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Australia: The Land Where Time Began |
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Anomalous Zones the Size of Continents of Low Seismic Velocity
at the Base of the Mantle
It has been revealed by seismic images of the interior of the Earth that
2 massive zones of slow seismic wave travel are located at the base of
the mantle, above the core. Surrounding these anomalous regions are
mantle materials that are believed to be composed of cooler rocks
associated with downward advection of former tectonic plates. It is
uncertain what the composition of these anomalous provinces is. These
zones have been depicted for a long time as mantle materials that are
related to convective upwelling that are warmer than the average mantle
materials. According to Garnero et
al. they may also be
chemically distinct from the mantle that surrounds them, and potentially
they may be partly composed of subducted or primordial material, and
they have therefore been termed thermochemical piles. The emerging view,
based on seismic, geochemical and mineral physics data, is that these
thermochemical piles appear to be denser than the surrounding mantle
materials, and they are dynamically stable and long-lived, and large
scale mantle flow has shaped them. The
composition of the piles is modified over time by stirring as well as by
chemical reactions with material from the surrounding mantle, the
underlying core, and potentially from volatile elements that have been
transported to the deep Earth by subducting plates, whether they are
remnants of a primordial layer or more dense materials that accumulated
later. It is suggested that upwelling mantle plumes may originate from
the thermochemical piles, therefore the unusual chemical composition of
the piles could possibly be the source of distinct signatures of trace
elements that have been observed in lavas from hotspots.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |