R. Robertson1 & G. Wadge2
1Seismic Research Unit, UWI
2ESSC, University of Reading
The eruption of Soufriere Hills Volcano, Montserrat that began on 18 July 1995 continues in late 1996. This is the fourth in a series of volcano-seismic crises at the volcano, spaced at approximately 30 year intervals over the last 100 years. The earlier crises (1897-98?, 1933-37 and 1966-67) did not result in eruption. Prior to the current eruption there were a series of earthquake swarms beginning in January 1992, the most intense of which in June 1994 prompted increased seismic monitoring effort by SRU. The most intense of the swarms occurred during the period late November - early December 1994 with a significant peak on 30 November. However, the start of the eruption was not preceded by escalating activity and no warning was given.
For the first 2 months activity was of a purely phreatic character centred on the Castle Peak dome within English's Crater and accompanied by swarms of volcano-tectonic earthquakes. A number of vents were active at different times producing steam, non-juvenile ash and blocks, though the first vent (18 July) on the northwestern sector of the original dome was the most significant. There is some indication that a NW-trending fissure may underline this vent at depth. On 25 September a small, spine-like extrusion began to form on the southwest side of the dome though the phreatic activity continued throughout October and November, by now involving mudflows which inundated the Hot River and Tar River valleys to the east. Erosion by these flows exposed a series of charcoal-bearing block-and-ash flow deposits which gave radiocarbon ages spanning a range of about 300-400 years BP. These are assumed to be the deposits produced by the eruption that formed the Castle Peak Dome.
About 15 November the first unequivocal lava appeared at the surface from the July 18 crater, accompanied by "hybrid" earthquakes and surface deformation. From November through to late January the lava formed a new dome at a relatively slow rate. The effusion rate then increased by about an order of magnitude, again accompanied by "hybrid" earthquakes. The old dome was progressively buried in succeeding months, leaving only Castle Peak itself visible, though the height of this (ca. 2650 ft asl) was overtopped by the new dome in late February and which later became the highest point on the island. The locus of dome building moved from week to week with a series of spines developing and collapsing. The dome overtopped the Gages Wall col on the English's Crater rim on 11 June but did not send any appreciable material into the Gages ghaut. On 27 March the first pyroclastic flow was produced by collapse of the dome. In the following 6 months many tens of such flows descended the eastern slope of the volcano, covering the Tar River catchment and forming a fan in the sea. This activity was particularly intense in August-September 1996 following a further increase in effusion rate and culminating in an explosive eruption on 17/18 September which destroyed about a quarter of the dome, blasting hot ballistics to the northeast and pumice over much of southern Montserrat. After 2 weeks quiescence lava reappeared on 1 October and quiet dome building resumed though at a lower rate.
In summary, the Soufriere Hills eruption has so far developed as a classic Peleean dome-forming eruption. By Lesser Antilles standards Soufriere Hills is a moderate-sized, moderately active calcalkaline volcano whose current magma is typical of its previous output and typical of hornblende andesites throughout the arc.