Earth's Core and Structure of the Crust

The radius of the earth at the equator is 6370 km and at the poles it is shorter by about 22 km; thus the earth is not quite a perfect sphere. It has a surface area of 510 x 106 km².

The earth's solid core is very dense material and rich in iron and nickel. This is inferred from the earth's bulk density, the magnetic field, and comparison with metallic meteorites. The outer part of the core, dimensions and liquid character are established from seismology: the interaction between the spinning solid mantle and liquid outer core is thought to drive the magnetic field. The core's average density is 10.7 gms/cm³ and its radius about 3470 km.

The earth's mantle, some 2300 km thick, lies between the core and the Moho layer (Mohorovicic discontinuity), and is chiefly composed of ultramafic peridotite and its high pressure cousin, eclogite, i.e. it is similar to 'garnet'. These rocks are rich in magnesium and iron and their bulk composition approximates to that of stony meteorites. A low seismic velocity zone of low rigidity defines the base of the lithosphere, the upper part of the mantle, which is 60 to 250 km thick. The asthenosphere is the weak zone within the upper mantle under the lithosphere where the mantle rocks deform by plastic flow in response to applied stresses of ca.100 MPa. The whole mantle forms some 84% of the volume of the earth, or 68% of its mass: its average density is about 4.5 gms/cm³.

The lithosphere has resulted from complex, poorly understood processes of differentiation from the upper part of the mantle, and is driven by thermal energy resulting in large part from radioactive decay. The relatively thin earth's crust 'floats' on the lithosphere; it comprises the thinner oceanic crust and the thicker continental crust and accounts for 1.4% of the solid earth. The surface of the crust is the Earth's surface. Parts of the crust are displaced relative to each other by what are thought to be convection currents in the upper mantle, resulting in 'plate tectonic' movements which slowly destroy and rebuild the earth's surface.

The distribution of the topographic level of the earth's surface shows that there are two dominant levels corresponding to the continents, with average height of about 1 km, and the ocean basins, with an average depth of about 4 km. The current proportion of total area occupied by the extremes of height and depth (mountain ranges and ocean trenches) is quite small. Elevations greater than about 3 km make up only some 1.6% of the total area of the earth's crust and depressions deeper than 5 km only 1%.

The ability of rocks to flow at depth means that material within the upper part of the mantle can be transferred so as to maintain isostatic equilibrium and allow each part of the crust to sink or rise to the appropriate level. The difference of over 4 km of mean elevation between the continents and oceans is therefore explained by the buoyancy of the thicker continental crust (Park, 1983).

The continents cover only 29% of the earth's surface, distributed in a rather uneven way with some 65% of the total land area being in the Northern Hemisphere. If the continental shelf and slope area are added to the land area of the continents, the total continental surface area is 40% compared with that of 60% for the ocean basins. This reflects the relative proportion of continental crust to oceanic crust.

The existence of such a large difference is explained primarily by the difference in thickness between the continental and oceanic crust. The principle of gravitational balance (isostasy) means that topographically higher sections must contain a greater proportion of lower density material to keep the total weight the same. At the base of the crust, the Moho marks a very significant change in composition and density of the rocks. The mean density of the crustal rocks is about 2.8 to 2.9, whereas the peridotic rocks of the upper mantle have a mean density of around 3.4. Continental parts of the crust have a average thickness of about 33 km and a mean composition close to that of granite (acid) whereas the ocean basin parts have an average thickness of only some 7 km and are formed mainly of rocks of gabbroic or basaltic (basic) composition.

The atmosphere and sea water must have also formed by global scale differentiation and their present compositions have resulted from various processes, including loss of gas consisting of the lightest elements from the interior of the earth and photochemical dissociation of early formed compounds. The ocean-atmosphere system provides important regulatory functions and feed-back interactions between the atmosphere, sea water and crust, which together with biological systems have maintained a near steady state chemical and thermal equilibrium through much of geological time (Dott and Batten, 1988).