BERKELEY, Calif., Aug. 26 (UPI) — The North American continent is not one single, solid slab, researchers say, but rather a layer cake of old and new material dating back 3 billion years.
Seismologists at the University of California, Berkeley, say a layer of 3-billion-year-old rock sits on top of much newer material less than 1 billion years old, ScienceDaily.com reported Thursday.
“This is exciting because it is still a mystery how continents grow,” study co-author Barbara Romanowicz, UC Berkeley professor of earth and planetary science, said.
“We think that most of the North American continent was constructed in the Archean (eon) in several episodes, perhaps as long ago as 3 billion years, though now, with the present regime of plate tectonics, not much new continent is being formed.”
Continents started forming some 3 billion years ago when the planet was much hotter and convection in the mantle more vigorous, Romanowicz said.
Continental rocks floated to the surface and eventually formed the lithosphere, Earth’s hard outer layer.
These pieces of the lithosphere, called cratons, stopped growing about 2 billion years ago as the planet cooled, though the modern era of plate tectonics has added new margins to the original cratons, slowly expanding the continents, Romanowicz said.
The original North American craton is located mostly in the Canadian part of North America with plate tectonic movement adding the rest over time.
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