Evolution of the continental crust
19 October 2006
The continental crust covers nearly a third of the Earth’s surface but when did it first form and how long did that take?
The Earth’s continental crust differs from the crust of other planets in our Solar System. It supports life, its formation modified the composition of the mantle and the atmosphere, and it remains a sink for CO2 through weathering and erosion. The continental crust has therefore had a key role in the evolution of this planet, and yet when and how it formed remain the topic of considerable debate.
The continental crust covers nearly a third of the Earth’s surface. It is buoyant – being less dense than the crust under the surrounding oceans – and is compositionally evolved, dominating the Earth’s budget for those elements that preferentially partition into silicate liquid during mantle melting.
Models for evolution of the crust can provide insights into how and when it was formed, and can be used to show that the composition of the material from which the continental crust is derived is similar to that of the average lower crust. From the late Archaean to late Proterozoic eras (some 3 – 1 billion years ago), much of the continental crust appears to have been generated in pulses of relatively rapid growth.
Reconciling the sedimentary and igneous records for crustal evolution indicates that it may take up to one billion years for new crust to dominate the sedimentary record. Combining models for the differentiation of the crust and the residence time of elements in the upper crust indicates that the average rate of crust formation is some 2 – 3 times higher than most previous estimates.