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paths of earthquake waves

See how volcanic forces continue to reshape the Earth’s crust.

The crust Earthquake waves can suddenly change direction at certain depths underground. This tells geologists that the waves are entering layers of different materials. Using this information, they have discovered that the Earth is made up of four main layers— the crust , mantle , outer core , and inner core. The crust makes up the Earth’s hard outer layer, or skin, and is very thin compared with the other layers. Under the oceans the crust is only about 4.3 miles (7 km) thick. Under the land areas, or continents, it is much thicker—up to 25 miles (40 km).

∆ Earthquake waves bend when they pass between the layers of the Earth. Study of these waves is called seismology.

The mantle and core Under the crust lies a very deep layer called the mantle, made up of heavier rock than the crust. In the upper part of the mantle the rock is partly molten and moves slowly, carrying the crust with it (see page 10). Underneath the mantle

is the metal core, made up mainly of iron and nickel. The inner part of the core is solid, but the outer part seems to be liquid. This probably explains why the Earth is magnetic. Scientists know that moving metals (the Earth is spinning) set up electric currents, and that these produce magnetism. Molten rock from the mantle sometimes forces its way to the surface. ∆

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