MVO/VSG - Open Scientific Meeting
27 November 1996

Erupted Volume, Growth Rate and Morphology of the Dome

Team Volume: J. Barclay1, S. Bower2, E.J. Calder1, P.D. Cole3, B. Derroux4, C. Harford1, R. Herd5, M. James6, A.-M. Lejeune1, G.E. Norton5, J.B. Shepherd6, G. Skerritt4, R.S.J. Sparks1, M.V. Stasiuk6, N.F. Stevens7, G. Thompson8, J. Toothill6, G. Wadge7, R. Watts5 and S.R. Young5

1Geology Department, Wills Memorial Building, University of Bristol, Bristol BS8 1RJ
2School of Mathematics, University of Bristol, Bristol BS8 1TW
3Department of Geology, University of Luton, Park Square, Luton, Beds. LU1 3JU
4Montserrat Volcano Observatory, Old Towne, Montserrat, West Indies
5British Geological Survey, Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG
6Environmental Science Division, IEBS Bldg, University of Lancaster, Lancaster LA1 4YQ
7Environmental Systems Science Centre, University of Reading, Reading RG6 6AB
8Department of Earth Sciences, University of Leeds, Leeds LS2 9JT

Introduction

Lava extrusion during the current episode of volcanic activity at the Soufriere Hills Volcano in Montserrat began on the 15th of November 1995 and has continued to the present day, forming a lava dome which has reached a maximum size of 30x106m3. Throughout its growth measurements have been made of the volume of the dome and its associated deposits and this has become a critical part of the monitoring effort, providing estimates of lava extrusion rate and current levels of activity.

History of Dome Growth

The history of the dome can be split into four periods, each characterised by differing extrusion rates and contrasting styles of activity.

  1. From the start of lava extrusion in November 1995 until the middle of January 1996, dome growth was relatively slow, with extrusion rates of 0.2-0.3m3s-1. During this time, lava was extruded quietly resulting in steady dome growth and frequent spine formation.
  2. In the middle of January 1995, an increase in extrusion rate to about 2m3s-1 occurred, accompanied by a swarm of hybrid earthquake activity. This higher rate of growth continued with low levels of seismic activity until July 1996, when a slight lessening in extrusion rate was observed, along with a shift in the active region of the dome from the north to the south. Throughout the whole of this period, dome growth was accompanied by intermittent rockfall events, which at times became semi-continuous and gave rise to small pyroclastic flows in the Tar River valley. The volume of the deposits formed by these flows has been estimated using DEMs.
  3. A series of volcano-tectonic earthquakes on the 20th/21st of July marked the start of the third period of dome growth, which was accompanied by renewed activity on the north-eastern flank of the dome and an increase in extrusion rate. Activity over the following two months was characterised by gravitational dome collapse on the north-eastern side, followed by high growth rates leading to instability and further collapse. During this time, the volume of the dome was highly variable due to the nature of the activity. Dome volume estimations were combined with measurements of the volumes of the deposits formed in the collapse events to yield an average extrusion rate for this time of around 4m3s-1, although growth rates immediately following the collapse of the 29th-31st of July may have reached as high as 10m3s-1. The cyclic pattern of behaviour continued until the 17th of September, when substantial rockfalls gave rise to an explosion which is estimated to have removed around 9x106m3 of material from
  4. On the 1st of October, a new dome was observed to be growing in the scar formed on the 17th of September. Initially this dome was extruded quietly and steadily with little seismic activity, but more recently a reduction in growth rate has been accompanied by intense VT activity.

Surface Morphology of the Dome

Throughout its growth, the dome has had an irregular and blocky surface and Peleean style morphology, with talus slopes mantling a central region of dense rock. The area of active growth on the dome has largely been concentrated in the north-east, but shifts to the south have occurred, notably before the increase in extrusion rate of the 20th/21st July. Both endogenous and exogenous growth has been observed. In the early stages of dome growth, exogenous growth was especially important and characterised by the extrusion of dense, smooth lava as spines, which subsequently collapsed to form large irregular blocks and slabs on the talus slopes. During early July 1996, a well-defined "half-crease" structure was observed on the summit of the dome, forming slabs of rock with smooth convex surfaces which were pushed southwards by further lava extrusion and fell down the southern flanks of the dome. The first scoriaceous material to be erupted was observed on the dome after the 21st of July, coincident with the increase in extrusion rate which occurred at this time. Throughout August and September the north-eastern side of the dome was dominated by the scars carved out by collapse events. New growth within these scars and on the summit of the dome was initially blocky and irregular until gravitational collapse turned the larger blocks into rubble. The last of these scars formed in the 17th/18th of September explosion and was subsequently filled by the October 1st dome, which was initially extruded as light grey lava which appeared to be more fluid in nature than previously erupted lava, but has since become more rubbly in appearance and now has a very similar morphology to earlier domes.

Implications for Hazard

In all but the earliest stages of its growth, the overall shape of the dome has been tightly controlled by the constraints of English's Crater. This has had important implications from a hazard point of view since pyroclastic activity has been largely confined to the Tar River valley. Comparisons with historic eruptions in the Soufriere Hills and with similar eruptions elsewhere in the world indicate that activity here may last for a number of years and it is important that measurements of erupted volumes and studies of the morphology of the dome are continued in order to provide us with an idea as to what form future activity may take.

Montserrat Volcano Observatory