TY - JOUR

T1 - An explosive-intrusive subglacial rhyolite eruption at Dalakvisl, Torfajokull, Iceland

AU - Tuffen, Hugh

AU - McGarvie, D. W.

AU - Pinkerton, H.

AU - Gilbert, Jennifer

AU - Brooker, R. A.

PY - 2008/5

Y1 - 2008/5

N2 - This paper describes unusual rhyolitic deposits at Dalakvisl, Torfajokull, Iceland that were emplaced during a Quaternary subglacial eruption. Despite its small volume (< 0.2 km(3)), the eruption mechanisms were highly variable and involved both explosive and intrusive phases. The explosive phase involved vesiculation-driven magma fragmentation at the glacier base and generated a pumiceous pyroclastic deposit containing deformed sheets of dense obsidian. Textures suggest that the obsidian was generated by the collapse of partly fragmented foam that was intruding the deposit and water contents indicate quenching at elevated pressures. In contrast, the intrusive phase of the eruption generated vesicle-poor quench hyaloclastites associated with a variety of peperitic lava bodies. The presence of juvenile-rich fluvio-lacustrine sediments is the first documented evidence that meltwater may pond close to the vent during subglacial rhyolite eruptions if the bedrock topography is favourable. In order to explain the variable eruption mechanisms, a conceptual model is presented in which the transition from an explosive to an intrusive eruption was controlled by the space available for fragmentation within the subglacial cavity melted above the vent. When the cavity became completely filled by volcanic deposits, the vent became blocked and rising magma was forced to intrude through poorly consolidated debris. This led to arrested fragmentation and welding of foam domains to form vesicle-poor obsidian lava; the transition to an intrusive eruption has taken place. Although this vent-blocking mechanism is particularly relevant to subglacial eruptions, it may also apply to subaerial rhyolitic eruptions, where patterns of explosive and effusive activity cannot be explained by shallow degassing processes alone. Meanwhile, the variable style of a small-volume subglacial rhyolite eruption further highlights the complex processes that mediate volcano-ice interactions.

AB - This paper describes unusual rhyolitic deposits at Dalakvisl, Torfajokull, Iceland that were emplaced during a Quaternary subglacial eruption. Despite its small volume (< 0.2 km(3)), the eruption mechanisms were highly variable and involved both explosive and intrusive phases. The explosive phase involved vesiculation-driven magma fragmentation at the glacier base and generated a pumiceous pyroclastic deposit containing deformed sheets of dense obsidian. Textures suggest that the obsidian was generated by the collapse of partly fragmented foam that was intruding the deposit and water contents indicate quenching at elevated pressures. In contrast, the intrusive phase of the eruption generated vesicle-poor quench hyaloclastites associated with a variety of peperitic lava bodies. The presence of juvenile-rich fluvio-lacustrine sediments is the first documented evidence that meltwater may pond close to the vent during subglacial rhyolite eruptions if the bedrock topography is favourable. In order to explain the variable eruption mechanisms, a conceptual model is presented in which the transition from an explosive to an intrusive eruption was controlled by the space available for fragmentation within the subglacial cavity melted above the vent. When the cavity became completely filled by volcanic deposits, the vent became blocked and rising magma was forced to intrude through poorly consolidated debris. This led to arrested fragmentation and welding of foam domains to form vesicle-poor obsidian lava; the transition to an intrusive eruption has taken place. Although this vent-blocking mechanism is particularly relevant to subglacial eruptions, it may also apply to subaerial rhyolitic eruptions, where patterns of explosive and effusive activity cannot be explained by shallow degassing processes alone. Meanwhile, the variable style of a small-volume subglacial rhyolite eruption further highlights the complex processes that mediate volcano-ice interactions.

KW - volcano-ice interaction

KW - rhyolite

KW - obsidian

KW - pumice

KW - explosive volcanism

KW - fragmentation

KW - subglacial volcanism

KW - MAGMA FRAGMENTATION

KW - SILICIC VOLCANISM

KW - FLOW DEPOSITS

KW - TUFF

KW - PRODUCTS

KW - DYNAMICS

KW - MODEL

KW - LAVA

KW - HYALOCLASTITES

KW - VATNAJOKULL

U2 - 10.1007/s00445-007-0174-x

DO - 10.1007/s00445-007-0174-x

M3 - Journal article

VL - 70

SP - 841

EP - 860

JO - Bulletin of Volcanology

JF - Bulletin of Volcanology

SN - 0258-8900

IS - 7

ER -