Monday 24 November, 2014

Revealed: How earth got tectonic plates

Published On: Mon, Apr 7th, 2014 | Geology | By BioNews

In what could be a probable answer to an enduring puzzle about how earth’s tectonic plates were created, a thrilling research indicates that these tectonic plates may have taken as long as one billion years to form.

Geophysicists have discovered that earth’s outermost layer, or lithosphere, was weakened by movement in viscous layers below it – a process when one plate dives below another.

Starting roughly four billion years ago, cooler parts of earth’s crust were pulled downwards into the warmer upper mantle, damaging and weakening the surrounding crust.

The process happened again and again until the weak areas formed plate boundaries.

“Over a much longer period, the same process could have created many tectonic plates. We have got a physical mechanism to explain how it could have happened,” study author David Bercovici from Yale University explained.

The process began about four billion years ago and caused complete fractures some three billion years ago.

To investigate how the plates formed, Bercovici and Yanick Ricard of University of Lyon in France developed a computer model of earth’s crust as it may have existed billions of years ago.

The model included a low-pressure zone at the base of the crust which caused a piece of the crust to sink into the upper mantle – mimicking conditions thought to have occurred early in the earth’s history.

As the process repeated over time, it created a large tectonic plate with an active subduction zone.

Prior studies suggested the age of the plates – based on evidence of subduction gathered from minerals – preserved in ancient rocks.

In geology, subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate and sinks into the mantle as the plates converge.

The oldest such specimens are four-billion-year-old zircons found in the Jack Hills of Australia that appear to have formed at temperatures and pressures that are indicative of subduction.

“The subsequent movement of the plates has erased much of the evidence of their origin,” said Paul Tackley, a geophysicist at Swiss Federal Institute of Technology (ETH) in Zurich, Switzerland in a report published in Nature.

According to Robert Stern, a geologist at University of Texas in Dallas, there is no firm evidence of plate tectonics earlier than one billion years ago, but the new mechanism behind plate formation is “the first interesting example of how it might have occurred”.

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