The 1995–1999 eruption of the Soufriere Hills volcano, MontINTRODUCTION serrat, has produced a crystal-rich andesite containing quenchThe Soufriere Hills volcano, Montserrat, located in the textured mafic inclusions, which show evidence of having been molten Lesser Antilles island arc in the West Indies, began to when incorporated into the host magma. Individual crystals in the erupt on 18 July 1995, after about 350 years of dormancy andesite record diverse histories. Amphibole phenocrysts vary from (Young et al., 1998b). The eruption was preceded by a pristine and unaltered to strongly oxidized and pseudomorphed by period of seismic swarms, which began in January 1992. anhydrous reaction products. Plagioclase phenocrysts are commonly Previous earthquake crises have occurred on Montserrat reverse zoned, often with dusty sieve textures. Reverse zoned rims are in 1896–1897, 1932–1935 and 1966–1967 (Powell, 1938; also common on orthopyroxene phenocrysts. Pyroxene geothermometry Shepherd et al., 1971). Initial phreatic and phregives an average temperature of 858 ± 20 °C for orthopyroxene atomagmatic activity was followed by extrusion of an phenocryst cores, whereas reverse zoned rims record temperatures andesitic lava dome in November 1995. Pyroclastic flows, from about 880 to 1050 °C. The heterogeneity in mineral rim generated by gravitational collapse of the unstable dome, compositions, zoning patterns and textures is interpreted as reflecting have been the dominant form of activity throughout the non-uniform reheating and remobilization of the resident magma eruption, but periods of explosive activity, producing body by intrusion of hotter mafic magma. Convective remobilization pumice flows and falls, have also occurred (Robertson et results in mixing together of phenocrysts that have experienced al., 1998; Young et al., 1998b). Dome growth ceased different thermal histories, depending on proximity to the intruding suddenly in March 1998 and further activity up to May mafic magma. The low temperature and high crystallinity are 1999 has mainly involved gravitational collapses of the interpreted as reflecting the presence of a cool, highly crystalline dome and periods of ash venting, with some small-scale magma body beneath the Soufriere Hills volcano. The petrological vulcanian explosive events. observations, in combination with data on seismicity, extrusion rate The Soufriere Hills volcano is the only active centre and SO2 fluxes, indicate that the current eruption was triggered by on Montserrat. The volcano is a composite of at least recent influx of hot mafic magma. five andesitic lava domes, flanked by pyroclastic deposits. Rea (1974) identified four other volcanic centres, all

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Remobilization of Andesite Magma by Intrusion of Mafic Magma at the Soufriere Hills Volcano, Montserrat, West Indies

During the eruption of the Soufriere Hills volcano, Montserrat (1995–99), and several other dome eruptions, shallow seismicity, short-lived explosive eruptions and ground deformation patterns indicating large overpressures (of several megapascals) in the uppermost few hundred metres of the volcanic conduit have been observed. These phenomena can be explained by the nonlinear effects of crystallization and gas loss by permeable flow, which are here incorporated into a numerical model of conduit flow and lava dome extrusion. Crystallization can introduce strong feedback mechanisms which greatly amplify the effect on extrusion rates of small changes of chamber pressure, conduit dimensions or magma viscosity. When timescales for magma ascent are comparable to timescales for crystallization, there can be multiple steady solutions for fixed conditions. Such nonlinear dynamics can cause large changes in dome extrusion rate and pulsatory patterns of dome growth.

Nonlinear dynamics of lava dome extrusion

[1] Experiments were conducted to study the temporal evolution of feldspar crystallization kinetics during isothermal decompression. Pinatubo dacite was held at 780°C, 220 MPa, fO2 = NNO + 2, H2O-saturated conditions for an equilibration period, decompressed to final pressures, Pf, ranging from 175 to 5 MPa, and then held for 0.3–931 hours. According to the plagioclase liquidus curve in PH2O-T space for the relevant melt composition, these decompressions impose effective undercoolings, ΔTeff, of 34–266°C. Growth of preexisting phenocrysts and newly formed sparse microlites dominate crystallization at 75 ≤ Pf < 150 MPa (ΔTeff = 34–93°C), and equilibrium crystal modes are achieved in <168 hours. Microlite nucleation is the dominant transformation process for 10 < Pf < 50 MPa (ΔTeff = 125–241°C), and chemical equilibrium is not attained by 168 hours under these conditions. Slow, steady decompressions typically produced normally zoned, euhedral, and planar-faceted feldspar crystals, although anhedral morphologies were produced at very low Pf. Contrary to expectation, slowly decompressed samples were usually further from chemical equilibrium than rapidly decompressed samples after similar durations below the initial pressure. Although counterintuitive, these trends are consistent with new constraints on the relative rates of feldspar nucleation and growth (controlled by ΔTeff and melt viscosity) experienced during each decompression path. Analysis of liquid to solid transformation kinetics using TTT-style diagrams shows that crystallization occurs most rapidly at ∼100 MPa by a crystal growth mechanism. The next most efficient crystallization conditions are at 25 MPa, in a crystal nucleation-dominated regime.

