Experimental Modeling of a Plume Forming in an Inclined Flat Crustal Layer of a Subducting Oceanic Plate

Abstract

The results of experimental studies of the thermal and hydrodynamic structure of the plume conduit melted out in a flat inclined layer above a local heat source are presented. For different thermal powers on the plume base, the evolution of the plume conduit structure and the plume breakthrough on the surface are presented. It was found that for heater power N = 5.6 W and N = 6.4 W and relative power Ka = 1.18, the plume does not reach the surface of the inclined layer. For N = 7.9 W (Ka = 1.42) and N = 8.9 W (Ka = 1.53), the plume reaches the surface, and the first plume cell (plume head) has a mushroom shape. Depending on the value of the Ka criterion (relative thermal power), the upper cell structure and the regime of plume breakthrough on the surface change. There is an ascending flow at the upper bounding surface of the inclined layer and a descending flow along the rest of the perimeter of the plume conduit. Longitudinal roll flows are also observed. The roll axes are oriented in the direction of the ascending flow. The roll flows transfer heat from the ascending flow to the descending one and hence reduce the unstable stratification existing in the layer. Due to these flows, the turbulent viscosity of the melt increases, which also reduces unstable stratification. In the conduit portion, where the rolls are weak, unstable stratification is additionally compensated by the plume conduit widening. Laboratory experiments have shown that the regime of plume breakthrough on the surface of an inclined flat layer depends on the Ka criterion, and this dependence differs from that of plume breakthrough on the surface in an unbroken solid massif