This report covers a six-and-one-half month period, starting from the major dome collapse on 20 July 1999 (BGVN 24:07) and extending to 4 February 2000. Tremor, explosions, ash venting, and pyroclastic flows occurred frequently during the period from the 20 July dome collapse through late November. Dome growth continued through the end of the interval. Ash clouds were frequent during this period, some rising to above 3 km and several with maximum heights exceeding 6 km. The volcano continued seismic and volcanic activity including rockfalls and abundant earthquakes of several types. The number and intensity of these have varied, but events of all types have been reasonably contiguous throughout the ensuing period. Visibility of the dome area was obscured until early November when the crater was visible for the first time during the early reporting period. Subsequent observations in late November revealed new dome material within the 3 July 1998 dome scarp, the first growth observed since March 1998. This new lava dome continued to grow. Episodes of tremor also began in November (see below).

Figure 46 shows a photo of the dome in December 1999. Early in that month it was estimated to be ~200 m long by 80 m wide and ~70 m high as its widest point. The two small spines at the top were ~8 m high.

Figure 46. Soufriere Hills breached crater and growing dome in December 1999 as seen from the SE side. Courtesy of MVO.

Dome growth continued through February; in the last week of January clear weather permitted sufficient measurements and photographs to estimate the dome's volume. Growth rates were above ~3 m³/s for December and early January. This appears consistent with increases in seismicity witnessed during 28 January to 4 February 2000. The first pyroclastic flow from the dome during the reporting interval occurred on 2 February; resulting deposits were seen on 3 February in the S part of the Tar River valley and most of the way across the delta.

During the more than six-month period since the collapse seen on 20 July 1999, rockfalls took place at rates of over 100/week (326 during the last week of January) to as few as 16/week. Weekly earthquake events also varied as shown in table 33. Small earthquake swarms (e.g., 20 events) occurred at numerous times, but were usually of low intensity. A swarm of 213 recorded events occurred during the period 3-8 November; those that could be located occurred at estimated depths between 1.5 and 2 km.

Table 33. A summary of Soufriere Hill's weekly earthquakes during 20 July 1999 through 4 February 2000 emphasizing the maximum and minimum number of events each week. Courtesy of MVO.

On 23 November 1999, cyclic bands of tremor began. They were continuing as of the end of this reporting period. Overall, more than 100 tremor episodes occurred during the reporting period. On average, a cycle included 5 hours and 40 minutes of seismic quiescence followed by 3 hours of tremor. These tremor bands were associated with periods of dome growth, with an increasing amplitude of the seismic peak within each cycle. The slow increase in background long-period seismicity suggested that the dome growth rate was increasing.

Average daily SO₂ flux estimates were made when weather conditions permitted measurement. The results typically varied from about 200 to 600 tons/day, with some peak values exceeding 1,500 tons/day.

Background. A recent reprint volume has been compiled to provide a single source of scientific information on the Montserrat eruption up to late 1997 (Young and others, 1998). In addition, Rozdilsky (1998) discusses some social impacts of the ongoing crisis. The destructive events that began in July 1995 led to the evacuation of ~70% of the population of 11,000 people under emergency conditions. In other words, 7,243 people from 22 settlement areas were evacuated. The settlements were subsequently damaged or destroyed; however, due to the evacuations less than 25 deaths were attributed to the dome collapse.

During 1999, extensive planning for redevelopment of the safer, northern portion of the island was undertaken by the governments of Montserrat (a British Overseas Territory) and the United Kingdom. As a future target, 10,000 persons (approximately the island's population prior to the volcanic crisis) could be relocated at seven new activity centers in N Montserrat.

References. Rozdilsky, J.L., 1999, Disaster recovery in an on-going hazard situation on Montserrat: the July 20, 1999 volcanic dome collapse (preliminary abstract): Ph.D. dissertation, Dept. of Resource Development and Interdepartmental Urban Affairs, Michigan State University.

Young, S. R., Voight, B., Sparks, R.S.J., Rowley, K., Robertson, R.E.A., Lynch, L.L., and W. P. Aspinal (conveners), 1998, Selected papers on the eruption of the Soufriere Hills Volcano, Montserrat: reprints from the Geophysical Research Letters, v. 25, nos. 218 and 219, published by the American Geophysical Union (1998), 3387-3700 p. (ISBN-0-87590-919-1).

Geological Summary. The complex, dominantly andesitic Soufrière Hills volcano occupies the southern half of the island of Montserrat. The summit area consists primarily of a series of lava domes emplaced along an ESE-trending zone. The volcano is flanked by Pleistocene complexes to the north and south. English's Crater, a 1-km-wide crater breached widely to the east by edifice collapse, was formed about 2000 years ago as a result of the youngest of several collapse events producing submarine debris-avalanche deposits. Block-and-ash flow and surge deposits associated with dome growth predominate in flank deposits, including those from an eruption that likely preceded the 1632 CE settlement of the island, allowing cultivation on recently devegetated land to near the summit. Non-eruptive seismic swarms occurred at 30-year intervals in the 20th century, but no historical eruptions were recorded until 1995. Long-term small-to-moderate ash eruptions beginning in that year were later accompanied by lava-dome growth and pyroclastic flows that forced evacuation of the southern half of the island and ultimately destroyed the capital city of Plymouth, causing major social and economic disruption.

Information Contacts: Montserrat Volcano Observatory (MVO), Mongo Hill, Montserrat, West Indies (URL: http://www.mvomrat.com/).