Adam J. Stinton

Heat flow in the Lesser Antilles island arc and adjacent back arc Grenada basin

Using temperature gradients measured in 10 holes at 6 sites, we generate the first high fidelity heat flow measurements from Integrated Ocean Drilling Program drill holes across the northern and central Lesser Antilles arc and back arc Grenada basin. The implied heat flow, after correcting for bathymetry and sedimentation effects, ranges from about 0.1 W/m2 on the crest of the arc, midway between the volcanic islands of Montserrat and Guadeloupe, to 15 km from the crest in the back arc direction. Combined with previous measurements, we find that the magnitude and spatial pattern of heat flow are similar to those at continental arcs. The heat flow in the Grenada basin to the west of the active arc is 0.06 W/m2, a factor of 2 lower than that found in the previous and most recent study. There is no thermal evidence for significant shallow fluid advection at any of these sites. Present-day volcanism is confined to the region with the highest heat flow.

Submarine record of volcanic island construction and collapse in the Lesser Antilles arc: First scientific drilling of submarine volcanic island landslides by IODP Expedition 340

IODP Expedition 340 successfully drilled a series of sites offshore Montserrat, Martinique and Dominica in the Lesser Antilles from March to April 2012. These are among the few drill sites gathered around volcanic islands, and the first scientific drilling of large and likely tsunamigenic volcanic island-arc landslide deposits. These cores provide evidence and tests of previous hypotheses for the composition and origin of those deposits. Sites U1394, U1399, and U1400 that penetrated landslide deposits recovered exclusively seafloor sediment, comprising mainly turbidites and hemipelagic deposits, and lacked debris avalanche deposits. This supports the concepts that i/ volcanic debris avalanches tend to stop at the slope break, and ii/ widespread and voluminous failures of preexisting low-gradient seafloor sediment can be triggered by initial emplacement of material from the volcano. Offshore Martinique (U1399 and 1400), the landslide deposits comprised blocks of parallel strata that were tilted or microfaulted, sometimes separated by intervals of homogenized sediment (intense shearing), while Site U1394 offshore Montserrat penetrated a flat-lying block of intact strata. The most likely mechanism for generating these large-scale seafloor sediment failures appears to be propagation of a decollement from proximal areas loaded and incised by a volcanic debris avalanche. These results have implications for the magnitude of tsunami generation. Under some conditions, volcanic island landslide deposits composed of mainly seafloor sediment will tend to form smaller magnitude tsunamis than equivalent volumes of subaerial block-rich mass flows rapidly entering water. Expedition 340 also successfully drilled sites to access the undisturbed record of eruption fallout layers intercalated with marine sediment which provide an outstanding high-resolution data set to analyze eruption and landslides cycles, improve understanding of magmatic evolution as well as offshore sedimentation processes.

Chapter 5 Vulcanian explosions at Soufrière Hills Volcano, Montserrat between 2008 and 2010

Abstract Vulcanian explosions generated at Soufrière Hills Volcano between 2008 and 2010 varied from simple events involving minimal pyroclastic density currents (PDCs) to complex events involving more than one explosion. Calculated volumes for the deposits of the PDCs formed by these explosions ranged up to 2.7×106 m3, with more than half the explosions having volumes greater than 1×106 m3. The deposits formed by the explosions varied in lithology, with some explosions generating pumice-rich PDCs (e.g. 29 July 2008 and 11 February 2010) showing development of sinuous lobes. These explosions are similar to those formed in 1997, with gas-rich, conduit-derived magma being the dominant driving mechanism. Other explosions were pumice-poor (c. 5 wt% pumice) and generated morphologically distinct PDC deposits. Many of the pumice-poor explosions were associated with lower tephra plumes of <8 km, but were some of the largest volume events in terms of PDC production and suggest a generation mechanism involving destruction of significant quantities of the lava dome. Analysis of video footage shows that PDC formation was pulsatory, probably related to destabilization of portions of the lava dome during the initial phases of the explosion.

