Physics
Scientific paper
Sep 1986
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1986pepi...43..261g&link_type=abstract
Physics of the Earth and Planetary Interiors, Volume 43, Issue 4, p. 261-273.
Physics
43
Scientific paper
Diapirs rising through the Earth's mantle and lithosphere are hotter, and generally also of a different chemical composition, than their surroundings. Experiments demonstrate that diapirs driven by a temperature difference alone entrain material from their surroundings and that their velocity of rise decreases as they enlarge. These diapirs are referred to as `thermals'. However, when the driving buoyancy is due only to a difference in chemical composition there is no entrainment and diapirs rise at a constant velocity (assuming the surroundings have a constant viscosity). The different flow is a result of radically different diffusion coefficients for heat and composition. Between these two extremes is a range of behaviours characterised by the density ratio Rρ = Δρi/ρ∞αΔT, where Δρi is the contribution to the total density difference resulting from the composition, or `intrinsic', difference and ρ∞αΔT is the contribution from the temperature difference. For Rρ of order one, buoyancy given to surrounding material by heat loss from the diapir must be taken into account and entrainment persists during a diapir's rise through a scale height for cooling. Beyond this height entrainment ceases (if the two buoyancy forces reinforce, Rρ > 0) or gravitational instability causes breakup of the diapir (if the buoyancy forces are opposed and thermal effects are initially dominant, Rρ < 0). Furthermore, only diapirs driven by an intrinsic density difference, but having temperatures high enough to produce a low-viscosity lubricating boundary layer, are able to exceed the Stokes terminal velocity.
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