Subglacial and ice-contact volcanism at the Öræfajökull stratovolcano, Iceland
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Eruptions of Öræfajökull have produced mafic and silicic magmas, and have taken place in both glacial and interglacial periods. The geology of the volcano records the differing response of magmas of contrasting composition to interaction with ice of variable thickness and gives insight into the development of a long-lived ice-covered stratovolcano. Vatnafjall, a ridge on the southeast flank of Öræfajökull, is the first area of the volcano to have been mapped in detail and the geological map is presented here alongside descriptions of each erupted unit. The oldest units comprise pillow lavas, hyaloclastite and jointed lava flows that were formed during subglacial basaltic eruptions involving abundant meltwater. The products of a subsequent explosive, initially phreatomagmatic, subglacial rhyolite eruption were confined by ice to form a tephra pile over 200 m thick that was intruded by dense rhyolite magma towards the end of the eruption. Confinement by ice caused a later trachydacite lava flow to form buttresses and a steep pillar. Whilst some of the meltwater produced infiltrated the lava (to generate red and black glassy breccias and cause localised steam explosions), it is likely that much of it drained down the steep topography. The most recently-erupted units are subaerial basaltic lava flows, the oldest of which were erupted during an interglacial period and have subsequently been partially eroded and scoured by advancing ice. Ice has been important in shaping the edifice by confining eruptive products to form constructional features and by later eroding parts of them to form deep valleys. Reconstructions of volcano-ice interaction allowed the local thickness of the glacier at the time of each eruption to be estimated, and demonstrates that the upper surface of the ice has varied in elevation by over ∼700 m.