Intraglacial Volcanoes of the Laugarvatn Region, Southwest Iceland, II

The constituents and structure of two basaltic volcanoes which grew in meltwater ponds in an ice sheet are described. Pillow lava and para-pillow lava, a newly denned variety of subaqueous lava flow, appear to have fabrics and fabric relationships similar to that of pahoehoe and aa, and are inferred to have similar modes of emplacement. Pillow breccias are attributed to gravitational collapse of pillow lava in varying states of cooling and crystallization. Flow-foot breccias are attributed to the flow into water of lava erupted in air. Vitric tuffs are inferred to be the product of explosive activity coinciding with emergence of the volcanoes from their meltwater ponds. The emergent explosive phase, which followed effusion of lava in water (pillow lava) and preceded effusion in air (sheet lava/flow-foot breccia), is attributed to explosive evolution of steam from water sucked into the conduit by rapidly rising lava. The onset of explosive activity at depths of less than about 200 m is related to increasing velocity of uprise of lava with decreasing depth of eruption.