Convective thermal evolution of the upper mantles of Earth and
Venus
F. Nimmo and D. McKenzie, Geophys. Res. Lett. 24, 1539-1542, 1997
On Earth the present-day rate of heat loss is about twice the
heat generation rate; on Venus it is about half. Though this
rough balance may be due to a feedback mechanism between mantle
temperature and heat loss, it is difficult to see how such a
mechanism can occur on timescales of 1 Ga or less when the
upper mantle of the Earth is thought to be cooling at about 40
degrees C Ga-1. On Venus a decrease in surface heat flux
presumably occurred at the end of the catastrophic resurfacing
event at similar to 500 Ma. Parameterized convection models
relate heat flux to Rayleigh number by the exponent beta. Such
models using a range of viscosities and values of beta from 0.2
to 0.3 show that the effect of a sudden decrease in surface
heat flux is to cause an independently convecting upper mantle
to increase in temperature by 100 - 500 degrees C over 1 Ga,
whereas, if whole mantle convection occurs, the temperature
change is less than 60 degrees C. An increase in mantle
temperature of 200 degrees C or more will affect mantle
viscosity, lithospheric thickness and melt generation rate, all
of which may affect the feedback mechanism.
Francis' Page
Department of Earth Sciences
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nimmo@esc.cam.ac.uk
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