Chapter 7 The 11 February 2010 partial dome collapse at Soufrière Hills Volcano, Montserrat

Abstract On 11 February 2010, a partial dome collapse, the largest since 20 May 2006, occurred at Soufrière Hills Volcano (SHV), Montserrat. The collapse is also the largest generated on the northern flank of SHV since the eruption began in 1995. Approximately 50×106 m3 was removed from the dome, resulting in widespread pyroclastic density currents (PDCs). Mapping revealed a complex stratigraphy that varied widely across the northern and NE flanks, and reflected the complex evolution of the collapse. The deposits included a range of fine-grained ash-rich and pumice-rich units deposited by dilute PDCs, and several types of coarse-grained, blocky deposits from dense PDCs. Several previously unaffected areas, including Bugby Hole, Farm River Valley, the village of Harris and Trants, suffered significant damage to the natural and built environments. The collapse lasted 107 min but the bulk of the activity occurred in a 15 min period that included five of the six peaks in PDC generation and two Vulcanian explosions. Although powerful, the PDCs generated were not associated with a lateral blast. The likely cause was the piecemeal collapse of a series of large, unstable lobes that had been extruded on the northern flank of the pre-existing dome.

Chapter 4 Ash venting occurring both prior to and during lava extrusion at Soufrière Hills Volcano, Montserrat, from 2005 to 2010

Abstract This paper describes ash-venting activity at Soufrière Hills Volcano, Montserrat that was precursory to the onset of three phases of lava extrusion in 2005, 2008 and 2009, and similar ash venting that occurred during the fifth phase of lava extrusion. We describe in detail a style of mild, tephra-generating activity termed ash venting and its associated tephra products. The nature of the seismicity associated with ash venting is compared with that of explosive activity. All explosive events, from small explosions to large Vulcanian explosions, have impulsive, low-frequency onsets. These are absent in ash-venting events, which have subtle, emergent onsets. Microscope and grain-size analyses show that ash-venting events and large Vulcanian explosions generate tephra that is similar in grain size (in medial and distal regions), although phreatic events in 2005 were finer grained. Ash-venting products are either composed of fine-grained, variably altered pre-existing material or juvenile material. There is a general correlation between the length of the pause and the length of the period of precursory activity prior to lava extrusion following it. Syn-extrusive ash venting is frequently associated with short-term increases in extrusion rate and is considered to be related to shear-induced fragmentation at the conduit margin.

Late Pleistocene stratigraphy of IODP Site U1396 and compiled chronology offshore of south and south west Montserrat, Lesser Antilles

Marine sediments around volcanic islands contain an archive of volcaniclastic deposits, which can be used to reconstruct the volcanic history of an area. Such records hold many advantages over often incomplete terrestrial data sets. This includes the potential for precise and continuous dating of intervening sediment packages, which allow a correlatable and temporally constrained stratigraphic framework to be constructed across multiple marine sediment cores. Here we discuss a marine record of eruptive and mass-wasting events spanning ∼250 ka offshore of Montserrat, using new data from IODP Expedition 340, as well as previously collected cores. By using a combination of high-resolution oxygen isotope stratigraphy, AMS radiocarbon dating, biostratigraphy of foraminifera and calcareous nannofossils, and clast componentry, we identify five major events at Soufriere Hills volcano since 250 ka. Lateral correlations of these events across sediment cores collected offshore of the south and south west of Montserrat have improved our understanding of the timing, extent and associations between events in this area. Correlations reveal that powerful and potentially erosive density-currents traveled at least 33 km offshore and demonstrate that marine deposits, produced by eruption-fed and mass-wasting events on volcanic islands, are heterogeneous in their spatial distribution. Thus, multiple drilling/coring sites are needed to reconstruct the full chronostratigraphy of volcanic islands. This multidisciplinary study will be vital to interpreting the chaotic records of submarine landslides at other sites drilled during Expedition 340 and provides a framework that can be applied to the stratigraphic analysis of sediments surrounding other volcanic islands.

Chapter 6 Dome growth and valley fill during Phase 5 (8 October 2009–11 February 2010) at the Soufrière Hills Volcano, Montserrat

Abstract Extrusion during Phase 5 (8 October 2009–11 February 2010) produced significant volumetric and geomorphic changes to the lava dome and surrounding valleys at the Soufrière Hills Volcano, Montserrat. Approximately 74×106 m3 of lava was extruded at an average rate of 7 m3 s−1 during the short period of activity. Addition of lava to the pre-existing dome resulted in a net volumetric increase of up to 38×106 m3. Pyroclastic density current (PDC) and ashfall deposits accounted for the remaining 36×106 m3. A series of thick, blocky lobes were extruded from a central vent. In addition, several short-lived spines and two large shear lobes were also extruded. Significant PDC activity resulted in substantial valley filling of up to 108 m. The large pre-existing dome significantly influenced the growth of lobes, such that many block-and-ash flows were generated from viscous lobes draped over the summit and upper slopes. Geomorphic changes caused by rapid filling of the surrounding valleys aided in both flow avulsion and the emplacement of deposits up to 6 km from the dome. These geomorphic changes have important consequences for hazards from PDCs.

