Drivers of Variation in the Eruptive and Magmatic Processes of a Persistently Active Volcano, Yasur, Vanuatu

Abstract

A study to better understand the shallow conduit processes and eruptive manifestations of Strombolian-style volcanism was conducted at Mt Yasur, on Tanna Island, Vanuatu. It comprised an 11-week, multi-component monitoring deployment that utilized seismometers, gas spectrometry, thermal-IR imagery and visual observations. From this, explosion parameters were quantified and related to global analogues. Strombolian-style explosions occurred at rates of 30 to 129 per hour, reaching heights of 30-520 m, and ranging in style from Types 1, 2a, 2b, and a new style defined as Type 3. Cycles of 10 to 20 days were observed to indicate changes in magma and volatile flux across the whole volcano. Other ~5-day fluctuations at individual vent zones reflected changes in magma and degassing percolation within each upper conduit branch. An overall decline in the strength of activity during the observation period signified a longer cycle only partly observed. Statistical modelling revealed that the two main craters (South and North) are separated deeply, but above the level of the magma reservoir. Two vent zones within South Crater (A and B) are closely connected and display alternating activity with some periods of simultaneous paired explosions. This indicated only shallow separation of conduits above the point of gas-slug formation. The most powerful volcanism occurred during periods of increased background activity and SO2 flux, associated with hot conduit pathways filled with mobile magma. Lower power and low frequency, ash-rich eruptions occurred (Type 2b) if cooling and stagnant magma was present within the conduit. A new explosion type, Type 3, occurred when vents were deeply buried by surface debris and with no background degassing. Precursory low-frequency seismic signals indicate that gas is trapped beneath the surface, and that gas-rupture mechanics for Type 3 differ from other explosion types. Overall proximal hazards are primarily from ejection of lava bombs. Mean explosion heights of 150 m and ~250 m are required to reach the rim and outer flanks (and car park), respectively. On 31% of days, bombs landed on the rim and on 2% of days, they reached the car park. The outcomes of this study included linking the variation in explosion types at steady-state Strombolian-style volcanic activity to the upper conduit magma state and identifying new hazard warning/mitigation strategies.