An inclined Vulcanian explosion and associated products

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 Soufrière 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.

Dome Collapse Interaction With the Atmosphere

“…Relative thermal intensity calculated by integrating the camera FOV above the 30 °C threshold (Figure ) indicates at least two clear thermal pulses at 17:20:14 and 17:21:40, lasting 74 and 42 s, respectively, which seems associated with two acoustic peaks at 41 and 28 Pa, respectively. Evidence of at least two pulses during the explosive phase were also reported by Cole et al, , on the base of thermal recordings. The explosive injection of mass and thermal energy into the atmosphere has triggered gravity waves, which amplitude increases to higher values (up to 22 Pa, Figure a) with respect to those observed during the PDCs activity (Figure ).…”

Reference data set of volcanic ash physicochemical and optical properties

“…The deviation in composition in the intermediate range (basaltic‐andesitic to dacitic ash) as shown in Figure can be the result of several processes such as fractional crystallization of a mafic parent magma, partial melting of crustal material, or magma mixing between felsic rhyolitic and mafic basaltic magmas in a magma reservoir. Our measured compositions of the individual ash sample are confirmed by earlier studies where the same samples were already analyzed and their compositions determined [e.g., Bayhurst et al ., ; Adams et al ., ; McGimsey et al ., ; Alfano et al ., ; Gislason et al ., ; Cole et al ., ].…”

“…The Soufríere Hills volcano (SOU) ashfall sample from the February 2010 dome collapse pyroclastic density column was collected ~5 km NE of the eruption by the Montserrat Volcano Observatory. The eruption plume reached altitudes up to 15 km, where an umbrella cloud was formed and dispersed by SE winds [ Cole et al ., ]. The Eyjafjallajökull (EYJ) ashfall sample from the 2010 eruption was collected ~35 km SW of the volcano by the Institute for Earth Sciences of the University of Iceland.…”

Ash aggregation during the 11 February 2010 partial dome collapse of the Soufrière Hills Volcano, Montserrat

“…1). These PDCs then continued north draining into the Farm River valley and moved eastwards and then northeastwards towards Trant's (Cole et al 2015). The deposits were emplaced by multiple PDC pulses.…”

“…The total activity can be summarized by six peaks in PDC generation and two Vulcanian explosions. The second, larger Vulcanian explosion was inclined 25° from the vertical in an N-NE direction (Cole et al 2015) and generated a tephra plume that rose to about 15 km above sea level. The origin of the dome collapse was the piecemeal failure of a series of large, unstable lobes that had been recently emplaced on the northern flank of the lava dome (Stinton et al 2014b).…”