The relationship between eruptive activity, flank collapse, and sea level at volcanic islands: a long-term (>1 Ma) record offshore Montserrat, Lesser Antilles

Hole U1395B, drilled southeast of Montserrat during Integrated Ocean Drilling Program Expedition 340, provides a long (>1 Ma) and detailed record of eruptive and mass-wasting events (>130 discrete events). This record can be used to explore the temporal evolution in volcanic activity and landslides at an arc volcano. Analysis of tephra fall and volcaniclastic turbidite deposits in the drill cores reveals three heightened periods of volcanic activity on the island of Montserrat (?930 ka to ?900 ka, ?810 ka to ?760 ka, and ?190 ka to ?120 ka) that coincide with periods of increased volcano instability and mass-wasting. The youngest of these periods marks the peak in activity at the Soufriere Hills volcano. The largest flank collapse of this volcano (?130 ka) occurred towards the end of this period, and two younger landslides also occurred during a period of relatively elevated volcanism. These three landslides represent the only large (>0.3 km3) flank collapses of the Soufriere Hills edifice, and their timing also coincides with periods of rapid sea-level rise (>5 m/ka). Available age data from other island arc volcanoes suggests a general correlation between the timing of large landslides and periods of rapid sea-level rise, but this is not observed for volcanoes in intra-plate ocean settings. We thus infer that rapid sea-level rise may modulate the timing of collapse at island arc volcanoes, but not in larger ocean-island settings.

Chapter 13 AVTIS observations of lava dome growth at Soufrière Hills Volcano, Montserrat: 2004 to 2011

Abstract To solve the problem of lava dome growth at Soufrière Hills Volcano (SHV) being invisible and unmeasured owing to cloud, we have designed, built and deployed a ground-based millimetre-wave radar/radiometer: the All-weather Volcano Topography Imaging Sensor (AVTIS). In this chapter, after an outline technical sketch of the instruments, we describe the campaigns between 2004 and 2011 used to test their capabilities. We then present results from the campaigns to illustrate how signals of volcanological interest can be retrieved. The primary measurements of AVTIS are range (to within, at best, about 1 m), and, from that, topography, topographical change and effusion rates, and surface temperature (to within a few degrees Celsius). Changes in radar reflectivity can indicate surface processes (e.g. mass wasting). Surface motion within the instantaneous field of view produces a Doppler signal that allows detection of rockfall. Attenuation of the signal by rain along the path can, when stacked temporally, give an image of rain cloud structure and, by calibration, a rate of rainfall. We regard a strategy of two radars – one permanantly mounted (at Windy Hill) autonomous instrument, and the other used as a rover – as being best for capturing dome growth.

An inclined Vulcanian explosion and associated products

Vulcanian explosions generate some of the most hazardous types of volcanic phenomena, including pyroclastic density currents. Non-vertical directionality of an explosion promotes asymmetrical distribution of proximal hazards around the volcano. Although critical, such behaviour is relatively uncommon and has been seldom documented. Here we present, for the first time, evidence both from geophysical monitoring and field survey data that records the occurrence of such an event. Thermal imagery captures a Vulcanian explosion at Soufrière Hills Volcano, Montserrat, which occurred during a large partial lava dome collapse in February 2010, and was inclined at about 25° from the vertical in a northerly direction. Pyroclastic products were preferentially distributed to the north and included: an unusual pumice boulder deposit that we propose was formed by a dilute pyroclastic density current; pumice flow deposits; and a proximal lapilli and block fallout lobe. The inclined nature of the explosion is attributed to the asymmetric geometry around the vent. The explosion-derived pyroclastic density currents had notably lower velocities than those associated with lateral blasts, which, we suggest, result from a separate and distinct mechanism. These inclined explosions present an additional mechanism that is able to generate directed pyroclastic density currents, with consequent implications for hazard assessment.