The Earth's Layers

The Earth's structure comprises three basic layers. The outermost layer, which covers the Earth like a thin skin, is called the crust. Beneath that is a thick layer called the mantle. Occupying the innermost region is the core. Each layer is subdivided into other, more complex, structures.

The crust of the Earth varies in thickness from place to place. The average thickness of the crust under the oceans is 3 miles (5 kilometers), but under the continents the average thickness is 19 miles (31 kilometers). This difference in thickness under the continents and under the oceans is a distinctive characteristic of the crust.

These two parts of the crust differ in other ways. Each has different kinds of rocks. Rocks, such as granite, that are found under the continents are less dense than rocks, such as basalt, that are found mostly in the ocean basins. Each part of the crust also has a different structure. The basaltic type of rock that covers most of the ocean floors also lies under parts of the continents. Scientists believe the lighter rocks of the continental landmasses float on the heavier rocks beneath.

Modern theories about the Earth's structure suggest that this is exactly what is happening. But to understand this theory of floating rocks, called isostasy (a general equilibrium in the Earth's crust), it is necessary to know something about the Earth's next deeper layer, the mantle. Researchers have drilled deep holes, such as those for oil wells, into the crust of the Earth both in the continents and in the ocean floor. But no hole has ever been drilled all the way through the crust into the mantle. All information about the inner Earth has come from measuring seismic waves and the vibrations caused by earthquakes.

The mantle is about 1, 800 miles (2, 900 kilometers) thick and is divided into three regions. The rocky mantle material is quite rigid. But if pressure is applied to it over a long period—perhaps millions of years—it will become slightly more elastic. So, if the distribution of rock in the crust changes gradually, as it does when material eroded off mountains is deposited in the ocean, the mantle will slowly give way to make up for the change in the weight of the rock above it. This is the theory of isostasy. Between the inner mantle and outer core scientists have found at least one so-called “continent”—areas about 200 miles (320 kilometers) wide that are denser than surrounding regions.

The core extends outward from the Earth's center to a radius of about 2, 160 miles (3, 480 kilometers). Obtaining information about the Earth's interior is so difficult that many ideas about its structure remain uncertain. Some evidence indicates that the core is divided into zones. The inner core, which has a radius of about 780 miles (1, 255 kilometers), is quite rigid, but the outer core surrounding it is almost liquid.

There is evidence that the Earth's inner core spins independently of the rest of the planet and at a speed that would allow it to lap the Earth's surface approximately once every 400 years. This knowledge could give scientists a better idea of how the core evolved over time, how the planet's magnetic field came to exist and periodically reverses itself, and how heat moves through the Earth.

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