Earth’s geological structure and processes - Fundamentals of the Physical Environment - page 274

assessment of Earth-crossing objects; until we do, solutions and mitigation will remain in
the realm of science fiction.
EARTH STRUCTURE AND INTERNAL ENERGY
CORE, MANTLE, CRUST, OCEAN, ATMOSPHERE AND
BIOSPHERE
Plate tectonics perpetuates the geological distillation and fractionation of planetary raw
materials which began as the planets condensed from interstellar gases and led to the
formation of Earth's six concentric geospheres (Figure 10.3). The innermost core formed
by the separation of a nickel-iron mixture from lighter silicon-rich material and generates
Earth's magnetic field. Its mean density of 10·7 gm cm −3 rises to almost 14 gm cm −3 at
Earth's centre, from which the core extends 3,460 km, concentrating 32·2 per cent of rock
mass in just 16·9 per cent of planetary volume. Seismic evidence described later (box, p.
197) indicates that the inner core is solid for 1,300 km, with a liquid outer core. Density
falls sharply at its boundary with the mantle , which extends for a further 2,970 km. The
mantle has a mean density of 4·5 gm cm −3 and is composed of minerals transitional
between the iron of the core and lighter oxides of silicon and aluminium,
Figure 10.3 Earth's internal structure and the relation
between outer spheres and crustal processes.