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An experimental study of the kinetics of decompression-induced crystallization in silicic melt

Dome growth at the Soufriere Hills volcano (1996 to 1998) was frequently accompanied by repetitive cycles of earthquakes, ground deformation, degassing, and explosive eruptions. The cycles reflected unsteady conduit flow of volatile-charged magma resulting from gas exsolution, rheological stiffening, and pressurization. The cycles, over hours to days, initiated when degassed stiff magma retarded flow in the upper conduit. Conduit pressure built with gas exsolution, causing shallow seismicity and edifice inflation. Magma and gas were then expelled and the edifice deflated. The repeat time-scale is controlled by magma ascent rates, degassing, and microlite crystallization kinetics. Cyclic behavior allows short-term forecasting of timing, and of eruption style related to explosivity potential.

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Magma flow instability and cyclic activity at soufriere hills volcano, montserrat, british west indies

The prevalence of andesitic and dacitic volcanic eruptions over the past 20 years has led to a new appreciation of processes typical of magmas of intermediate composition. Extensive syn-eruptive crystallization, driven by decompression and volatile exsolution, is one such process. A water-saturated melt that is decompressed isothermally from its liquidus must crystallize in response to the diminishing capacity of the melt to retain volatiles (particularly H 2 O). Only rapid magma ascent allows such a melt to reach the Earth9s surface without crystallizing. Intermediate rates of ascent permit varying amounts of syn-eruptive crystallization, which in turn changes magma rheology and affects continued magma progress toward the surface. Feedback among magma decompression, vesiculation, and crystallization is poorly understood, particularly with regard to the kinetics of crystallization. Here we present two complementary approaches to the study of syn-eruptive, degassing-induced crystallization. The first involves projection of matrix glass compositions onto the well-understood Qz-Ab-Or ternary, which allows relative (quartz-undersaturated melt) or absolute (quartz-saturated melt) determination of magma equilibration (or ‘closure’) pressure. We show that glass composition (groundmass crystallinity) changes as a function of decompression rate, and that either very slow ascent or rapid ascent followed by arrest and shallow cooling can lead to extensive cotectic precipitation of quartz + feldspar. The second approach involves quantification of plagioclase textures, which provides a direct measurement of the relative importance of crystal nucleation and growth ( J/G ). This parameter can, in turn, be linked to the effective undercooling (supersaturation) experienced during decompression. Finally, we use phenocryst melt inclusion data to suggest that a substantial amount of phenocryst crystallization may also be explained by decompression of water-saturated melt.

Degassing and crystallization of ascending andesite and dacite

The recent eruption of the Soufriere Hills Volcano in Montserrat (July, 1995, to present; October, 1997) has produced a hornblende-bearing, andesitic lava dome. It is possible to petrologically estimate changes in ascent rates of amphibole-bearing magmas. For certain rates of decompression, a breakdown rim of fine-grained, anhydrous reaction products forms where amphibole is in contact with melt. The thickness of the rim varies with ascent rate. Most of the amphibole phenocrysts in the magma storage region lack breakdown rims. About 10% have 200–400 µm-thick, coarse-grained breakdown rims that are interpreted to be relicts of a past heating event. Study of a time series of new dome andesites showed that ascent rate increased from December, 1995 (∼0.001 m/s), to July, 1996 (∼0.008 m/s), while eruptive style remained extrusive. Ascent rate increased to >0.012 m/s in August, 1996, and the first major explosive eruption occurred on 17–18 September, 1996.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/98GL00912

M. J. Rutherford

Papers

Petrologic determination of ascent rates for the 1995–1997 Soufriere Hills Volcano andesitic magma