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The interior

More than 90 percent of the Earth's mass is composed of iron, oxygen, silicon, and magnesium, elements that can form the crystalline minerals known as silicates. However, in terms of chemical and mineralogical composition, as in physical properties, the Earth is far from homogeneous. Apart from the superficial lateral heterogeneities near the surface (i.e., in the compositions of the continents and ocean basins), the Earth's principal differences vary with distance toward the centre owing to increasing temperatures and pressures and due to the original segregation of materials into a metal-rich core, a silicate-rich mantle, and the more buoyant, highly refined crustal rocks. The Earth is geochemically differentiated to a great extent. Crustal rocks contain about twice as much of the rock-forming element aluminum as does the rest of the solid Earth and nearly 50 times as much uranium. On the other hand, the crust, which accounts for a mere 0.4 percent of the Earth's mass, contains less than 0.1 percent of its average abundance of iron. Virtually all the iron is concentrated in the Earth's core.

The increasing pressure with depth causes phase changes in crustal rocks at depths of roughly 60 kilometres, marking the boundary of the upper mantle. This transition area, called the Mohoroviić discontinuity, is prominently revealed by seismic wave analysis. It is believed that most basaltic magmas are generated near the base of the upper mantle at a depth of about 400 kilometres. The upper mantle, which is rich in the greenish mineral olivine, shows significant lateral inhomogeneities. Nearly 50 percent of the body of the Earth, down to a depth of 2,890 kilometres, consists of the lower mantle, which is composed chiefly of magnesium- and iron-bearing silicates, including high-pressure phases of olivine and pyroxene. The mantle is not static but rather slowly convects. One important feature is the production of temporary superplumes (huge, rising jets of hot, partially molten rock), which may originate as deep as the heterogeneous Dð layer near the core-mantle interface. Much larger than ordinary thermal plumes, such as that associated with the Hawaiian Island chain in the central Pacific, superplumes may have had profound effects on the Earth's geologic history and even on its climate. One outburst of global volcanism that began about 125 million years ago and lasted through most of the Cretaceous Period may have been associated with melting at the tops of one or more giant plumes that rose in the mantle beneath the Pacific and Indian plates.

About one-third of the Earth's mass is contained in its core, most of which is liquid iron alloyed with some lighter, cosmically abundant components (e.g., sulfur or oxygen). Its liquid nature is revealed by the failure of shear-type seismic waves to penetrate the core. However, a small central part of the core, below 5,150-kilometre depth, is solid. Temperatures in the core are extremely hot, ranging from 4,000 to 5,000 K at the outer part of the core to 5,500 to 7,500 K in the Earth's centre, probably hotter than the surface of the Sun. (Uncertainties in temperature arise from questions as to which compounds form alloys with iron.) The core's reservoir of heat may contribute as much as one-fifth of all the internal heat that ultimately flows to the surface of the Earth.

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