| Topic: |
Science > Physics |
| User: |
"Sam Wormley" |
| Date: |
06 Feb 2008 04:25:58 AM |
| Object: |
Physicists get to the bottom of volcano hotspots |
Physicists get to the bottom of volcano hotspots
http://physicsworld.com/cws/article/news/32760
One of the mysteries of geophysics is how heat is transported from
the Earth's core, through the mantle and into the crust. A
controversial aspect of this process is the idea that massive plumes
of hot material from deep in the mantle well up towards the surface,
causing volcano hotspots such as Iceland and Hawaii. Now, two
physicists in the US have shown that -- at least in theory --
such mantle plumes could exist for long enough to cause hotspots.
The mantle is a region of dense rock that extends 50-3000 km below
the surface of the Earth. Although the mantle is solid, geophysicists
believe that it can also flow in response to the extreme temperatures
and pressures generated by the Earth's core. As a result, convective
processes -- such as mantle plumes -- are likely to play an
important role in how heat is transported through the mantle.
The theory of mantle plumes got a boost in 1999 when Anne Davaille at
the University of Paris 7 simulated them in a small tank containing
two unmixed liquids of slightly different densities. Two liquids were
used to mimic one of the many boundaries between rock layers that are
believed to exist in the mantle.
When the denser liquid at the bottom of the tank was heated, Davaille
observed several different types of behaviour ranging from violent
mixing to the emergence of thin tendrils of denser liquid that
extended up into the less dense fluid.
The tendrils formed on the boundary between the two fluids where the
lower fluid pushed into the upper fluid to create a cone-shaped
intrusion with a wide base and narrow top from which the tendril
emerged. While the tendrils persisted for long periods of time, it
wasn't clear from the experiment whether the tendrils would occur on
the density, time and length scales relevant to the Earth's mantle.
Now, Laura Schmidt and Wendy Zhang at the University of Chicago have
come up with a new mathematical model of how tendrils form in
Davaille's experiments (Phys Rev Lett 100 044502). The researchers
believe that their model can be scaled up to explain how mantle
plumes many kilometres across could arise.
Schmidt and Zhang began with equations that described the shape and
flow of the tendrils, which they solved using a combination of
analytical and numerical techniques. Their calculations suggest that
the cones occur at "stagnation points" which endure for long periods
of time and anchor the tendrils. Such stagnation points have also
been observed in a similar experiment done by Schmidt at the
University of Chicago.
Schmidt told physicsworld.com that the tendrils remain stable for
long periods of time because they remain isolated from larger-scale
convective flow in the tank. This enduring quality of the tendrils is
important, because mantle plumes responsible for hotspots would have
to endure for more that 100 million years.
According to Schmidt, the calculations suggest that the tendrils "are
a generic feature of convecting multi-layer liquids, and would be
present in a layered mantle". She describes the tendrils as "a
possible mechanism behind hotspot persistence".
Schmidt and Zhang now plan to do a more rigorous check of their
theory against experiment by seeing if their model can predict the
rate of flow through tendrils of different sizes. They are also doing
a more detailed study of what factors affect the stability of the
tendrils.
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