DESCRIPTION:
Soufriere Hills Volcano, Montserrat, West Indies

[Map,16K,InlineGIF]
Map, Soufriere Hills Volcano and Montserrat Island, West Indies, 1997

Soufriere Hills Volcano
Location: Montserrat Island
Latitude: 16.72 N
Longitude: 62.18 W
Height: 915 meters (3,010 feet)
Type: Stratovolcano
Latest Eruptions: Current eruption began July 18, 1995, and continues -- (written August, 1997)

Soufriere Hills volcano sits on the north flank of the older South Soufriere Hills volcano, located at the south end of Montserrat Island (13 x 8 kilometers). The summit area consists primarily of a series of ESE-trending lava domes. Block-and-ash flow and surge units associated with dome growth predominate in flank deposits. Pyroclastic-flow deposits associated with the formation of English's Crater have been dated at around 19,000 years BP (before present). A series of eruptions dated at 16,000-24,000 years BP pre-dates the Castle Peak dome in the crater by an unknown period of time. English's Crater is breached to the east. Periods of increased seismicity below Soufriere Hills were reported in 1897-98, 1933-37, and again in 1966-67. There were no reported historical eruptions, but some deposits and features have a young appearance. A radiocarbon date of ~320 ± 54 years BP from a northeast-flank pyroclastic-flow deposit is significantly younger than other radiocarbon dates from the volcano, and could have resulted from the latest activity of Castle Peak.

The complex andesitic Soufriere Hills volcano occupies the southern half of the island of Montserrat. The summit area consists primarily of a series of lava domes emplaced along a ESE-trending zone. Prior to 1995, the youngest dome was Castle Peak, which was located in English's Crater, a 1-kilometer-wide crater breached widely to the east. Block-and-ash flow and surge deposits associated with dome growth predominate in flank deposits. Non-eruptive seismic swarms occurred at 30-year intervals in the 20th century, but the first historical eruption on Montserrat did not take place until 1995. Long-term small-to-moderate ash eruptions were accompanied by lava dome growth and pyroclastic flows that initially forced evacuation of the southern half of the island and then destroyed the capital city of Plymouth.

Historical unrest on Montserrat has been in the vicinity of the Soufriere Hills, the youngest of many volcanic centers on this small island. The Soufriere Hills are a complex of andesitic domes; older andesitic domes and andesitic to basaltic lavas form nearby hills.

Historical unrest on Montserrat has been in the vicinity of the Soufriere Hills, the youngest of many volcanic centers on this small island. The Soufriere Hills are a complex of andesitic domes; older andesitic domes and andesitic to basaltic lavas form nearby hills. In mapping to date (MacGregor, 1938; Rea,1974), no evidence has been found that the Soufriere Hills lie within a caldera. Similarly, there is no suggestion from either mapping or geophysical studies that the Soufriere Hills are underlain by any large magma reservoir. A strong NW-SE lineament connects several vents, including the Soufriere Hills.

Lavas on Montserrat in general contain 50-63 percent SiO2; three samples from the Soufriere Hills proper had 58-63 percent SiO2. A few blocks of lava and pumice show compositional banding, 58 percent versus 62 percent SiO2 (Rea, 1974).

The southern half of Montserrat, in the vicinity of the Soufriere Hills Volcano, has experienced three volcano-seismic crises within the past century. A series of earthquakes occurred during 23-27 April 1897; one particularly strong event ( Modified Mercalli Intensity=8) occurred on 25 April 1897. Even stronger earthquakes of this episode occurred in October 1900, near the end of the unrest. Houses, churches, and windmills were heavily damaged on and near the northern and western flanks of the volcano, and solfataric activity intensified about the time of the earthquakes. Another strong earthquake occurred near Guadeloupe on 29 April 1897, suggesting that the Montserrat activity was related to regional tectonic strain. Perhaps coincidentally, the 1897-1900 earthquakes followed the heaviest rains and flooding in recent years, in 1896.

Beginning in March 1933, solfataras on the lower slope of Soufriere Hills began to show intensified activity. Earthquake swarms began in July 1933, and increased in frequency and intensity to a peak in November 1935. Several thousand shocks were felt on the island during 1933-1937. Those measured were shallow (1-2 kilometers, Perret, 1939). At least 28 were of "the seventh or eight magnitudes of the seismic scale" (Perret, 1939); the two strongest earthquakes of the series (12 December 1934 and 6 May 1935) had modified Mercalli intensities of 8 (Robson, 1964). Earthquake damage and solfataric activity were in essentially the same zone, north and northwest of the Soufriere Hills, as they had been in the 1897-1900 episode. The frequency of earthquakes varied seasonally, with maxima in May and November-December of each year. Increased H2S emission often preceded the 1933-1937 earthquake swarms by a few days, but it also intensified after the larger earthquakes. On 10 November 1935, a strong tectonic earthquake occurred 100 kilometers north of Montserrat; activity at Montserrat decreased thereafter, although it did not fully return to background levels until late 1937.

Earthquakes were felt in January and February 1966 (maximum Magnitude=4.0), and a seismic array was installed in March and April 1966. The epicentral region was broad and crudely elongate in a NW-SE direction, coincident with the lineament connecting the Soufriere Hills and several other vents. It is not certain whether this represents a slight southward shift relative to the activity of 1933-37, because the latter activity was not precisely located. In the 1966-67 swarm, earthquake depths varied systematically, averaging 5.2 kilometers in April and May 1966, rising to a minimum of 2.8 kilometers (average) in July-September, deepening to an average of 13.1 kilometers in April 1967, and ending at an average of 9.7 kilometers in November 1967. The total seismic energy release was 3 x 10^16 ergs, equivalent to a Magnitude 4.5 earthquake. The b-value of the earthquakes was 0.8 throughout the entire series. Seismic energy release reached seasonal maxima in May 1966, November-December 1966, and October-November 1967 (similar to the seasonality observed during 1933-37).

Three water-tube tiltmeters, located on the lower flanks of Soufriere Hills about 3-4 kilometers from the summit, indicated inflation centered just south of the summit area, coincident with the center of seismicity. Inflation was greated in July-October 1966 (maximum recorded rate of tilt=12 microradians/month). Inflation continued at a lower rate from October 1966 until January 1967, deflation occurred during January-March 1967, and inflation occurred again during March-September 1967. The net change was inflationary. Fumarolic activity increased sympathetically with inflation.

The Royal Society mission (Powell, 1937, 1938), Perret (1939), and Shepherd and others (1971) have unanimously inferred that the unrest has been due to episodic, small magmatic intrusions beneath the Soufriere Hills. Perret thought a (phreatic) explosion from one of the solfataras would be the most likely outcome, whereas Shepherd and others considered an eruption from the summit of the Soufriere Hills to be more likely.

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