EARTH/Crust 403 illustrates the complexity of the problems It is not possible to explain all the isotopic complexities by simple mixing of one enriched and one depleted reservoir, and the answer would seem to be that, even taking into account crustal contamination on ascent, there are a number of different reservoirs, a fact that argues for a heterogeneous mantle Yet the exact petrological, spatial, and physical nature of the source regions remains as elusive as ever See Also Africa: Rift Valley Earth: Crust Earth Structure and Origins Igneous Rocks: Kimberlite Mantle Plumes and Hot Spots Plate Tectonics Solar System: Meteorites Tectonics: Earthquakes Further Reading Agee CE (2000) Mantle and core composition In: Hancock PL and Skinner BJ (eds.) The Oxford Companion to the Earth, pp 654 657 Oxford: Oxford University Press Anderson DL (1989) Theory of the Earth Oxford: Blackwell Scientific Publications Anderson DL (1999) A theory of the Earth In: Craig GV and Hull JH (eds.) James Hutton Present and Future, pp 13 35 Special Publications 150 London: Geological Society Clark SP and Ringwood AE (1964) Density distribution and constitution of the mantle Reviews of Geophysics 2: 35 88 Hamilton WB (2002) The closed upper mantle circulation of plate tectonics In: Stein S and Freymueller JT (eds.) Plate Boundary Zones, pp 359 410 Geodynamics Series 30 American Geophysical Union Holmes A (1965) Physical Geology Edinburgh, London: Nelson Mason B (1966) Principles of Geochemistry New York, London, and Sydney: John Wiley and Sons Price NJ (2001) Major Impacts and Plate Tectonics London and New York: Routledge Ringwood AE (1975) Composition of the Earth New York: McGraw Hill Rubey WW (1955) Development of the hydrosphere and atmosphere with special reference to the probable composition of the early atmosphere In: Poldervaart A (ed.) Crust of the Earth, pp 631 650 Colorado Geo logical Society of America Special paper 62 Part IV Geological Society of America Van Andel TH (1994) New Views on an Old Planet Cambridge: Cambridge University Press Wylie PJ (1967) Ultramafic and Related Rocks New York, London, Sydney: John Wiley and Sons Crust G J H McCall, Cirencester, Gloucester, UK ß 2005, Elsevier Ltd All Rights Reserved Introduction Holmes, in 1965, defined the ‘crust’ or ‘lithosphere’ as the outer shell of the solid Earth, the two terms being synonymous at that time He described it as being composed of a great variety of rocks, with a blanket of loose soil or superficial deposits (e.g., alluvium, desert sands) The term dates back to Descartes (1596–1650), who saw the crust as a shell of heavy rocks which was covered by lighter sands and clays and rested on a metallic interior Leibnitz (1646–1716) believed that the Earth had cooled from an incandescent state and that the crust was the first consolidated rocky part covering a still molten interior However, it became clear from the work of Kelvin (1862) that the Earth’s tidal behaviour precluded a still molten interior beneath a thin solid crust The plate tectonics paradigm appeared in the late 1960s and required a rigid lithosphere extending to ca.100 km (see Plate Tectonics), including part of the upper mantle, above the convecting asthenosphere This redefinition made the terms ‘crust’ and ‘lithosphere’ no longer synonymous The crust is the rocky upper layer of the Earth with a loose superficial cover, and extends to the Mohorovicˇic´ discontinuity (Moho) at 5–35 km below the surface, whereas the lithosphere extends down to 100 km and includes the crust as its upper layer and also the uppermost part of the mantle beneath The crust is thus the top compositional layer of the lithospheric plates, which move about the Earth’s surface, the remainder of the plates being formed of the uppermost, rigid part of the mantle The oceanic crust and continental crust are of different character (Figure 1) The oceanic crust, at its thinnest, only extends down to km minimum, but locally may be thicker, up to 15 km It forms 59% of the total crust by area The continental crust extends down to 30–80 km and forms 79% of the crust by volume Under islands, continental margins, and island arcs, the crust is transitional and is 15–30 km thick