TRANSITIONS IN THE STYLE OF MANTLE CONVECTION AT HIGH RAYLEIGH NUMBERS

The pattern and style of mantle convection govern the thermal evolutio n, internal dynamics, and large-scale surface deformation of the terre strial planets. In order to characterize the nature of heat transport and convective behaviour at Rayleigh numbers, Ra, appropriate for plan etary mantles (between 10(4) and 10(8)), we perform a set of laborator y experiments. Convection is driven by a temperature gradient imposed between two rigid surfaces, and there is no internal heating. As the R ayleigh number is increased, two transitions in convective behaviour o ccur. First we observe a change from steady to time-dependent convecti on at Ra approximate to 10(5). A second transition occurs at higher Ra yleigh numbers, Ra approximate to 5 x 10(6), with large-scale time-dep endent flow being replaced by isolated rising and sinking plumes. Corr esponding to the latter transition, the exponent beta in the power law relating the Nusselt number Nu to the Rayleigh number (Nu similar to Ra-beta) is reduced. Both rising and sinking plumes always consist of plume heads followed by tails. There is no characteristic frequency fo r the formation of plumes. (C) 1998 Elsevier Science B.V. All rights r eserved.