Subglacial rhyolite volcanism at Torfajokull, Iceland.
Subglacial rhyolite eruptions at Torfajokull, Iceland have produced a variety of
volcanic edifices during the last glacial period (115-11 ka). These range from small-volume
(<0.1 km3) volcanoes, such as Bhlhnukur and Dalakvislfell, to larger volume
(-1 km3) flat-topped tuyas such as South-east Rauoufossafjoll. 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 Bhlhnukur was primarily caused
by quenching, when rising magma encountered meltwater. Fragmentation at Southeast
Rauoufossafjoll was apparently more energetic, and generated phreatomagmatic
ash over 300 m thick. Dalakvisl 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 Blahnukur apparently terminated before the
glacier surface had been pierced, whereas the eruption at South-east Rau6ufossafjoll
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 Blahnukur. 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 Rauoufossafjol1.