The North American continent is not one thick, rigid slab, but a layer cake of ancient, 3 billion-year-old rock on top of much newer material probably less than 1 billion years old, according to a new study by seismologists at the University of California, Berkeley.The finding, which is reported in the Aug. 26 issue of Nature, explains inconsistencies arising from new seismic techniques being used to explore the interior of the Earth, and illuminates the mystery of how the Earth’s continents formed.

This graphic shows the thickness (in kilometers) of the North American lithosphere. The blue area is about 250 km thick and, based on new findings reported in Nature, is composed of a 3-billion-year old craton underlain by younger lithosphere deposited as ocean floor subducted under the continent within the past billion years. The green, yellow and red areas are younger and thinner continental lithosphere added around the margins of the original craton, also by subducting sea floor. The thick broken line indicates the borders of the stable part of the continent. (Credit: Barbara Romanowicz and Huaiyu Yuan, UC Berkeley)

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