4~dating has facilitated a substantial reinterpretation of the volcanic evolution of Montserrat. Three volcanic centres with non-overlapping volcanic activity are identified: Silver Hills (c. 2600 to c. 1200 ka); Centre Hills (at least c. 950 to c. 550ka); South Soufri~re Hills-Soufri6re Hills (at least c. 170ka to present). The geochronological data show that old xenocrysts are common in the porphyritic andesite, implying that reliable ages are best obtained by dating the groundmass. Soufri~re Hills evolved from early eruptions dominated by two-pyroxene andesite to eruptions of hypersthene-hornblende andesite at c. 110 ka. Between the two varieties of andesite there was an interlude of mafic volcanism at c. 130 ka to form South Soufri~re Hills. There is evidence of tectonic uplift of early products of the complex along with older submarine volcanic rocks. Consideration of stratigraphy and age data indicates that only a proportion of the dome-forming eruptions are recorded as domes in the geological record. Older products are removed from the subaerial edifice by sector-collapse events. The timeaveraged eruption rate of the South Soufri6re Hills-Soufri6re Hills centre is estimated at 0.005 m 3 S -1 (C. 0.15 km 3 ka -l) (dense rock equivalent). The ongoing eruption is very similar in style to previous activity at Soufri6re Hills, and future activity is likely to pose similar hazards. Soufri6re Hills have been characterized by alternations of periods of enhanced activity and periods of dormancy, both lasting of the order of 10 4 years. During periods of elevated activity several major dome-forming eruptions are separated by quiescent interludes lasting less than c. l03 years. The ongoing eruption may mark the onset of a fourth period of enhanced volcanic activity at Soufri6re Hills.

In 1997 Soufriére Hills Volcano on Montserrat produced 88 Vulcanian explosions: 13 between 4 and 12 August and 75 between 22 September and 21 October. Each episode was preceded by a large dome collapse that decompressed the conduit and led to the conditions for explosive fragmentation. The explosions, which occurred at intervals of 2.5 to 63 hours, with a mean of 10 hours, were transient events, with an initial high-intensity phase lasting a few tens of seconds and a lower-intensity, waning phase lasting 1 to 3 hours. In all but one explosion, fountain collapse during the first 10-20 seconds generated pyroclastic surges that swept out to 1-2 km before lofting, as well as high-concentration pumiceous pyroclastic flows that travelled up to 6 km down all major drainages around the dome. Buoyant plumes ascended 3-15 km into the atmosphere, where they spread out as umbrella clouds. Most umbrella clouds were blown to the north or NW by high-level (8-18 km) winds, whereas the lower, waning plumes were dispersed to the west or NW by low-level (<5 km) winds. Exit velocities measured from videos ranged from 40 to 140 ms-1 and ballistic blocks were thrown as far as 1.7 km from the dome. Each explosion discharged on average 3 x 105m3 of magma, about one-third forming fallout and two-thirds forming pyroclastic flows and surges, and emptied the conduit to a depth of 0.5-2 km or more. Two overlapping components were distinguished in the explosion seismic signals: a low-frequency (c. 1 Hz) one due to the explosion itself, and a high-frequency (>2 Hz) one due to fountain collapse, ballistic impact and pyroclastic flow. In many explosions a delay between the explosion onset and start of the pyroclastic flow signal (typically 10-20 seconds) recorded the time necessary for ballistics and the collapsing fountain to hit the ground. The explosions in August were accompanied by cyclic patterns of seismicity and edifice deformation due to repeated pressurization of the upper conduit. The angular, tabular forms of many fallout pumices show that they preserve vesicularities and shapes acquired upon fragmentation, and suggest that the explosions were driven by brittle fragmentation of overpressured magmatic foam with at least 55 vol% bubbles present in the upper conduit prior to each event.

Eruption of the Soufri+re Hills Volcano on Montserrat allowed the detailed documentation of a Pel~an dome-forming eruption. Dome growth between November 1995 and March 1998 produced over 0.3km 3 of crystal-rich andesitic lava. Discharge rates gradually accelerated from < 1 m 3 s -1 during the first few months to >5 m 3 s -1 in the later stages. Early dome growth to September 1996 was dominated by the diffuse extrusion of large spines and mounds of blocky lava. A major dome collapse (17 September 1996) culminated in a magmatic explosive eruption, which unroofed the main conduit. Subsequent dome growth was dominated by the extrusion of broad lobes, here termed shear lobes. These lobes developed through a combination of exogenous and endogenous growth over many weeks, with movement accommodated along curved shear faults within the dome interior. Growth cycles were recognized, with each cycle initiated by the slow emplacement of a large shear lobe, constructing a steep flank on one sector of the dome. A growth spurt, heralded by the onset of intense hybrid seismicity, pushed the lobe rapidly out, triggering dome collapse. Extrusion of another lobe within the resulting collapse scar reconstructed the steep dome flanks prior to the next cycle.

Pyroclastic flows were formed at Soufrière Hills Volcano by lava-dome collapse and by fountain collapse associated with Vulcanian explosions. Major episodes of dome collapse, lasting tens of minutes to a few hours, followed escalating patterns of progressively larger flows with longer runouts. Block-and-ash flow deposit volumes range from <0.1 to 25 x 106 m3 with runouts of 1-7 km. The flows formed coarse-grained block-and-ash flow deposits, with associated fine-grained pyroclastic surge deposits and ashfall deposits. Small flows commonly formed lobate channelized deposits. Large block-and-ash flows in unconfined areas produced sheet-like deposits with tapering margins. the development of pyroclastic surges was variable depending on topography and dome pore pressure. Pyroclastic surge deposits commonly had a lower layer poor in fine ash that was formed at the current front and an upper layer rich in fine ash. Block-and-ash flows were erosive, producing striated and scoured bedrock surfaces and forming channels, many metres deep, in earlier deposits. Abundant accidental material was incorporated. Pyroclastic flow deposits formed by fountain collapse were pumiceous, with narrow sinuous, lobate morphologies and well developed levees and snouts. Two coastal fans formed where pyroclastic flows entered the sea. Their seaward extent was limited by a submarine slope break.

Abstract. The andesite lava currently erupting at the Soufriere Hills volcano, Montserrat, contains ubiquitous mafic inclusions which show evidence of having been molten when incorporated into the andesire. The andesire phenocrysts have a range of textures and zonation patterns which suggest that non-uniform reheating of the magma occurred shortly before the current eruption. Reheating resulted in remobilisation of the resident magma and may have induced eruption.