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On the purpose of toroidal motion in a convecting mantle
Author(s) -
Bercovici David
Publication year - 1995
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 - 10.1029/95gl03082
Subject(s) - toroid , physics , dissipation , mechanics , dissipative system , mantle convection , convection , mantle (geology) , classical mechanics , newtonian fluid , geophysics , geology , lithosphere , thermodynamics , tectonics , plasma , paleontology , quantum mechanics
The purpose of toroidal flow, i.e., strike‐slip motion and plate spin, in the plate‐tectonic style of mantle convection is enigmatic. It is a purely horizontal, dissipative flow field that makes no apparent contribution to the release of heat. However, when plate‐like toroidal motion is allowed to arise as a phenomenon of non‐Newtonian mantle dynamics, it in fact acts to reduce the net amount of viscous dissipation. We show that for a non‐Newtonian flow driven by an existing poloidal field (a source‐sink field), the generation of toroidal motion interacts with the nonlinear rheology to cause less viscous dissipation than if there were no toroidal motion. With power‐law rheologies, the generation of toroidal motion causes up to a 25% reduction in viscous dissipation. With a self‐lubricating rheology, which has been shown to induce the most plate‐like behavior, toroidal motion causes as much as an 80% reduction in viscous dissipation. Thus, basic non‐Newtonian fluid dynamical theory of the formation of plate tectonics shows that toroidal motion is far from superfluous but in fact facilitates the efficiency of convective flow at the surface.