424 EARTH STRUCTURE AND ORIGINS Table Volumes, masses, and densities of the various concentric shells of the Earth (from C M R Fowler (2000) Earth Structure In: Hancock PL and Skinner BJ (eds.) Oxford Companion to the Earth, pp 276 280 Oxford: Oxford University Press) Depth (km) Crust Upper mantle Lower mantle Outer core Inner core Whole Earth à Moho 670 2891 5150 Mohoà 670 2891 5150 6371 Volume (1018 m3) Volume (% of total) Mass (10 21 kg) Mass (% of total) Density (10 kg m 3) 10 297 600 169 1083 0.9 27.4 55.4 15.6 0.7 100 28 1064 2940 1841 102 5975 0.5 17.8 49.2 30.8 1.7 100 2.60 3.35 4.38 9.90 12.76 2.90 3.99 5.56 12.16 13.8 The Moho is 25 35 km deep under the continents and km deep under the oceans the north magnetic pole, first located by James Clark Ross in 1831 in the Boothia Peninsula, north-east Canada, shows secular displacement and is at present accelerating northwards towards Siberia (Figures 6A and 6B) The magnetic field is believed to be due to convection in the liquid outer core (Figure 7) (see Magnetostratigraphy) Ozone Layer There is a layer of ozone (O3), a blue-green poisonous gas, in the stratosphere at an altitude of between 15 and 40 km (Figure 8) This is important because it absorbs the carcinogenic part of the solar spectrum (ultraviolet B) It is produced by photochemical reactions, and its concentration displays natural variations, being more abundant at the poles and less so in equatorial regions, though it is created at the equator and destroyed at the poles, where the concentration shows seasonal variation Chlorofluorocarbon compounds, used as refrigerants and aerosol sprays, are believed to threaten the ozone layer; stratospheric jet planes would also so, but relatively few are in use Plate Tectonic Movement and Mantle Convection Figure Magnetic polarity divisions of geological time (chrons, subchrons, transitional zones, and excursions) (reproduced from Harland WB, Armstrong RL, Cox AV, et al (1990) A Geologic Timescale 1989 Cambridge: Cambridge University Press) switching positions (Figure 5) Such reversals may occur over periods of a few hundred thousand years, but there have been longer periods without reversal of around 10 Ma The intensity of the dipole field varies, being particularly strong during long stable periods and weakening just before reversal The position of Space exploration has revealed no other body (planet, satellite, or asteroid) with evidence of lateral plate-tectonic displacement attributable to mantle convection, with accompanying ridge eruptivity, spreading away from ridges, subduction at plate margins, and continental collision (Figure 9) The upper mantle, though solid, is capable of very slow thermal convection over the vast periods of geological time Mantle convection and plate tectonics probably did not operate in the Archaean and earliest Proterozoic (before 2000 Ma), though some minor shuffling of numerous small plates may have occurred There is evidence that it operated through most of the Proterozoic and Phanerozoic, and the Earth’s surface at present consists of relatively few