Mar. 31, 2013 ? Earth is dynamic. What we perceive as solid ground beneath our feet, is in reality constantly changing. In the space of a year Africa and America are drifting apart at the back of the Middle Atlantic for some centimeters while the floor of the Pacific Ocean is subducted underneath the South American Continent. "In 100 million years' time Africa will be pulled apart and North Australia will be at the equator," says Prof. Dr. Falko Langenhorst from the Friedrich Schiller University Jena (Germany). Plate tectonics is leading to a permanent renewal of the ocean floors, the mineralogist explains. The gaps between the drifting slabs are being filled up by rising melt, solidifying to new oceanic crust. In other regions the slabs dive into the deep interior of Earth and mix with the surrounding Earth's mantle.
Earth is the only planet in our solar system, conducting such a 'facelift' on a regular basis. But the continuous up and down on Earth's crust doesn't run smoothly everywhere. "Seismic measurements show that in some mantle regions, where one slab is subducted underneath another one, the movement stagnates, as soon as the rocks have reached a certain depth," says Prof. Langenhorst. The causes of the 'congestion' of the subducted plate are still unknown. In the current issue of Nature Geoscience, Prof. Langenhorst and earth scientists of Bayreuth University now explain the phenomenon for the first time.
According to this, the rocks of the submerging ocean plate pond at a depth of 440 to 650 kilometers -- in the transition zone between the upper and the lower Earth mantle. "The reason for that can be found in the slow diffusion and transformation of mineral components," mineralogist Langenhorst explains. On the basis of high pressure experiments the scientists were able to clarify things: under the given pressure and temperature in this depth, the exchange of elements between the main minerals of the subducted ocean plate -- pyroxene and garnet -- is slowed down to an extreme extent. "The diffusion of a pyroxene-component in garnet is so slow, that the submerging rocks don't become denser and heavier, and therefore stagnate," the Jena scientist says.
Interestingly there is congestion in Earth's mantle exactly where the ocean floor submerges particularly fast into the interior of Earth. "In the Tonga rift off Japan for example, the speed of subduction is very high," Prof. Langenhorst states. Thereby the submerging rocks of the oceanic plate stay relatively cold up to great depth, which makes the exchange of elements between the mineral components exceptionally difficult. "It takes about 100 Million years for pyroxene crystals which are only 1 mm in size to diffuse into the garnet. For this amount of time the submerging plate stagnates," Langenhorst describes the rock congestion. It can probably only diffuse at the boundary of the lower Earth mantle. Because then pyroxene changes into the mineral akimotoite due to the higher pressure in the depth of 650 kilometers. "This could lead to an immediate rise in the rock density and would enable the submerging into greater depths."
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The above story is reprinted from materials provided by Friedrich-Schiller-Universitaet Jena.
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Journal Reference:
- W. L. van Mierlo, F. Langenhorst, D. J. Frost, D. C. Rubie. Stagnation of subducting slabs in the transition zone due to slow diffusion in majoritic garnet. Nature Geoscience, 2013; DOI: 10.1038/NGEO1772
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Source: http://feeds.sciencedaily.com/~r/sciencedaily/most_popular/~3/lA2XiL0tTGo/130331165559.htm
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Editor?s note:?Derek?Andersen?is the founder of?Startup Grind, a 40-city community bringing the global startup world together while educating, inspiring, and connecting entrepreneurs. As an?Electronic Arts’ intern eight years ago, I asked Bing Gordon then the chief creative officer?and the only remaining early founding team member, a question about vision. ?How can I know where the puck is going to be?? While he delivered a satisfactory response, two weeks later I received an email from Bing saying, ?I answered that question poorly a few weeks and I wanted to try again.? A few weeks ago?Bing joined me?at Startup Grind in Silicon Valley where he delivered some great advice that has become one of his trademarks. In 2010?Mark Pincus called KPCB general partner Bing Gordon?(look for a bald guy on the front row) one of the world?s ?great CEO coaches? supporting founders on the boards of companies like Amazon, Zynga, Klout, and Zazzle. Here?are some excepts from our recent?interview. Derek: Tell us about your family and where you grew up? BING: So I grew up in a suburb of Detroit.? My dad was a first generation Scotsman and his dad was a janitor.? And he was somebody that believed the grass was always greener and didn?t have, kind of, context or resources.? Thanks, Dad!? We were the first to move in to a subdivision built out of farmlands surrounding Detroit, so I grew up kind of in the creek.? Playing sports with my brother who remembers growing up in the House of Pain. ?So I had a good Midwestern upbringing.? I didn?t work in an office before going to Stanford business school, but I did think I was a pretty damn good teenage caddy. I played hockey and lacrosse at the university level and played both, kind of, for most of my adult life. Derek: What was your plan heading to college? BING: Well I went to Yale thinking I was going to be a math major and a writer, and I got there and Yale was lousy at math and it seemed socially irrelevant, so I kind of became an athlete-near-college-dropout.? I realized I was flunking a third of my classes going into the final.? My proud accomplishments in college other than sports achievements was I wrote poetry.? Kind of light verse, in a coffee shop, and Peter Faulk when he was doing Columbo came, and liked it so much he took
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Public release date: 26-Mar-2013
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