Chloe L. Harford

Submarine evidence for large-scale debris avalanches in the Lesser Antilles Arc

Results from a recent marine geophysical survey demonstrate the importance of the process of flank collapse in the growth and evolution of volcanoes along an island arc. The Aguadomar cruise, aboard the French R/V L’Atalante, surveyed the flanks of the Lesser Antilles Arc between the islands of Montserrat and St. Lucia. Analysis of the data shows that flank collapse events occurred on active volcanoes all along the arc and resulted in debris avalanches, some of them being of large magnitude. The debris avalanche deposits display hummocky topography on the swath bathymetry, speckled pattern on backscatter images, hyperbolic facies on 3.5 kHz echosounder data and chaotic units on air gun seismic profiles. They extend from horseshoe-shaped structures previously identified on the subaerial part of the volcanoes. In the southern part of the arc, large-scale debris avalanche deposits were identified on the floor of the Grenada Basin west of active volcanoes on Dominica, Martinique and St. Lucia. The extent of debris avalanche deposits off Dominica is about 3500 km2. The debris avalanches have resulted from major flank collapse events which may be mainly controlled by the large-scale structure of the island arc and the presence of the deep Grenada Basin. In the northern part of the arc, several debris avalanche deposits were also identified around the island of Montserrat. With smaller extent (20–120 km2), they are present on the east, south and west submarine flanks of Soufriere Hills volcano which has been erupting since July 1995. Flank collapse is thus a recurrent process in the recent history of this volcano. The marine data are also relevant for a discussion of the transport mechanisms of debris avalanches on the seafloor surrounding a volcanic island arc.

Magma production and growth of the lava dome of the Soufriere Hills Volcano, Montserrat, West Indies: November 1995 to December 1997

From November 1995 to December 1997 a total volume of 246 × 106 (DRE) m³ of andesite magma erupted, partitioned into 93 × 106 m³ of the dome, 125 × 106 m³ of pyroclastic flow deposits and 28 × 106 m³ of explosive ejecta. In the first 11 weeks magma discharge rate was low (0.5 m³/s). From February 1996 to May 1997 discharge rates have averaged 2.1 m³/s, but have fluctuated significantly and have increased with time. Three pulses lasting a few months can be recognised with discharge rates reaching 3 to 8 m³/s. Short term pulsations in growth lasting a few days reach discharge rates of over 10 m³/s and there are periods of days to a few weeks when dome growth is < 0.5 m³/s. Discharge rate increased from May 1997 with an average rate of 7.5 m³/s to December 1997. The observations indicate an open magmatic system.

Episodes of cyclic Vulcanian explosive activity with fountain collapse at Soufrière Hills Volcano, Montserrat

Abstract 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.

Deposits from dome-collapse and fountain-collapse pyroclastic flows at Soufrière Hills Volcano, Montserrat

abstract 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.

Geomorphological evolution of Montserrat (West Indies): importance of flank collapse and erosional processes

Analysis of topography and new swath bathymetry as well as geophysical data provides information about aerial and submarine morphological features and mass transfer processes on Montserrat. The island has a characteristic shallow (<100 m) submarine shelf, interpreted as having been formed through erosion with a depth controlled by glacio-eustatic sea-level variation. Several debris avalanche deposits are identified on the lower submarine flanks of Soufrière Hills volcano, and there is evidence of lateral collapses at the older volcanic centres. The morphological evolution of Montserrat is interpreted in terms of three stages. The first stage comprises submarine growth. The second stage, subaerial growth, is represented by the active South Soufrière Hills–Soufrière Hills volcanic centre. During the current eruption of Soufrière Hills volcano (1995–2002) more than half of the lava erupted was transported into the sea. Flank collapses occurred several times during this stage, such as the Englishs Crater event (c. 4000 years ago) or the Boxing Day event during the current eruption (26 December 1997). Montserrats older volcanic centres, the Centre Hills and Silver Hills, illustrate the third stage of evolution, extinction and erosion. Magma production, long-term erosion and total sedimentation rates on Montserrat have been estimated as 0.17 km3 ka−1, 0.0125 km3 ka−1 and 0.11 km3 ka−1 (i.e. 1.1 cm ka−1), respectively.

The explosive eruption of Soufriere Hills Volcano, Montserrat, West Indies, 17 September, 1996

On 17 September 1996 the Soufriere Hills Volcano started a 9 hour period of dome collapse involving 11.7 × 106 m³ (DRE) of lava. After 2.5 hours of quiescence a sustained explosive eruption began. Estimated eruption parameters are: plume height at least 11.3 km and a maximum of 15 km; 180 m/s for launch velocities of ballistic clasts; peak explosion pressure of 27.5 MPa; magma water contents of 2.5–5%; magma discharge rates from 2300 to 4300 m³/s; ejecta volume of 3.2 (± 0.9) × 106 m³ (DRE). Ejecta consists of pumice (ρ =1160 kg/m³), higher density vesiculated ejecta (ρ =1300 to 2000 kg/m³), dense glassy clasts (ρ =2600 kg/m³), breccias cut by tuffisite veins and hydrothermally altered lithics. The ejecta are interpreted as a conduit assemblage with evacuation of the conduit down to depths of about 3 to 5 km. The eruption was triggered by unloading of a gas pressurised conduit due to dome collapse.

