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A possible role of weak zone at plate margin on secular mantle cooling
Author(s) -
Honda S.
Publication year - 1997
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/97gl02959
Subject(s) - nusselt number , boundary layer , convection , thermodynamics , mantle (geology) , rayleigh number , natural convection , viscosity , isothermal process , mantle convection , geophysics , geology , mechanics , physics , turbulence , paleontology , subduction , tectonics , reynolds number
We study a simple 2‐D model of secular mantle cooling by convection in which the viscosity depends exponentially on the temperature. To understand the effects of weak zones near the sinking and rising areas, their position and viscosity are imposed. The heat transport efficiency is measured by introducing local Rayleigh ( Ra l ) and Nusselt ( Nu l ) numbers defined at each thermal boundary layer. We find that the Nu l ‐ Ra l relationship for the bottom boundary layer is similar to that of the steady‐state convection with constant viscosity in a rectangular box. The existence of weak zones predicts a higher Nu l for the top thermal boundary layer, suggesting that this will enhance the heat transport of the top layer. If we change Ra l of the top layer into the higher of the two Ra ls , which is usually that of bottom layer, the resultant Nu l ‐ Ra l approaches that of steady‐state constant viscosity convection. This may imply that the heat transport of convection with secular cooling may be controlled by the boundary layer which has higher Ra l . We also suggest that the commonly used parameterization, in which the convecting medium is treated as having a constant viscosity equal to that of its isothermal core, may overestimate the heat transport efficiency.