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How Melting Glaciers, Alter Earth's Surface, Spur Quakes Volcanoes

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  • npat1
    Fw: [fuelcell-energy] ... How Melting Glaciers Alter Earth s Surface, Spur Quakes, Volcanoes June 9, 2006; Page A11 Imagine the surface of Earth as a giant
    Message 1 of 1 , Jun 11, 2006
      Fw: [fuelcell-energy]
      ---------- Forwarded Message ----------
      How Melting Glaciers
      Alter Earth's Surface,
      Spur Quakes, Volcanoes
      June 9, 2006; Page A11

      Imagine the surface of Earth as a giant trampoline that accumulated a
      slab of ice over the winter, and you can get a sense of what a
      growing number of scientists say is in store for the planet as
      glaciers keep melting.

      Once the trampoline's ice turns to water that drips over the edges in
      the warm days of spring, the concave elastic slowly rebounds to its
      original flat shape. That's how Earth responds as glaciers retreat,
      and the consequences promise to be ... interesting.

      The reason is that one cubic meter of ice weighs just over a ton, and
      glaciers can be hundreds of meters thick. When they melt and the
      water runs off, it is literally a weight off Earth's crust. The crust
      and mantle therefore bounce back, immediately as well as over
      thousands of years. That "isostatic rebound," according to studies of
      prehistoric and recent earthquakes and volcanoes, can make the
      planet's seismic plates slip catastrophically, and cause magma
      chambers that feed volcanoes to act like bottles of shaken seltzer.

      "It's unavoidable that glacial retreat will induce tectonic
      activity," says geoscientist Allen Glazner of the University of North
      Carolina, Chapel Hill.

      The connection between melting glaciers and earthquakes isn't to be
      confused with a myth that zipped through cyberspace after the 2004
      Asian tsunami. It claimed that global warming (which is not even two
      degrees above historical averages so far) heated magma, causing
      seismic plates to shift. No.

      Instead, the world-wide melting of glaciers portends a seismically
      active future because of isostatic rebound and also because the
      meltwater from liquefying glaciers adds mass atop oceanic plates.
      That creates a teeter-totter effect, further destabilizing the
      planet's crust. "Recent findings reinforce the idea that the solid
      earth and the climate are inextricably linked," says Prof. Glazner.

      That link has reared its ugly head in the past, especially during
      periods of rapid climate change such as the end of ice ages. When ice
      sheets retreated 10,000 years ago, for instance, Iceland experienced
      a surge in volcanic eruptions. Volcanoes in the Mediterranean,
      Antarctica and eastern California also seem to have been awakened by
      retreating ice.

      When he analyzed 800,000 years of activity from about 50 volcanoes in
      eastern California (the age of rocks formed from volcanic ash can be
      determined by radioactive dating), Prof. Glazner found that "the
      peaks of volcanic activity occurred when ice was retreating globally.
      At first I thought it was crazy, but other scientists also found
      evidence that climate affects volcanism." The likely mechanism:
      glacial retreat lifts pressure that had kept the magma conduit closed.

      The retreat of ice sheets 10,000 years ago also triggered a wave of
      powerful earthquakes in Scandinavia. Since isostatic rebound
      continues for thousands of years, it may still be contributing to
      quakes in eastern Canada, says geoscientist Patrick Wu of the
      University of Calgary.

      That area has no plate boundaries, the usual site of quakes. But
      9,000 years ago, when glaciers retreated, the region began
      experiencing frequent "intraplate" quakes up to magnitude 7, he
      finds. They continue to this day, with quakes such as the 6.3 Ungava
      event that struck northern Quebec on Christmas Day 1989.

      "The pressure of the ice sheet suppresses earthquakes, so removing
      that load triggers them," says Prof. Wu. That creates weakened zones
      that remain vulnerable to seismic activity to this day, including in
      northern Europe. "Present-day earthquakes may have their origin in
      postglacial rebound," he says.

      In southwest Alaska, where the Pacific plate thrusts under the
      continental plate, the immense mass of glacial ice counters the
      tendency of the plates to slip catastrophically. As global warming
      melts the glaciers, however, the ice load is diminishing. As a
      result, Earth is springing back there, too, removing the check on
      seismic activity. The magnitude 7.2 temblor that shook the area in
      1979 is linked to a bounce-back of the crust, conclude geoscientists
      Jeanne Sauber of NASA's Goddard Space Flight Center and Bruce Molina
      of the U.S. Geological Survey.

      "In southwest Alaska, glaciers have been thinning and retreating by
      hundreds of meters for 100 years," says Dr. Sauber. "Huge ice loads
      suppress earthquakes for a while, but when you remove the ice it is
      easier for them to occur." She therefore calls them "promoted"
      earthquakes. Glacial retreat, she says, "is another factor that has
      to be looked at when we assess seismic hazard."

      Alaska isn't the only place where glacial retreat due to the current
      warming coincides with active faults. It does so in the Andes, the
      Swiss Alps, the New Zealand southern Alps, the Rocky Mountains, the
      Himalayas and the edges of Greenland (where scientists recently
      reported that the melting of the ice sheet has accelerated), says
      geologist Bill McGuire of University College London.

      He wrote in the magazine New Scientist that "it shouldn't come as a
      surprise that the loading and unloading of the Earth's crust by ice
      or water can trigger seismic and volcanic activity." He told me
      that "no one knows how much unloading there can be before you trigger
      certain faults."

      � You can email me at sciencejournal@...1.

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