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CCS Seminar
Professor Paul Hall
Boston University - Department of Earth Sciences
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FRIDAY - February 29, 2008
12:00 noon
Physics Research Building - Room 595
3 Cummington Street
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"Computational Geodynamics: Exploring the Earth's Deep Interior from the
Comfort of the Computer Lab"

Abstract: The development of the theory of plate tectonics in the late
1960's revolutionized our understanding of the solid Earth, providing a
framework through which phenomena such as earthquakes and volcanoes can
be understood as the surface manifestation of convective motion in the
Earth's deep interior. Over the 40 years since the advent of this new
paradigm, the exquisitely slow motions of the Earth's rocky mantle have
been investigated using a variety of both geophysical and geo-chemical
methods, and they are now thought to be intimately tied to the origin
and evolution of life, as well as to a number of mass extinction events
that nearly destroyed life on Earth. However, because the Earth's
interior is physically inaccessible, we are unable to directly observe
convection in the mantle, and therefore many basic questions about the
morphology and timescale of these motions remain unresolved. In an
effort to answer these questions, Earth scientists have increasingly
turned to computational fluid dynamics (CFD) to develop models of mantle
convection.

By treating the mantle as a highly viscous fluid, using material
properties obtained from high-pressure, high-temperature mineral physics
experiments and applying boundary and initial conditions derived from
geophysical and geochemical observations, it is possible to produce
meaningful CFD models of flow in the Earth's mantle. This type of
computational modeling poses many challenges that are not normally
encountered within more traditional applications of CFD. For example,
the relevant physical processes being modeled cover a vast range of
length scales (from the flow of magma along the boundaries of individual
mineral grains (L < 0.000001 m) to the dimensions of individual
convective cells (L > 1,000,000 m)), and pressures (from 0 GPa at the
surface to >100 GPa at the base of the mantle). Furthermore, ductile
deformation within the mantle is governed by a range of mechanisms, from
diffusion creep (e.g., Coble, Nabarro-Herring) to dislocation creep,
resulting in an effective viscosity that is highly non-linear and varies
with temperature, pressure, composition, grain size and strain rate.
Finally, mantle minerals undergo a variety of both solid-solid and
solid-liquid phase transitions as they move around within the mantle,
significantly altering their physical properties quite abruptly over
very short distances. By way of illustration, a case study using a
finite element CFD model in conjunction with a Lagrangian particle
method to model the interaction between a thermo-chemically buoyant
mantle plume and a mid-ocean ridge system (an analog for the creation of
the Easter Island - Salas y Gomez seamount chain) will be presented.


Cheryl Endicott
Administrative Assistant
Center for Computational Science
3 Cummington Street
Boston, MA 02215
tel: 617-358-1470
fax: 617-358-2487
http://ccs.bu.edu

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