Subglacial rhyolite volcanism at Torfajökull, Iceland

Abstract

Subglacial rhyolite eruptions at Torfajökull, Iceland have produced a variety of volcanic edifices during the last glacial period (115-11 ka). These range from small-volume (3) volcanoes, such as Bláhnúkur and Dalakvíslfell, to larger volume (~1 km3) flat-topped tuyas such as South-east Rauđufossafjöll. Lithofacies associations at each volcano record distinct phases of volcano-ice interaction beneath temperate glaciers at least 350 m thick. All eruptions began with the construction of a pile of glassy fragmental material within a subglacial cavity. Fragmentation at Bláhnúkur was primarily caused by quenching, when rising magma encountered meltwater. Fragmentation at Southeast Rauđufossafjöll was apparently more energetic, and generated phreatomagmatic ash over 300 m thick. Dalakvíslfell is intermediate between the other two localities. Most fragmental deposits are massive, suggesting that a sustained meltwater lake did not develop during eruptions, in contrast with evidence from many basaltic volcanoes. Instead, meltwater drained away in a number of discrete channels, some of which have been identified. The eruption at Bláhnúkur apparently terminated before the glacier surface had been pierced, whereas the eruption at South-east Rauđufossafjölll produced a cap of flat-lying subaerial lava flows about 1.5 km in length. Numerical models are presented, in which simple patterns of ice melting and deformation are used to simulate the evolving size of subglacial cavities during eruptions. The radius of the cavity is compared to the radius of the growing subglacial volcano. The models predict that, at low magma discharge rates and beneath thick ice, cavities will become completely filled with volcanic debris and the eruption will be dominantly intrusive, forming the types of lithologies observed at Bláhnúkur. Cavities never become filled at higher magma discharge rates, and an explosive phreatomagmatic eruption is predicted, which would form the types of lithologies observed at South-east Rauđufossafjöll