Tectonic congestion on the road from subductionPlate tectonics shape our planet. They make it habita

Tectonic congestion on the road from subductionPlate tectonics shape our planet. They make it habitable. Mountains, oceans, atmosphere and pastureland, all are shaped by the relentless and restless, yet barely discernable, recycling of the solid Earth. Ocean floor is created at mid-ocean ridges, but later, like a giant conveyor belt, may be subducted beneath the continents at the ocean margin. Subducted and drawn into Earth’s mantle, rock and all it contains (including water … the subject of a future post, maybe) sinks to the depths. Old ocean floor, progressively denser than the mantle it is sinking into, falls towards Earth’s core under the pull of gravity.Over our lifetimes such movements are almost invisible. The tectonic plates move at the same sorts of speed as our fingernails grow. But the movement is jerky on human timescales. When we see it, we see it as earthquakes (like the Tohoku event - subduction of the Pacific into the Japanese trench) or as spasms of volcanicity. However, over the deep time of Earth processes the movement is continuous, the mountains (and mantle) flow, and when viewed on millennia or more they move across the face of the globe and into the depths of the planet in a continuous cycle of destruction and rebirth.Geophysicists can use the seismic waves from earthquakes to peer into Earth’s depths. Just as a doctor might use X-rays in CT scans of our bones and internal organs, so seismologists use tomography to build 3-D pictures of Earth’s interior. When they do, the pathways of cold dense subducted oceanic crust can be seen. The road to subduction leads to the depths, but seems to stutter on its downward plummet as it passes through the “transition zone”, a region between 410 and 660 km beneath our feet. Cold slabs of oceanic lithosphere stagnate above 660 km, and rest awhile.In a paper published in the latest issue of Nature Geoscience, Willem van Mierlo, Dan Frost and Dave Rubie of the Bayerisches Geoinstitut, Bayreuth, Germany, together with Falko Langenhorst at Jena, provide an explanation for the slab stagnation. Gravity, and hence density, drives the sinking slab down. With increasing depth the minerals in the slab transform to increasingly denser crystal structures. The atomic make-up of the minerals and rocks changes, to progressively denser minerals, like garnet, at depth. It is the same sort of process as we see when carbon, as graphite transforms to diamond in the depths of the Earth. But these transformations depend on re-arranging the atoms in the minerals, and this takes time.The Bayreuth results show that diffusion times, at the conditions of key minerals like pyroxenes in the transition zone, can be immense … hundreds of millions of years for pyroxene to dissolve into garnet. The slowness of atomic diffusion at depth is the reason that the slab’s descent slows. The persistent pyroxene adds to the slab’s buoyancy. Slowing the descent is like making a slab “traffic jam” at 660 km: congestion, like bunching of traffic on an over-busy highway.After time, any pyroxene that has not turned to garnet changes instead to another denser mineral, akimotoite. The brake is released and the slab continues on its way, possibly to the very base of Earth’s mantle almost 3000 km down, where, it is postulated, it may end its journey in a “slab graveyard”.Maybe next time you are caught in traffic on the highway you will feel even closer to Earth?~SATRImage: Left: Seismic tomography of remnants of Farallon slab, which has been subducted beneath the North American plate (NAM). Right: Plate tectonic interpretation of the history of the Farallon plate (FAR), linked originally to the Pacific plate (PAC). (Image from Fukao, link below)Links:Stagnant Slab: A Review by Yoshio Fukao, Masayuki Obayashi, Tomoeki Nakakuki, and the Deep Slab Project Group, Annu. Rev. Earth Planet. Sci. 2009. 37:19–46. http://www.annualreviews.org/doi/pdf/10.1146/annurev.earth.36.031207.124224Geodynamic models of deep subduction. Ulrich Christensen Physics of the Earth and Planetary Interiors 127 (2001) 25–34. http://lnu.edu.ua/faculty/geology/phis_geo/fourman/library-Earth/Geodynamic%20models%20of%20deep%20subduction.pdf -- source link

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