Plate tectonics started at least 3.5 billion years ago

Yosemite National Park’s Half Dome

ROCK OF AGES  Plate tectonics may have started as early as 3.5 billion years ago on Earth. That recycling of Earth’s surface leads to the separation of light-colored continental crust (exposed here at Yosemite National Park’s Half Dome) from darker, denser oceanic crust.  

© Basil Greber

Most estimates put the onset of plate tectonics at around 3 billion years ago. But a new study in the Sept. 22 Science asserts that the large plates that make up Earth’s outer crust began shifting 500 million years earlier.

Nicolas Greber of the University of Geneva and colleagues say previous studies got it wrong because scientists looked at silicon dioxide in shales, which bear the detritus of continental rocks. These rocks’ silicon dioxide composition hints at when continental rocks began diverging in makeup from oceanic rocks due to plate tectonics.

But weathering wreaks havoc on the silicon dioxide in shale. So, Greber’s team turned to titanium. The proportion of titanium isotopes in shale is a useful stand-in for silicon dioxide concentrations because titanium isn’t as easily altered by weathering. Those data helped the team estimate that plate tectonics was already going strong by 3.5 billion years ago.

Carolyn Gramling is the earth & climate writer. She has bachelor’s degrees in geology and European history and a Ph.D. in marine geochemistry from MIT and the Woods Hole Oceanographic Institution.

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