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Seismology – the study of earthquake waves – provides a window into the planet's interior. Careful measurement of the way these waves are bent inside the Earth and the time they take to travel through the planet (and, hence, their speed) has enabled geophysicists to determine the density and physical properties of the interior. The data show that the interior is rather like an onion, with a number of concentric layers.

At the surface is a thin, rocky crust which is 20–60 km deep beneath the continents and 8–10 km deep on the ocean floor. The continental crust is less dense (average 2.7 g/cm³) than the oceanic crust (average 3.0 g/cm³). Whereas the continents have a very complex structure and variable composition, the oceanic crust has a simple, layered structure and a uniform composition of basaltic lava. The continental crust is generally much older, with parts of Canada and Australia dating back more than 3,500 million years. In contrast, the oceanic crust is geologically young, with a maximum age of about 200 million years.

At the base of the crust is the so‐called Mohorovičić discontinuity (usually shortened to “Moho”). This boundary marks a sudden change to the mantle, which is mostly solid and composed of dense rock rich in the mineral olivine – a silicate containing magnesium and iron.

Geophysicists refer to the crust and the solid portion of the upper mantle as the lithosphere. The lithosphere is divided into many separate slabs, known as plates, that move independently. These lie on top of a mainly solid, “plastic” layer known as the asthenosphere, which extends to a depth of about 250 km.

Figure 3.19 Earth's interior is composed of four main layers. At the center is a solid, iron‐rich core, surrounded by a liquid outer core, which is the source of Earth's magnetic field. Above this is the mantle, where giant convection cells are thought to exist. The low density, solid crust “floats” on top of the “plastic” upper mantle.

(Lawrence Livermore National Laboratory)

The temperature of the rocky materials that make up the asthenosphere tends to be just below their melting point. This gives them a plastic‐like quality comparable to glass. An increase in the temperature or pressure causes the material to deform and flow. If the pressure on the material is greatly reduced, so is its melting point, and the material may begin to melt quickly, providing a supply of magma for volcanoes.

The solid silicate mantle, which makes up 84% of Earth's total volume, extends to a depth of 2,900 km, where it meets the even denser, metallic core. Large convective cells in the mantle circulate heat and are probably partly responsible for the motions of the crustal plates.

The outer core, which is about 2,300 km thick, seems to be composed mainly of an iron–nickel alloy with trace amounts of lighter elements, such as sulfur. Swirls and eddies in the liquid outer core are responsible for generating Earth's magnetic field. The solid inner core, which is composed of iron with some nickel, makes up the final 1,200 km to the planet's center.