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Four-Billion-Year-Old Rocks Reveal Clues about Early Earth

May 29, 2014 05:09 PM EDT
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It may just look like any old rock, but a four-billion-year-old chunk of an ancient protocontinent that scientists recently discovered may reveal clues about how Earth's first continents formed.

University of Alberta geochemistry student and lead author Jesse Reimin dug up the primordial rock after three years of sampling, and hopes it can tell him and other more about early Earth's environment.

"The timing and mode of continental crust formation throughout Earth's history is a controversial topic in early Earth sciences," Reimink said in a statement.

Continents form during a process known as subduction, in which one tectonic plate shifts beneath another into Earth's mantle and causes magma to rise to the surface. At least, that's how continents today progress, but scientists are not sure whether plate tectonics existed 2.5 billion to four billion years ago or if another process was at play.

One theory is the first continents formed in the ocean as liquid magma rose from Earth's mantle before cooling and solidifying into a crust. Iceland is believed to have formed this way.

Reimink and others spent their days collecting some of Earth's oldest rocks - between 3.6 and four billion years old - from the Acasta Gneiss Complex in Canada. Unfortunately, rocks this old usually undergo multiple metamorphic events, concealing their geochemistry secrets.

This team was lucky enough to find better-preserved rocks - dubbed "Idiwhaa" meaning "ancient" in the local Tlicho dialect - that unveiled crust-forming processes similar to present-day Iceland.

"This provides the first physical evidence that a setting similar to modern Iceland was present on the early Earth," Reimink said.

These ancient rocks are among the oldest samples of protocontinental crust that we have, he adds, and may have helped jump-start the formation of the rest of the continental crust.

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