The origin and growth of continents

Abstract

Abstract Geophysical evidence for continental origin and growth may be summarized as follows: 1. (1) The overall mass of the crust and mantle to a depth of about 500 km is the same on a broad scale beneath oceans and continents. 2. (2) The heat flow in shield areas is less than in orogenic areas and the mantle under shield areas is cooler than under the oceans. 3. (3) The average continental crust is about 40 km thick. Shield areas are 40–45 km thick and orogenic areas have thinner crust, typically about 35 km thick, but much thinner (20 km) in areas such as the Pacific coastal region of California. Geological evidence indicates that: 1. (1) mountainous areas are destroyed by erosion in a few million years; 2. (2) shield areas remain stable for long periods; 3. (3) the continents have grown throughout geological time and are now more extensive than previously; (4) calc-alkaline volcanic rocks, in particular andesites, are characteristic of orogenic areas. Geochemical evidence shows that: 1. (1) the continental crust has an overall composition (for major elements) equivalent to an intermediate rock with a silica content about 60%; 2. (2) the sedimentary rare-earth pattern is typical for the crust exposed to weathering, and may be typical of the whole crust; 3. (3) the Sr isotope data indicate that the amount of Rb available is sufficient only for about 20 km of granodiorite, so that the lower crust must contain much lower concentrations of Rb; (4) there has been a strong vertical concentration of the heat producing elements in the earth. A simplified model for continental origin and growth is proposed, with the following stages: 1. (1) An initial geochemical fractionation leads to the concentration of the lithophile elements in the upper 1,000 km. 2. (2) Lateral variations in the distribution of the heat producing elements in the uppermost mantle localise the sites of continental nuclei. 3. (3) Continental areas grow mainly by the addition of andesites and associated calc-alkaline rocks in orogenic areas. Granodiorites and granites are mainly formed by partial melting, leaving a lower anorthositic crust. The overall composition of the continental crust is close to that of calc-alkaline or orogenic andesites. 4. (4) Depletion of the subcontinental mantle in heat-producing elements terminates the process. 5. (5) Erosion of shield areas removes K, U and Th and reduces the heat flow to levels below those of present orogenic areas.