SO₂ emissions at Semeru volcano, Indonesia: Characterization and quantification of persistent and periodic explosive activity

We present the first measurements of SO₂ emissions at Semeru volcano, Indonesia, using an SO₂ camera. Activity at Semeru is characterized by quiescent degassing interspersed with short-lived explosive events with low ash burden. The interval between explosions was measured at 32.1±15.7min in a webcam survey of the volcano between the months of June and December 2013. We distinguish between two types of events: shorter events (type I: ~5min duration) with emissions returning quickly to baseline levels, and longer events (type II: ~15min duration) often showing multiple pulses and a longer period of increased emissions before a return to quiescent levels. Type I events represent >90% of the activity and release an average of 200-500kg of SO₂ per event. The single type II event we documented with the SO₂ camera released a total of 1460kg of SO₂. We estimate the daily average emissions of Semeru to be 21-71t d^-1 of SO₂, amounting to a yearly output of 8-26Gg (8000-26,000metric tons), with 35-65% released during explosive events. The time series patterns of degassing are consistent with the existence of a viscous plug at the top of the conduit, which seals the conduit immediately prior to explosive events, causing pressurization of the underlying magma followed by a sudden release of gas and fragmented magma.