4/1/16, "Massive Ancient Tectonic Slab Found Below the Indian Ocean," Earth and Space Science News, EOS, Cody Sullivan. Source: Geophysical Research Letters
"Scientists discover a surprisingly positioned tectonic plate, buried
below the southern Indian Ocean, that spans the entire mantle."
"A team of researchers recently discovered an ancient relic hidden
within Earth: a tectonic plate resting beneath the southern Indian
Ocean. Scientists have found other tectonic plates that sank below
Eurasia and North America, but here Simmons et al.
describe the unique structure of this newly discovered slab, which they
named the Southeast Indian Slab (SEIS). The slab has at least one
feature scientists have rarely seen before: It maintains its slab-like
structure all the way from the upper mantle near Earth’s crust down to
the region where the mantle meets the planet’s superheated core.
Farallon plate beneath North America is a well-known example of this—but
it was expected to exist and sank much more recently than the SEIS. In
addition, not only does the SEIS traverse the entire mantle, but it also
becomes more vertical along one end, so much so that it stands almost
vertically between the crust and core along the eastern edge, whereas
the western portion is more horizontal.
Researchers can make out structures beneath Earth’s crust by
examining the speed at which seismic waves generated by earthquakes and
similar Earth-shattering events—known as P and S waves—travel
through Earth. Here the researchers used wave data from 12,607 seismic
events dating back to the 1960s, collected by 7783 seismic stations
around the world, to develop the model that identified the ancient slab.
Once this tectonic slab was identified, the team looked at the
region’s tectonic history over millions of years to determine where and
when this plate was on the surface. They determined that the slab was
once along the eastern portion of the early supercontinent of Gondwana.
Then, sometime during the Triassic or Jurassic period, which stretched
from 250 million years ago to 145 million years ago, the slab plunged
underneath another plate. They further concluded that the subduction, or
the sinking of the Southeast Indian Slab beneath another plate,
terminated around 130 to 140 million years ago in the Mesozoic era,
around the same time that the tectonic plates under eastern Gondwana
began to separate and split up the continent.
Tectonic plates usually sink down into the mantle at a rate of about 1
centimeter per year or more; they don’t necessarily melt but instead
bunch up at the base of the mantle and eventually assimilate or become
undetectable as their temperature increases. However, if the researchers
accurately estimated the timing of their newly discovered slab’s
subduction, this slab must have stalled in a transition zone before
descending deeper down into the mantle, allowing the slab to persist in
the mantle longer than any other known plate. (Geophysical Research Letters, doi:10.1002/2015GL066237, 2015)
—Cody Sullivan, Freelance Writer
Citation: Sullivan, C. (2016), Massive ancient tectonic slab found below the Indian Ocean, Eos, 97, doi:10.1029/2016EO049219. Published on 1 April 2016."
Image caption: "Seismic wave velocity structure in the deep Earth revealed through
seismic tomography. Earthquakes generate seismic energy near their
epicenters (yellow markers), and the energy is recorded at seismic
stations around the world (red markers). Seismic waves (depicted as
yellow rays emanating from an earthquake beneath Spain) are disrupted as
they travel through fast (blue) and slow (red) structures in the Earth.
By mapping these anomalous structures on a global scale, researchers
have uncovered a previously unidentified tectonic plate that sank into
Earth’s mantle more than 130 million years ago beneath the southern
Indian Ocean. Credit: Nathan Simmons, using MATLAB."