Abstract This paper describes ash-venting activity at Soufrière Hills Volcano, Montserrat that was precursory to the onset of three phases of lava extrusion in 2005, 2008 and 2009, and similar ash venting that occurred during the fifth phase of lava extrusion. We describe in detail a style of mild, tephra-generating activity termed ash venting and its associated tephra products. The nature of the seismicity associated with ash venting is compared with that of explosive activity. All explosive events, from small explosions to large Vulcanian explosions, have impulsive, low-frequency onsets. These are absent in ash-venting events, which have subtle, emergent onsets. Microscope and grain-size analyses show that ash-venting events and large Vulcanian explosions generate tephra that is similar in grain size (in medial and distal regions), although phreatic events in 2005 were finer grained. Ash-venting products are either composed of fine-grained, variably altered pre-existing material or juvenile material. There is a general correlation between the length of the pause and the length of the period of precursory activity prior to lava extrusion following it. Syn-extrusive ash venting is frequently associated with short-term increases in extrusion rate and is considered to be related to shear-induced fragmentation at the conduit margin.

Abstract Vulcanian explosions generated at Soufrière Hills Volcano between 2008 and 2010 varied from simple events involving minimal pyroclastic density currents (PDCs) to complex events involving more than one explosion. Calculated volumes for the deposits of the PDCs formed by these explosions ranged up to 2.7×10 6 m 3 , with more than half the explosions having volumes greater than 1×10 6 m 3 . The deposits formed by the explosions varied in lithology, with some explosions generating pumice-rich PDCs (e.g. 29 July 2008 and 11 February 2010) showing development of sinuous lobes. These explosions are similar to those formed in 1997, with gas-rich, conduit-derived magma being the dominant driving mechanism. Other explosions were pumice-poor ( c. 5 wt% pumice) and generated morphologically distinct PDC deposits. Many of the pumice-poor explosions were associated with lower tephra plumes of <8 km, but were some of the largest volume events in terms of PDC production and suggest a generation mechanism involving destruction of significant quantities of the lava dome. Analysis of video footage shows that PDC formation was pulsatory, probably related to destabilization of portions of the lava dome during the initial phases of the explosion.

Abstract Extrusion during Phase 5 (8 October 2009–11 February 2010) produced significant volumetric and geomorphic changes to the lava dome and surrounding valleys at the Soufrière Hills Volcano, Montserrat. Approximately 74×10 6 m 3 of lava was extruded at an average rate of 7 m 3 s −1 during the short period of activity. Addition of lava to the pre-existing dome resulted in a net volumetric increase of up to 38×10 6 m 3 . Pyroclastic density current (PDC) and ashfall deposits accounted for the remaining 36×10 6 m 3 . A series of thick, blocky lobes were extruded from a central vent. In addition, several short-lived spines and two large shear lobes were also extruded. Significant PDC activity resulted in substantial valley filling of up to 108 m. The large pre-existing dome significantly influenced the growth of lobes, such that many block-and-ash flows were generated from viscous lobes draped over the summit and upper slopes. Geomorphic changes caused by rapid filling of the surrounding valleys aided in both flow avulsion and the emplacement of deposits up to 6 km from the dome. These geomorphic changes have important consequences for hazards from PDCs.

Abstract To solve the problem of lava dome growth at Soufrière Hills Volcano (SHV) being invisible and unmeasured owing to cloud, we have designed, built and deployed a ground-based millimetre-wave radar/radiometer: the All-weather Volcano Topography Imaging Sensor (AVTIS). In this chapter, after an outline technical sketch of the instruments, we describe the campaigns between 2004 and 2011 used to test their capabilities. We then present results from the campaigns to illustrate how signals of volcanological interest can be retrieved. The primary measurements of AVTIS are range (to within, at best, about 1 m), and, from that, topography, topographical change and effusion rates, and surface temperature (to within a few degrees Celsius). Changes in radar reflectivity can indicate surface processes (e.g. mass wasting). Surface motion within the instantaneous field of view produces a Doppler signal that allows detection of rockfall. Attenuation of the signal by rain along the path can, when stacked temporally, give an image of rain cloud structure and, by calibration, a rate of rainfall. We regard a strategy of two radars – one permanantly mounted (at Windy Hill) autonomous instrument, and the other used as a rover – as being best for capturing dome growth.