Geoscience Reference
In-Depth Information
4
Rheological Properties of Minerals
and Rocks
SHUN-ICHIRO KARATO
Department of Geology and Geophysics, Yale University, New Haven, CT, USA
Summary
substantially. The deep mantles of the Moon and
Mars are inferred to have low viscosities due pre-
sumably to the high water content. Super-Earths'
deep mantle may have low viscosities caused by
the extremely high pressure (to
Plastic deformation in the Earth and planetary
mantle occurs mostly either by diffusion or by
dislocation creep. In both mechanisms, the rate of
deformation increases strongly with temperature
but decreases with pressure at modest pressures.
Addition of water and grain-size reduction en-
hance deformation. Influence of partial melting
is modest for a small amount of melt. Models on
the rheological structures of the Earth's mantle
can be developed by including experimental re-
sults on all of these effects combined with models
of temperature, water and grain-size distribution.
The rheological properties in the upper mantle
are controlled mostly by temperature, pressure
and water content (locally by grain-size reduc-
tion). The strength of the lithosphere estimated
from dry olivine rheology for homogeneous de-
formation is too high for plate tectonics to occur.
The influence of orthopyroxene to reduce the
strength is suggested. Transition from the litho-
sphere to the asthenosphere occurs largely by
the increase in temperature but partly by the
change in the water content. Rheological proper-
ties of the transition zone and the lower mantle
are controlled by phase transformations. How-
ever, a transition to high-density structures does
not necessarily increase the viscosity. The grain-
size reduction caused by a phase transformation
has a stronger effect and weakens the material
1TPa) that may
enhance deformation by the transformation to a
compact crystal structure, or the transition in
diffusion mechanism from vacancy to interstitial
mechanism, and metallization.
∼
4.1 Introduction
4.1.1 Rheological properties and the dynamics
and evolution of terrestrial planets
Terrestrial planets are formed hot and they release
their energy by mantle convection. Forces asso-
ciated with mantle convection deform planetary
surface and its interior, and convection moves
materials to different thermodynamic conditions
leading to phase transformations including melt-
ing. Consequently, mantle convection is the sin-
gle most important process on terrestrial planets
that dictates their surface tectonics, deep man-
tle and core dynamics as well as their thermal
evolution (e.g., Schubert
et al
., 2001).
Mantle convection is possible because minerals
and rocks behave like a viscous fluid at the geo-
logical timescale under the mantle conditions but
the rheological properties of minerals and rocks
