The physical properties of the Earth's deep interior are unavoidably obscure. Direct human observation is limited to mines in the top 3 km or less, while deep drilling has provided core samples and instrument access to maximum depths of around only 12 km (Kozlovsky, 1987). Deeper physical evidence, down to several hundred kilometres, is obtained from xenoliths and other volcanic products ejected onto the surface from the mid-crust to upper mantle, but this still represents a very small portion of the 6370 km radius of the Earth. The bulk of our knowledge of the Earth's interior composition and physical properties has been deduced by indirect means. Major internal boundaries and structural features (Figure 2.1) have been delineated using seismic methods and supported by density models deduced from gravity readings (e.g. Ringwood, 1969, 1979; Brown and Mussett, 1993).

One conclusion from velocity modelling is that the interior of the Earth is significantly hotter than the surface. The internal heat is derived both from primordial sources related to the formation of the globe and from secondary processes generating heat internally.