Читать книгу Geochemistry - William M. White - Страница 24

The Earth consists of three principal layers: the core, the mantle, and the crust (Figure 1.12). The core, roughly 3400 km thick and extending about halfway to the surface, consists of Fe–Ni alloy and can be subdivided into an inner and outer core. The outer core is liquid while the inner core is solid. The mantle is nearly 3000 km thick and accounts for about two-thirds of the mass of the Earth, with the core accounting for the other third. The crust is quite thin by comparison, nowhere thicker than 100 km and usually much thinner. Its mass is only about 0.5% of the mass of the Earth. There are two fundamental kinds of crust: oceanic and continental. Ocean crust is thin (about 6 km) and, with the exception of the eastern Mediterranean, is nowhere older than about 180 million years. The continental crust is thicker (about 35–40 km thick on average) and relatively permanent, with an average age of ∼2 billion years.

Both the crust and the mantle consist principally of silicates. The mantle is comparatively rich in iron and magnesium, so ferromagnesian silicates, such as olivine and pyroxenes, dominate. Rocks having these compositional characteristics are called ultramafic. The continental crust is poor in iron and magnesium, and aluminosilicates such as feldspars dominate. Rocks of this composition are sometimes referred to as felsic (or silicic). The oceanic crust is intermediate in composition between the mantle and continental crust and has a mafic composition, consisting of a roughly 50:50 mix of ferromagnesian minerals and feldspar. These differences in composition lead to differences in density, which are ultimately responsible for the layering of the Earth: the density of the layers decreases outward. The continental crust is the least of dense of these layers. The fundamental reason why continents stick out above the oceans is that continental crust is less dense than oceanic crust. Not shown in the figure are two additional layers that are even less dense: the oceans or hydrosphere, which cover about two-thirds of the surface, and the atmosphere, which extends about 100 km above the surface.

Figure 1.12 The Earth in cross-section. The outer rocky part of the planet, the mantle and crust, consists principally of silicates and is 2885 km thick. The core, divided into a liquid outer core and a solid inner core, consists of iron–nickel alloy and is 3486 km thick.