An inclined Vulcanian explosion and associated products

R C Stewart,A J Stinton,C Bonadonna,H M Odbert,P D Cole

doi:10.1144/jgs2014-099

Abstract

Vulcanian explosions generate some of the most hazardous types of volcanic phenomena, including pyroclastic density currents. Non-vertical directionality of an explosion promotes asymmetrical distribution of proximal hazards around the volcano. Although critical, such behaviour is relatively uncommon and has been seldom documented. Here we present, for the first time, evidence both from geophysical monitoring and field survey data that records the occurrence of such an event. Thermal imagery captures a Vulcanian explosion at Soufriere Hills Volcano, Montserrat, which occurred during a large partial lava dome collapse in February 2010, and was inclined at about 25° from the vertical in a northerly direction. Pyroclastic products were preferentially distributed to the north and included: an unusual pumice boulder deposit that we propose was formed by a dilute pyroclastic density current; pumice flow deposits; and a proximal lapilli and block fallout lobe. The inclined nature of the explosion is attributed to the asymmetric geometry around the vent. The explosion-derived pyroclastic density currents had notably lower velocities than those associated with lateral blasts, which, we suggest, result from a separate and distinct mechanism. These inclined explosions present an additional mechanism that is able to generate directed pyroclastic density currents, with consequent implications for hazard assessment.

Highlights

Vulcanian explosions occur at many volcanoes around the world (Morrisey & Mastin 2000)
We propose that the asymmetry formed as a result of excavation of the dome collapse amphitheatre in the northern flank of the lava dome
We have demonstrated evidence that Vulcanian explosions can have a notable non-vertical component that results in directed products

Summary

Discussion

This is the first example where observational data from an explosion together with detailed studies of the resulting products record evidence of an inclined Vulcanian explosion. Deposits from andesitic volcanoes that produce abundant block and ash flows, such as other Lesser Antillean volcanoes, sometimes contain lenses and layers of more pumiceous material (Roobol et al 1987; Smith & Roobol 1990) We proffer that such deposit types may be related to inclined explosions similar to those described here rather than being formed by segregation within a dense pyroclastic density current. Helens in 1980 (Moore & Sisson 1981), and Soufrière Hills Volcano in 1997 (Ritchie et al 2002; Sparks et al 2002) Such lateral blasts are generally associated with large-scale catastrophic failure of part of the edifice of the volcano, which instantaneously unroofs pressurized magma and generates an immediate laterally directed explosion and high-velocity pyroclastic density currents (typically in excess of 100 m s−1). Depending on the volume of material ejected, explosion-derived pyroclastic density currents may or may not be formed, with lateral momentum introduced by the non-vertical component of explosive thrust

Findings

Conclusion

Acknowledgements and Funding