HR: 0800h
AN: U21A-0005    [Abstracts]
TI: Whole Mantle Thermal Convection At The High Rayleigh Number
AU: * Kutsov, A M
EM: alex-m-kutsov@yandex.ru
AF: Faculty of natural-science disciplines, 72 Kommunisticheskii av., Yuzhno-Sakhalinsk, 693000, Russian Federation
AB: Rayleigh number for mantle approximately in 10000 times exceeds critical value, the order 10³. Hence, according to the theory convection, the mantle not simply is in a condition convection instability, convection in it has intensive character. Chaotic time dependence is easily raised in a liquid heated up from within as it is necessary to bear heat from bowels of cells. The similar behaviour also takes place at heating from below, at enough high Rayleigh numbers. Transition from convection with chaotic time history, but stationary enough cells (so-called poorly turbulent mode) to convection with not organized picture of cells (so-called strongly turbulent mode) takes place at infinite Prandtle number with heating from below at Ra=10⁷ or at lower Rayleigh number if takes place internal heating. Rayleigh numbers for mantle convection in a fluid layer heated from below Ra_q=4.8×10⁶ and in a layer, heated from within Ra_H=3×10⁹. These values in 10⁴ and 10⁶ times more critical Rayleigh number. Rayleigh number for single-layered mantle convection based on superadiabatic differences of temperatures between a surface and core-mantle boundary has the order 10⁷, approximately in 10⁴ times more critical. In the geological past, in particular in early Archean, Rayleigh number for the Earth was probable even more as the mantle was more hot, and, hence, less viscous, than now. For Rayleigh number corresponding single-layered whole mantle convection at heating from below the initial stage of development non-stationary mantle convection is investigated. As algorithm it was applied so-called Semi-Implicit Method for Pressure-Linked Equations (SIMPLE). Calculations were spent in square and rectangular areas. For the account of real parities of the vertical and horizontal sizes of the mantle, and also for exception of influence of lateral borders, modelling was spent in rectangular area with the aspect ratio 10:1 on a grid 502×52 points. Bottom and lateral borders rely impenetrable without sticking. The top border is considered free. Temperatures on the top and bottom border are accepted fixed and equal, accordingly, T₀ and T₁, and on lateral borders equality is necessary to zero of a thermal stream. The bottom border corresponds to a surface of the core. The top border are associated with oceanic lithosphere. As initial temperature distribution it is accepted corresponding weak one-cellular convection. Modelling covers an interval of dimensional time from 0 up to 670 million years. Was it is calculated 2500 planforms convection (on number of steps on time). As a result of the lead researches it is established, that in the given system arises unstable convection therefore collapse convection cells and on their place there are numerous ascending and descending plumes which, reaching a surface, spread in a horizontal direction. The descending and ascending jets of streams arising as a drop, move horizontally, and some nearby streams are united in one, more powerful. Two kinds of plumes it was observed in model: developed plumes stretched through all depth of the layer, having rather steady position, and fine mobile plumes drawn to greater subvertical plumes. The heat transfer strongly depends on the wavelength of convection. Not looking at that movement takes the form of casually arising plumes, inversion of temperature arising at the first stage (owing to a choice of entry conditions) in many respects defines character of process at an initial stage. The benthonic part well transfers possible character of the processes occuring on core-mantle boundary. Presence of numerous descending streams, apparently, is caused by isoviscous conditions.
DE: 8121 Dynamics: convection currents, and mantle plumes
SC: Union [U]
MN: 2007 Fall Meeting