Simulation results for the reference model. The simulation shows variability in vertical plume volume flux (a) associated with destabilization of plume conduits (b) that tilt >67° from the vertical. Statistically significant periods (99.9 per cent statistical significance) of plume flux variability over a running window (considered volume flux for the red dot is indicated by the red arrows in a) indicate a general increase in periodicity through time (c). Melt produced by discrete volume flux pulses (d) are distinguished by alternating shades of blue. (e) Overall periodicity taken over the entire time of UMP (dashed line, α = 0.01 indicating 99.9 per cent statistical significance). (f–j) Model snapshots at different times (indicated by green triangles in a and b), where the contour for volume flux calculations is shown in purple. Contour for plume parameters is shown in blue. White contours show the non-adiabatic temperature from 25 to 200 K, at 25 K intervals. Arrows show the direction of the velocity field (not scaled by magnitude).