THE GEOLOGY OF MONTSERRAT

Montserrat island (with an area of 102 sq.km and a pre-1995 eruption population of 10,500,  is composed of three major volcanic centers, ranging in age from Pleistocene to Recent  The northern end of the island is formed by the Silver Hill complex described by Rea (1970, 1974) as composed of two-pyroxene andesite lavas and agglomerates.  39Ar/40Ar dating of these lavas have yielded ages of 2,580±60 ka and 1,160±46 ka (Harford et al., 2002). (The notation ka means thousands of years, so these ages could also be written as 2,640,000 to 2,520,000 years and 1,206,000 to 1,114,000 years). The central part the island is dominated by the Centre Hills complex characterized by steep-sided peaks (remnants of Pelean domes) surrounded by aprons of block and ash flow and surge deposits, a distinctive pumice lapilli fall deposit, reworked equivalents of the pyroclastic deposits as well as mudflow/lahar deposits. Petrographically the complex is composed of two-pyroxene and hornblende-hypersthene andesites  (Rea, 1970, 1974), and has yielded 39Ar/40Ar ages of 954±12 ka, 871±10 ka, 826±12 ka, 663±49 ka, and 550±23 ka (Harford et al., 2002).

The southern part of the island consists of the South Soufriere Hills – Soufriere Hills complex (SSH-SH complex) of two morphologically distinct centers (see map). The SSH center measures 4 km east-west by 2.5 km north-south and is bounded by the White River in the west and tributaries of Dry Ghaut in the north. It has a summit crater with a north-south dimension of about 1 km and is open to the east. The crater is occupied by the Raspberry Hill dome complex composed of two nested domes, the oldest in the west and the youngest in the east (Rea, 1970). The deposits from the SSH have been 39Ar/40Ar dated at 131±7 ka ( Landing Bay ), 130±5 ka (Roche's Bluff), 129±14 ka (Shoe Rock), 128±17 ka (Roche's Bluff), and 128±27 ka (White River Pyroclastic Fall series)(Harford et al., 2002).
The active SH center, measuring 9 km east-west by 6 km north-south is larger and more complex than the SSH, and before the present eruption rose to an altitude of just over 1000m at Chance's Peak. The superstructure of this center comprises four domes, Gage's, Chance's, Galway 's and Perche's.  On its eastern flanks is the steep-walled elliptical depression of English's Crater that contained the Castle Peak dome. English's Crater is now believed to be a sector or flank collapse scar with a corresponding submarine avalanche deposit offshore in the Atlantic . The Castle Peak dome was destroyed during the present eruption and has now been replaced by the 1995-present dome. Since European settlement in 1635 AD (Fergus, 1994), southern Montserrat has undergone four episodes of volcanic activity. The first three were volcano-seismic crises, in 1897-98 (Robson and Tomblin, 1966; Wadge and Isaacs, 1988), 1933-37 (Perret, 1939) and 1966-67 (Shepherd et al., 1971). The fourth episode which started in 1992 also involved an initial increase in seismic activity (Ambeh et al., 1995), but on August 18, 1995 explosive pyroclastic activity centered on English's Crater commenced (Young et al., 1998), and is still on.  39Ar/40Ar dating has provided weighted mean ages of 151±4 ka for Gage's dome, 112±9 ka for Galway 's dome, and 24±2 ka for Perche's dome (Harford et al., 2002). No ages were obtained for Chance's dome but it is believed by Harford, based on petrography, to be younger than 110 ka  (Harford et al., 2002). An age of 331±35 ka (Harford et al., 2002) from the xenocrystal component of Gage's dome suggests that the current superstructure of the SH is underlain by deposits from an earlier phase of activity.
The four small hills, Garibaldi Hill, Richmond Hill and St. George's Hill on the southwest side of the island, and Roche's Bluff on the southeastern coast are now regarded as uplifted volcanic sequences (Harford et al., 2002) rather than parasitic centers (Rea, 1970,1974). Garibaldi Hill has been 39Ar/40Ar dated at 282±8 ka, and is thought to represent part of the older superstructure of the SH. The deposits below Roche's Bluff lava flow (dated at 130 ka) have yielded an 39Ar/40Ar age of 1021±20 ka and are now interpreted as forming part of an intensely disrupted submarine fan from the Centre Hills complex (Harford et al., 2002), rather than part of a parasitic center of the SSH (Rea, 1970, 1974).

Montserrat island is built on the south-central part of an elliptical submarine bank which at the 200m isobath measures 15 km east-west by 25 km north-south. The submarine bathymetry around the island, as well as the subaerial morphology and the location of epicenters associated with both the 1966-67, and 1992-95 seismic activity suggests that the island is traversed by a number of fault zones. One of these is morphologically represented by a linear topographic feature that crosses the island in an ENE-WSW direction from Farm River on the east coast to Bransby Point on the west. Earthquake epicenters, recorded between 1966 and 1967 (Tomblin, 1972), in the vicinity of Antigua , could define the easterly submarine extension of this fault. A possible westerly extension forms the northern boundary of the Montserrat Rise or Spur, a major submarine ridge extending 50 km WSW of the island (Fig. 3). Earthquake epicenters, particularly for the period July 27 to August 6, 1995 formed a belt parallel to this proposed fault trace (Aspinall et al., 1998).

The surface expression of a second possible fault zone which trends NW-SE can be seen as a scarp on the south side of Roche's Bluff on the southeast coast and is particularly noticeable in the cliff line on the west coast at Old Road Bluff.  The trace of this fault zone passes directly beneath SH. South of this fault line an 800m wide belt of topographic lows can be distinguished, represented by Dry Ghaut in the east, and the Belham River on the west. It has been proposed that prehistoric activity within this fault zone was responsible for the uplift of St. George's , Richmond and Garibaldi Hills as well as Roche's Bluff (Harford et al., 2002). More recent activity is suggested by the locations of earthquake epicenters during both the 1966-67 seismic crisis (Shepherd et al., 1971), and the initial stages of the current eruption, particularly during the period July 29 to August 15, 1995 (Aspinall et al., 1998). The intense hydrothermal alteration of the uplifted tuffs and sediments exposed on the east coast near Roche's Bluff may also be associated with this fault. The trend of this fault is parallel to the trend of the "en echelon" fault zone formed by the Redonda, Bouillante-Montserrat, and Les Saintes fault systems described by Feuillet et al. (2002). GPS ground deformation studies of the Soufriere Hills volcano from October 1995 to July 1996 have suggested that up to 1m of expansion recorded during the current eruption was caused by the shallow (<3km) injection of a vertical dike into this zone (Mattioli et al., 1998).
Bathymetic studies around the island by Deplus and other (2001) have identified three separate debris avalanche deposits  that radiate away from the island.  The source areas of a fan to the southwest is just north of the now destroyed capital of Plymouth, a second to the south has a source in the White River area and a third to the west has a source in the tar River area which has to be the sector collapse scar of English's Crater.   The debris fans are smaller than those around south Dominica and are in shallower water at depths of less than 1000m.   The fans extend up to 20km from the shoreline and have megablocks up to 400m across and 40m high.

MONTSERRAT ERUPTION SEQUENCE OF EVENTS 1992-2005