ISSN:
1365-246X
Source:
Blackwell Publishing Journal Backfiles 1879-2005
Topics:
Geosciences
Notes:
The influence of internal heat sources on mantle convection is investigated using numerical calculations of 2-D thermal convection in an infinite Prandtl number, incompressible fluid. The geometry is a cylindrical annulus with inner and outer radii in proportion to the whole mantle. Time-dependent calculations are made starting from random initial conditions, with Rayleigh numbers RaT (based on boundary-temperature difference) and RaH (based on internal-heat production) in the range 103≤RaT≤ 107 and 0 ≤RaH≤ 24 RaT. At fixed RaT, increasing RaH results in transitions in flow structure from steady cells, to a pattern of stationary cells with time-variable amplitude, and finally to thermally turbulent convection with a non-stationary cell count. For RaT 〈 105, the equilibrium cell-aspect ratio increases with RaH, from near unity (10-cell solution) at RaH= 0 to three (four-cell solution) at RaH= 8RaT. Above RaT= 105, the flow is fully time dependent and consists of unequal, non-stationary cells separated by migrating boundaries. Recurring plumes develop from instabilities in both the surface and the basal boundary layers, travel with and modify the large-scale circulation. For RaT 〉 105 and RaH 〉 RaT approximately, the travelling plumes disrupt the large-scale circulation, producing turbulent convection. At RaT= 107 the flow is fully developed thermal turbulence, and for RaH 〉 0, consists of a rapidly fluctuating, irregular flow driven by transient rising and sinking sheets of buoyant fluid. Large fluctuations in total kinetic energy occur in this regime, with periodicities ranging from 40 to 1400 Myr. The transition to thermal turbulence occurs in these calculations at Rayleigh numbers well below the value estimated for subsolidus convection in the mantle, suggesting thermally turbulent convection may occur in the mantle, a consequence of internal heat sources. Thermal turbulence offers an explanation for long-term fluctuations in the rate of subduction, sea-floor spreading and global volcanic activity.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1111/j.1365-246X.1994.tb04671.x
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