Chapter 5

Continents in depth: the picture in 2000

At the same time as plate tectonics was driving understanding of Earth structure and providing context for a wealth of observations in sedimentary basins and mountain belts, the chemical and mineralogical composition of the oceans and continents was becoming clearer. Continental and oceanic lithosphere were considered to have different thicknesses and different mechanical behaviour. The observed differences were assumed to arise from thermal and compositional variations. In particular, the mantle beneath the continents was thought to be stronger than the lower crust that lay above it, making the lower crust a weak layer sandwiched between stronger rocks.