Pulsatory magma supply to a phonolite lava lake

A few lava lakes, like that at Erebus volcano, Antarctica, have been continuously active for decades, reaching a steady-state. We report spectroscopic and thermal observations from Erebus that reveal remarkable, phase-locked cycles of lava lake convection and gas plume composition. We argue that the observed fluctuations in gas ratios, including the SO2/CO2 content in the plume, identify two end-member contributions to the Erebus emission: a sustained source of CO2-rich gas percolating through permeable conduit magma, and a shallower source of H2O-rich gas exsolved from magma pulses that periodically enter the lava lake. The unstable magma flow may reflect the viscosity stratification between rising and descending magma in the conduit, and the resulting oscillatory behaviour of the phonolite lake exemplifies the relative roles of closed- and open-system degassing in persistently active volcanoes.