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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS011 Poster presentation 2182
Mantle Convection Models with Temperature and Depth-dependent
Thermal Expansivity
Mrs. Sanaz R. Ghias
Earth & Space Science Graduate Student IASPEI
Gary T. Jarvis
This study investigates the effects of temperature- and depth-dependent thermal expansivity in 2D
mantle convection models in both plane-layer and cylindrical-shell geometries. For simplicity, most
previous mantle convection models have used a constant coefficient of thermal expansion, a, although
there are some limited earlier studies of the effects of temperature and depth-dependent coefficient of
thermal expansion in plane layer models. We consider a to have the form a(z,T) = az(z)aT(T) and
employ the results of separate mineral physics experiments to determine the functional forms of az(z) =
1/ (1+2.0255e-4*z)3, and aT(T) = a0 + a1T + a2 /T 2. Note that az(z) decreases with depth while
aT(T) increases with T. We find that the depth-dependence and temperature-dependence of a each
have a significant effect on the mean surface heat flux (or Nusselt number) and the mean surface
velocity of the convecting system. For a = az(z), the decrease of a with depth causes a decrease of
surface heat flux by about 25% and a decrease in mean surface velocity by about 40%, relative to the
constant-a case, in either geometry. Consequently, studies of the effects of depth-dependence of a
alone would seriously underestimate these surface parameters. However, when a = az(z)aT(T) (i.e., the
temperature-dependence of a is also included) our predicted values of surface heat flow and velocity
increase; aT(T) compensates for the effects of a(z). Compared to models with a = az(z), for Earth-like
conditions our predicted values increase by 34% and 75%, respectively, in Cartesian coordinates, and
by 23% and 60%, respectively, in cylindrical geometry.
Keywords: convection, thermal expansivity