Growth patterns and emplacement of the andesitic lava dome at Soufrière Hills Volcano, Montserrat

Abstract

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.

Magma flow instabilities in a volcanic conduit: Implications for long-period seismicity

“…In addition, all these models consider a yield strength that produces a 240 continuous shear band along the entire length of the upper conduit. This suggests that 241 the lava extruded at the free-surface is always along shear boundaries when LP 242 seismicity occurs, which is not the case (Miller et al, 1998;Watts et al, 2002). Hence, 243…”

“…At the 163 peak of tilt deformation and during dome deflation lava is commonly extruded via slip 164 along ductile shear faults with grooved surfaces. The generation of a new lava flow 165 pathway is marked by acceleration in extrusion rate and an increase in the amount of 166 rockfall activity from the advancing lava (Watts et al, 2002). Changes in extrusion rate 167 are commonly related to major switches in the direction of lava dome growth.…”

“…Resulting 168 in the development of shear lobes, that move from the vent along shear boundaries 169 (Watts et al, 2002). Although enhanced extrusion rates from crystal rich flows suggest 170 the development of shear boundaries during these cyclic events (Green and Neuberg,171 2006), observations by Watts et al (2002) show that the whole range of structures 172 extruded on Soufriere Hills Volcano, from spines to low-viscosity flows, can form with 173 hybrid earthquakes present. This suggests that shear boundaries observed at the conduit 174 exit are not a necessity during hybrid earthquake activity.…”

Modelling the lava dome extruded at Soufrière Hills Volcano, Montserrat, August 2005–May 2006

“…During moderate extrusion rates shear lobes dominated and grew for weeks to months; importantly they included both exogenous and endogenous growth. Periods of higher extrusion rates suppressed microlite crystal growth and resulted in the extrusion of lower viscosity lava ('pancake lobes'; Watts et al, 2002). Dome growth can switch abruptly from entirely endogeneous to dominantly exogenous growth and a well-documented example of this occurred between October andDecember 1996 (Hale and.…”

“…The solidus temperature is not well constrained at low pressures, but during the growth of the lava dome on SHV in early October 1996, Watts et al (2002) noted that the rubbly carapace was only a few metres thick. This suggests that the solidus pressure can be relatively small.…”