Planetary Crust Acts Like A Yo-Yo

Italian Western Alps | Courtesy of Daniella Rubatto

New research out of The Australian National University has recorded for the first time what was once thought to be only theoretical; yo-yo subduction in the Earth’s crust.

“Understanding subduction zones is critical as they are the areas where plates meet and merge, where volcanos form, where lava is generated and where earthquakes happen,” said Dr Daniela Rubatto from the ANU Research School of Earth Sciences, who along with co-workers from ANU and colleagues at the Uniersity of Bern and University of Torino, collaborated on the research. “This discovery also has the potential to change the way we look at plate collision zones and mountain building.”

Previous thought had believed that subduction only took place once.

“The typical idea is that the convergence of plates creates a force that pushes rocks down from the surface of the Earth making the whole system move down like a conveyor belt. Most of the material will sink forever in the planet’s mantle and only a fraction will make it back to the surface.”

“However we have now determined that the Earth’s crust is pushed down from the planet’s surface to a depth of 60 to 100 km, comes back towards the Earth’s surface, is pushed down again and then finally ends up back on the surface where it becomes part of mountain belts.”

“Our findings suggest that within a subduction zone individual rock units move independently, faster and in a more complex way: they go down and up and down again and up again, like a yo-yo.”

“On top of this we can prove that all this happens in a relatively short geological time of less than 20 million years. To put this time into context a mountain belt normally takes 50-100 million years to form and the rocks around Canberra have been moving up or down only a few kilometres in the last 400 million years.”

“We also often observe consequences of deep movements in the Earth like earthquakes, tsunamis and so on. Now we can track these movements in detail, in time and space. Our new methodology of investigating rocks is like being finally able to read a flight recorder. So far we have known where the plane started, that it had a stop over, and finally landed at its destination. Now we can read the actual route of the plane, how fast it went, and how long it took – and find out that it went to a destination twice.”

The new research is a breakthrough, and according to Dr Rubatto, it is the result of a combination of selecting the right sample of rock, the ability to make accurate chemical measurements of tiny minerals with the ANU-built SHRIMP ion microprobe, and some luck.

Source: The Australian National University

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