are able to draw conclusions about what the core of the Earth must look like.

Geologists also can learn a lot about the core of our planet by looking at Earth’s magnetic field. The Magnetic field is created by massive circulations of hot liquid mantle beneath the Earth’s surface.

The Earth's magnetic field

These clues lead geologists to believe that the Earth is made of four distinct layers. These layers are the crust, the mantle, the outer core, and the inner core.