The volcanic evolution of Montserrat using 40 Ar/ 39 Ar geochronology

Abstract 40Ar/39Ar 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. 550 ka); South Soufrière Hills-Soufrière Hills (at least c. 170 ka 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. Soufrière Hills evolved from early eruptions dominated by two-pyroxene andesite to eruptions of hypersthene-hornblende andesite at c. llOka. Between the two varieties of andesite there was an interlude of mafic volcanism at c. 130ka to form South Soufrière 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 time-averaged eruption rate of the South Soufrière Hills-Soufrière Hills centre is estimated at 0.005 m3 s-1 (c. 0.15 km3 ka-1) (dense rock equivalent). The ongoing eruption is very similar in style to previous activity at Soufrière Hills, and future activity is likely to pose similar hazards. Soufrière Hills have been characterized by alternations of periods of enhanced activity and periods of dormancy, both lasting of the order of 104 years. During periods of elevated activity several major dome-forming eruptions are separated by quiescent interludes lasting less than c. 103 years. The ongoing eruption may mark the onset of a fourth period of enhanced volcanic activity at Soufrière Hills.

Recent remobilisation of shallow-level intrusions on Montserrat revealed by hydrogen isotope composition of amphiboles

Abstract Ion probe measurements of hydrogen isotopes were made on amphiboles representing different stages of the ongoing eruption of the Soufriere Hills Volcano, Montserrat. The majority (80%) of the andesitic amphiboles show relative intra- and inter-crystal δD homogeneity, with a mean of −38±12‰, consistent with known primary magmatic values. The remainder (20%) of amphiboles show marked δD heterogeneity, with a mean of −6±30‰. The heterogeneous amphiboles have ∼100 μm rims with δD values similar to the homogeneous crystals, but core values which are significantly heavier than primary magmatic values. Early in the eruption 50% of amphiboles are isotopically heterogeneous but post spring 1996 all amphiboles are homogeneous. We interpret these ion probe data in terms of development of a shallow andesitic magma chamber by repeated emplacement of andesitic magma in the shallow crust, over at least the last 100 yr. We suggest such shallow intrusions, emplaced during the volcano–seismic crises preceding the ongoing eruption, solidified. Some regions interacted with hydrothermal fluids and isotopic exchange took place generating the heavy hydrogen isotope signatures in the heterogeneous amphiboles. On onset of the ongoing eruption, such old igneous material was remobilised, with intimate mixing between the new magma batch and the old intrusions indicated by the close proximity of homogeneous and heterogeneous crystals. The isotopically primary magmatic rims on the heterogeneous crystals indicate hydrogen isotope exchange over a period of a few weeks, consistent with timescales of magma ascent at the Soufriere Hills Volcano. Once the old igneous material was flushed out, by spring 1996, the eruption proceeded by extruding material dominated by the new batch of magma. This resulted in a marked increase in extrusion rate.

Eyewitness accounts of the 25 June 1997 pyroclastic flows and surges at Soufrière Hills Volcano, Montserrat, and implications for disaster mitigation

Abstract Eyewitness and survivor accounts allow reconstruction of the sequence of events on 25 June 1997, when a sustained partial collapse of the lava dome occurred leading to the death of 19 people. An unsteady pyroclastic flow was generated with three distinct pulses. The third flow pulse caused most of the damage to infrastructure and most, if not all, of the casualties. Pyroclastic surges detached along most of the path of the third flow pulse, and one travelled 70 m up an adjacent hillside. Observations were made that will be important for the development of mitigation measures at future events involving high-temperature flows and surges. Temperatures remained high (300-400°C) at the periphery of the most voluminous and extensive surge, even though dynamic pressure and velocity were low, causing the death of seven victims. Some people survived at the margins of the surge zone but suffered serious burns when they were forced to walk across the hot surge deposits to safety. Deflagration of buildings and vegetation was immediate within the pyroclastic surge and intense fires burned long after the volcanic activity had ceased. Fires could be a serious secondary hazard in an urban area. Search-and-rescue efforts were hampered in the immediate aftermath of the pyroclastic flows and surges by smoke and ash in the atmosphere. The hot, locally gas-rich surge deposits posed a major hazard to search-and-rescue workers and volcanologists for days afterwards. Despite the efforts of officials, scientists and concerned members of the public, about 80 people were in Zones A and B of the Exclusion Zone on 25 June 1997. Our findings suggest that many had become accustomed to the pyroclastic flows and had become overconfident in their own ability to judge the threat by observing repeated flows that had gradually increased runout but remained restricted to valleys. Many people had contingency plans and believed that there would be observable or audible warning signs from the volcano if the activity were to escalate significantly. However, there were no such discernible warnings and individual contingency plans proved inadequate. Public education should concentrate on correcting such public misapprehension of hazardous phenomena and attendant risks in future volcanic crises.