Most of what we know about the structure of the earth has come about as a result of what were initially relatively simple seismic studies, the study of earthquake waves as they pass through the earth.

The earth's crust is a thin skin which covers the whole surface but forms only 0.2% of the total mass of the earth. Oceanic crust is very thin (less than 10km thick) and dense and is composed largely of basic igneous rocks covered by a thin layer of deep-sea sediments. The crust of the continents is thicker (20 to 90 km, average about 35 km) and composed of a complicated mixture of igneous, metamorphic and sedimentary rocks.

The boundary between the crust and the underlying mantle is known as the Mohorovicic Discontinuity, or "Moho" for short. Mohorovicic discovered in 1909 that seismic velocities undergo a sharp increase in this area, where they pass through the crust and hit the mantle. This is due to the differing chemical composition of the two zones. The mantle is primarily composed of peridotite, an ultrabasic igneous rock rich i n iron and magnesium. It is generally divided into three zones, becoming increasingly dense at the lower depths.

The earth's core has a radius of 3500 km and it forms 33% of the mass of the planet. Seismic evidence indicates that while the inner core is solid, the outer core is liquid and likely convects from heat supplied to it from the inner core. Iron is the main constituent of the earth's core. Evidence from meteorites suggests that the inner solid core consists of a nickel-iron alloy.

Additional information can be obtained from the Univ. of Arizona's Nine Planets Page on the Earth

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