Rhyolitic volcano-ice interactions in Iceland

Iceland contains an abundance and diversity of rhyolitic edifices produced during volcano-ice interactions (glaciovolcanism) that is unmatched by any other volcanic province, with an estimated 350 km(3) erupted during the past 0.8 Ma from at least fifteen volcanic systems. This review summarises research undertaken during the past decade and provides a new summary of the distribution of rhyolitic glaciovolcanic rocks erupted during the past 0.8 Ma. Descriptions of effusion-dominated edifices focus on the two best-studied edifices - the c.0.6 km(3) and 570 m high Prestahnukur edifice and the smaller-volume (<0.1 km(3)) drape of Blahnukur. Both show little or no evidence of magmatic fragmentation (i.e. driven by volatile exsolution), but both show abundant evidence of quench fragmentation and meltback of ice walls. A particular feature of Prestahnukur is its sheet-like lava bodies which are interpreted as sills that intruded the ice-edifice interface. Rhyolitic tuyas are products of sustained eruptions into thick ice, and form distinctive steep-sided edifices with flat or broad dome-like tops. They are 300-700 m high with volumes up to 2 km(3) (though 0.5-1.0 km(3) is more typical). Initial vigorous phreatomagmatic eruptions within well-drained ice vaults build steep-sided piles of tephra confined by ice walls. Gradual upwards increases in inflated clasts point to the decreasing involvement of meltwater and the increasing ability of the magma to vesiculate. As the eruption progresses and terminates, non-explosive degassing produces lava caps up to 300 m thick. Oraefajokull is a stratovolcano illustrating complex volcano-ice interactions generated when rhyolitic lavas erupt onto steeper slopes and encounter ice of variable thickness and properties (e.g. fracturing). Eruptive units record dramatic ice thickness fluctuations of up to 500 m in valley glaciers between eruptions, which emphasises that useful palaeoenvironmental information can be gleaned from detailed studies of volcano-ice interactions. Considerable value is added when palaeoenvironmental information is combined with Ar-Ar dating. For example Ar-Ar dating of two rhyolitic tuyas at Torfajokull reveal that they erupted prior to the last glacial maximum when temperatures were at least 4 degrees C cooler than the present day. Ar-Ar dating of Prestahnukur suggests it erupted during the last interglacial-glacial transition into over 700 m of ice, which corroborates studies arguing for rapid accumulation of land-based ice during global cooling. (C) 2008 Elsevier B.V. All rights reserved.