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News item - Geological processes decrease density of continents

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  • Todd S. Greene
    Continents loss to oceans boosts staying power Study measures effects of chemical weathering on the composition of continents by Jade Boyd (Rice University
    Message 1 of 1 , Apr 8, 2008
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      Continents loss to oceans boosts staying power
      Study measures effects of chemical weathering on the composition of
      by Jade Boyd
      (Rice University press release, 4/1/2008)

      New research suggests that the geological staying power of continents
      comes partly from their losing battle with the Earth's oceans over
      magnesium. The research finds continents lose more than 20 percent of
      their initial mass via chemical reactions involving the Earth's crust,
      water and atmosphere. Because much of the lost mass is dominated by
      magnesium and calcium, continents ultimately gain because the lighter,
      silicon-rich rock that's left behind is buoyed up by denser rock
      beneath the Earth's crust.

      The Earth's continents seem like fixtures, having changed little
      throughout recorded human history. But geologists know that continents
      have come and gone during the Earth's 4.5 billion years. However,
      there are more theories than hard data about some of the key processes
      that govern continents' lives.

      "Continents are built by new rock that wells up from volcanoes in
      island arcs like Japan," said lead author Cin-Ty Lee, assistant
      professor of Earth science at Rice University. "In addition to
      chemical weathering at the Earth's surface, we know that some
      magnesium is also lost due to destabilizing convective forces beneath
      these arcs."

      Lee's research, which appeared in the March 24 issue of the
      Proceedings of the National Academy of Science, marks the first
      attempt to precisely nail down how much magnesium is lost through two
      markedly different routes -- destabilizing convective forces deep
      inside the Earth and chemical weathering reactions on its surface. Lee
      said the project might not have happened at all if it weren't for some
      laboratory serendipity.

      "I'd acquired some tourmaline samples in San Diego with my childhood
      mentor, Doug Morton," Lee said. "We were adding to our rock
      collections, like kids, but when I got back to the lab, I was curious
      where the lithium, a major element in tourmaline, needed to make the
      tourmalines came from. I decided to measure the lithium content in the
      granitic rocks from the same area, and that's where this started."

      In examining the lithium content in a variety of rocks, Lee realized
      that lithium tended to behave like the magnesium that was missing from
      continents. In fact, the correlation was so close, he realized that
      lithium could be used as a proxy to find out how much magnesium
      continents had lost due to chemical weathering.

      Continents ride higher than oceans, partly because the Earth's crust
      is thicker beneath continents than it is beneath the oceans. In
      addition, the rock beneath continents is made primarily of
      silicon-rich minerals like granite and quartz, which are less dense
      than the magnesium-rich basalt beneath the oceans.

      Lee said he always assumed that processes deep in the Earth, beneath
      the volcanoes that feed continents, accounted for far more magnesium
      loss than weathering. In particular, a process called "delamination"
      occurs in subduction zones, places where one piece of the Earth's
      crust slides beneath another and gets recycled into the Earth's magma.
      As magma wells up beneath continent-feeding volcanoes, it often leaves
      behind a dense, magnesium-rich layer that ultimately founders back
      into the Earth's interior.

      In previous research, Lee found that about 40 percent of the magnesium
      in basaltic magma was lost to delamination. He said he was thus
      surprised to find that chemical weathering alone accounted for another
      20 percent.

      "Weathering occurs in just the top few meters or so of the Earth's
      crust, and it's driven by the hydrosphere, the water that moves
      between the air, land and oceans," Lee said. "It appears that our
      planet has continents because we have an active hydrosphere, so if we
      want to find a hydrosphere on distant planets, perhaps we should look
      for continents."

      The research was sponsored by the National Science Foundation (NSF)
      and the Packard Foundation. Co-authors include Morton, of the U.S.
      Geological Survey (USGS) and the University of California–Riverside;
      the NSF's William Leeman, professor emeritus of Earth science at Rice;
      Ronald Kistler of the USGS; former Rice postdoctorate Arnaud Agranier,
      now of the University of West Brittany in Brest, France; and former
      Rice undergraduate Ulyana Horodyskyj, now a graduate student at Brown
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