Geophysics in the news: Time to forget everything you thought you knew about mantle plumes? For some years now mantle plumes have been a controversial topic, and a newly published paper in the journal Proceedings of the National Academy of Sciences debunks the theory. In fact, these narrow jet streams, or mantle plumes, may not even exist. Mantle plumes have never had a sound physical or logical basis, says Professor Don Anderson of the California Institute of Technology. According to the currently held theory, internally-generated heat generates hot, narrow plumes that move up through the mantle to erupt at the surface. While it isnt clear exactly how these jets are created, scientists have long assumed that they exist. Supposedly they are no more than 300 kilometers wide and located almost 3,000 kilometers underground - nearly halfway to the planets center. Much of solid-Earth science for the past 20 years - and large amounts of money - have been spent looking for elusive narrow mantle plumes that wind their way upward through the mantle, Anderson explained. Instead, researchers behind this latest study argue that there are broad upwellings, rather than narrow jets, driving volcanic behavior. After analyzing global seismic activity, the research team failed to find evidence of mantle plumes, but did discover large, slow, upward-moving chunks of mantle a thousand kilometers wide. Anderson describes their behavior like that of a lava lamp, in which the heat that is transferred upward via jets - like blobs of wax - is balanced by the slower downward motion of cooled, broad, uniform chunks of mantle. Whats new is incredibly simple: upwellings in the mantle are thousands of kilometers across, Anderson said. While this may not seem like a huge revelation, it disproves what scientists had long believed about the mechanisms behind a volcanic eruption. What is driving this motion is not heat from the core, but cooling at the Earths surface. This cooling and plate tectonics drives mantle convection, the cooling of the core, and Earths magnetic field - volcanoes are simply a side effect. However, many geologists continue to support the mantle plume theory...so this controversial topic has much more life in it yet! The open source paper can be found on: pnas.org/content/early/2014/09/05/1410229111.full.pdf+html
Posted on: Wed, 10 Sep 2014 10:08:49 +0000
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