Privileged Planet: deep plate studies (Introduction)

by David Turell , Sunday, October 29, 2023, 18:59 (181 days ago) @ David Turell

A deeper analysis using garnets:

https://www.sciencedaily.com/releases/2023/10/231027110729.htm

"Examining how plates move in Earth's mantle and how mountains form is no easy feat. Certain rocks that have sunk deep into Earth's interior and then returned from there can deliver answers.

"Geoscientists analyze rocks in mountain belts to reconstruct how they once moved downwards into the depths and then returned to the surface. This history of burial and exhumation sheds light on the mechanisms of plate tectonics and mountain building. Certain rocks that sink far down into Earth's interior together with plates are transformed into different types under the enormous pressure that prevails there. During this UHP metamorphosis (UHP: Ultra High Pressure), silica (SiO2) in the rock, for example, becomes coesite, which is also referred to as the UHP polymorph of SiO2.

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"The research team analyzed whiteschist from the Dora Maira Massif in the Western Alps, Italy. "Whiteschists are rocks that formed as a result of the UHP metamorphosis of a hydrothermally altered granite during the formation of the Alps," explains Duretz. "What is special about them is the large amount of coesite. The coesite crystals in the whiteschist are several hundred micrometers in size, which makes them ideal for our experiments." The piece of whiteschist from the Dora Maira Massif contained pink garnets in a silvery-white matrix composed of quartz and other minerals.

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"According to Duretz, the previous assumption that UHP rock reaches a depth of 120 kilometers seems less probable in view of this rapid decompression because the ascent from such a depth would take place over a long period of time, which does not equate with the high decompression rate, he says. "We rather presume that our whiteschist lay at a depth of only 60 to 80 kilometers," says the geoscientist. And the processes underway in Earth's interior could also be quite different than assumed in the past. That rock units move continuously upwards over great distances, from a depth of 120 kilometers to the surface, also seems less probable than previously thought. "Our hypothesis is that rapid tectonic processes took place instead, which led to minimal vertical plate displacements." We can imagine it like this, he says: The plates suddenly jerked upwards a little bit in Earth's interior -- and as a result the pressure surrounding the UHP rock decreased in a relatively short time."

Comment: We are on a perfect planet for life. Contingencies or serendipity? Or God.