Influence of a basal thermal anomaly on mantle convection

Abstract

We perform laboratory experiments to study the effects of a variable basal thermal anomaly on convection. In the regime of cellular convection ( Ra < 10 7), the convection pattern changes as the horizontal temperature variation in the bottom boundary increases. When the temperature variation is less than the critical value, there is no effect to the convection pattern. Above this critical value, an upwelling is fixed at the site of the thermal anomaly. For a larger temperature variation, the upwelling region becomes wider. For the cases above the critical value, the time-averaged temperature in the isothermal core above the thermal anomaly becomes higher than that in the other regions. In the regime of plume dominant convection ( Ra ≥ 10 7), when the horizontal temperature variation exceeds the critical value, the location of a hot plume is similarly affected. For this case, the plume generated by the thermal anomaly straddles around the site of the thermal anomaly rather than being fixed. For a larger temperature variation, multiple plumes cluster together which also straddle around the anomaly. The straddling nature of the hot plumes generated by the thermal anomaly causes the time-averaged temperature above the thermal anomaly to remain unchanged. We also find that different from the cellular convection cases, the temperature variation less than critical is capable of generating intermittent hot plumes, but they do not dominate the convection pattern. The critical horizontal temperature variation needed to affect the convection pattern is found to be scaled by the maximum standard deviation of the time variation of the temperature σ max * around the lower thermal boundary layer in the absence of thermal anomaly. We estimate the possible temperature variation which can be generated by a partially molten region at the CMB. We find that assuming that σ max * for mantle is the same as that obtained from the experiment, a region less viscous than the surrounding region by an order of magnitude, can generate a hotspot.