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David Hutt of the British Broadcasting Company and his crew recently came to the
Supercomputing Institute to interview Professor David Yuen of the Geology and Geophysics
Department of the University of Minnesota and Fellow of the Supercomputing Institute
on the topic of mantle plumes and massive basaltic magma outpouring resulting from
plumes impinging on the oceanic plate under the Pacific Ocean and the continental
region under Siberia 225 million years ago. Recent geophysical evidence shows layering
in the lower part of the mantle transition zone down to a depth of 1,000 km. Seismic
observations have revealed a reflector surface at around 900 or 1,000 km depth for
which a possible explanation can be given in terms of a new phase transition of the
lower-mantle constituent minerals. Furthermore, new attempts on the inversion of
the oceanic geoid show the existence of a second low viscosity zone somewhere between
660 and 1,000 km depth. The existence of the second low viscosity zone may be linked
to the mid-mantle phase transitions.
High-resolution, three-dimensional numerical simulations by Dr. Laszlo Cserepes,
a collaborator in Professor Yuen’s research group, from the Eotvos University in
Budapest, Hungary were visualized by undergraduate research assistants from Professor
Yuen’s group, Josh Collins and Brigit Schroeder. These simulations (see figures)
include phase and viscosity stratification of the transition zone in three dimensions
in a 4 x 4 x
1 rectangular box. A hypothetical weak endothermic transition at 1,000 km was assumed
in some of the models. When the 1,000 km endothermic phase transition is included,
these instabilities can grow only at a few places, but they form strong downwellings.
Two distinct types of penetrative, deep downwellings can be present at the same time:
one that crosses the whole transition zone, and another that crosses only the 660
km discontinuity and stops at 1,000 km (at least temporarily). This can explain seismological
observations that suggest that subducted slabs can be retarded not only by the 660
km boundary, but also by some deeper obstacle near 1,000 km depth.
These visualizations were filmed by David Hutt’s crew for display. The hour long
special has already been shown in England and is now being distributed to television
networks around the world.
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| Three-dimensional numerical model
of thermal convection in a box in which a compressible fluid and the anelastic-liquid
approximation of the governing equations are assumed. This model shows one phase
transition at 660 km. | | Three-dimensional numerical model
of thermal convection in a box in which a compressible fluid and the anelastic-liquid
approximation of the governing equations are assumed. This model shows two phase
transitions at 660 and 1,000 km. |
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