Laboratory convection experiments: Effect of lateral cooling and generation of instabilities in the horizontal boundary layers

Abstract

Convection experiments are carried out in a tank with two isothermal heat sinks: The top plate and one of the sidewalls. Heating is supplied either by an isothermal bottom plate or by generation within the fluid. This situation is similar to that of the earth's subcontinental mantle in the presence of a neighboring subducting oceanic lithosphere. There, the mantle material loses heat not only to the base of the continental lithosphere but also to the cold dipping oceanic slab. The thermal structure of the convective fluid is observed by a variety of techniques, including differential interferometry and strioscopy. Two parameters characterize the observations: the usual ‘vertical’ Rayleigh number and a newly defined ‘lateral’ Rayleigh number. The lateral cooling induces a large roll with axis parallel to the cold wall. The variation of its width relative to the values of the two Rayleigh numbers has been determined. An application to the earth's upper mantle would predict rolls 5 times wider than high. The circulation in the large roll remains two‐dimensional for Rayleigh numbers up to about 3 × 104. Beyond this value, boundary layer instabilities are observed within the persisting large roll. Their period has been determined. The interferometric method is shown to be very useful for visualizing other time‐dependent processes such as the growth of the induced large rolls. This growth is rapid enough to allow us to propose the existence of such large rolls in the earth and argue that their action could have led to continental break‐up.