Unrecognized underwater volcanic activity
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15 July 07
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|Unrecognized underwater volcanic activity|
13 Jul 07 – "Many earthquakes in the deep ocean are much smaller in magnitude than expected. Geophysicists from the Woods Hole Oceanographic Institution (WHOI) have found new evidence that the fragmented structure of seafloor faults, along with previously unrecognized volcanic activity (italics added), may be dampening the effects of these quakes.
"Examining data from 19 locations in the Atlantic, Pacific, and Indian oceans, researchers led by graduate student Patricia Gregg have found that "transform" faults are not developing or behaving as theories of plate tectonics say they should. Rather than stretching as long, continuous fault lines across the seafloor, the faults are often segmented and show signs of recent or ongoing volcanism (italics added) Both phenomena appear to prevent earthquakes from spreading across the seafloor, thus reducing their magnitude and impact.
"Gregg, a doctoral candidate in the MIT/WHOI Joint Program in Oceanography and Oceanographic Engineering, conducted the study with seismologist Jian Lin and geophysicists Mark Behn and Laurent Montesi, all from the WHOI Department of Geology and Geophysics. Their findings were published in the July 12 issue of the journal Nature.
"Oceanic transform faults cut across the mid-ocean ridge system, the 40,000-mile-long mountainous seam in Earth’s crust that marks the edges of the planet’s tectonic plates. Along some plate boundaries, such as the Mid-Atlantic Ridge, new crust is formed. In other regions, such as the western Pacific, old crust is driven back down into the Earth.
"If you imagine the mid-ocean ridge as the seams on a baseball, then transform faults are the red stitches, lying mostly perpendicular to the ridge. These faults help accommodate the motion and geometry of Earth’s tectonic plates, cracking at the edges as the different pieces of rocky crust slip past each other.
‘The researchers [examined] gravity data collected over three decades by ships and satellites, along with bathymetry maps of the seafloor. Conventional wisdom has held that transform faults should contain rocks that are colder, denser, and heavier than the new crust being formed at the mid-ocean ridge. Such colder and more brittle rocks should have a "positive gravity anomaly."
But Gregg "was surprised to find that the faults were not exerting extra gravitational pull. On the contrary, many seemed to have lighter rock within and beneath the faults.
"What we found was the complete opposite of the predictions," said Gregg.
"It is also possible that magma, or molten rock, from inside the earth is rising up beneath the faults. (italics added) Earthquakes stem from the buildup of friction between brittle rock in Earth’s plates and faults. Hot rock is more ductile and malleable, dampening the strains and jolts as the crust rubs together and serving as a sort of geological lubricant.
"The findings by Gregg, Lin, and colleagues may also have
implications for understanding the theory of plate tectonics, which says
that new crust (2,150-degree magma) is only formed at mid-ocean
ridges. By traditional definitions, no crust can be created or destroyed
at a transform fault. The new study raises the possibility that new
crust (2,150-degree magma) may be forming along these faults and
fractures at fast-spreading ridges such as the East Pacific Rise.
This story was originally entitled "Fragmented Structure Of Seafloor Faults May Dampen Effects Of Earthquakes." I think the discovery of so much unexpected underwater volcanic activity is the real news here.
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