ABSTRACT
The word crust originally arose from Kelvin’s idea of a solidified layer on a molten earth but is now understood to be a compositionally distinct veneer on the outer solid silicate part of the Earth (the mantle), from which it has been derived by volcanism. Two distinct crustal types are recognised, continental and oceanic, with average properties summarised in Table 8.1, although there are regional variations, summarised in Table 8.2, that depart strongly from the averages and, in some cases, appear as compromises between the two types. Both have depth variations approximated as layers, numbered 1 to 3 or 4 from the surface, and both are distinct, in composition and properties, from the underlying mantle.
