Glaciovolcanism in Iceland: Observations of frequent eruptions over the last three decades, styles of activity, influence of ice thickness and impact on the glaciers

Due to its northerly latitude, about 10% of Iceland is covered by glaciers and a substantial part of the most active volcanoes are ice covered.  As a result, volcano-ice interaction in various forms is very common in Iceland.  Steep-sided mountains (elongated ridges and tuyas) formed in volcanic eruptions during the repeated Pleistocene glaciations dominate the landscape in many parts of the volcanic zones.  Over the last 30 years, when active monitoring has taken place, six eruptions, ranging in composition from basalt to trachyte have occurred in glaciers in Iceland.  The 1996 Gjálp eruption within the Vatnajökull glacier occurred where the initial thickness was 600-750 meters.  As a result, the bulk of the activity was fully subglacial, ice flow into the depressions formed was substantial, and the observed subaerial phase was relatively modest.  The eruptions in Grímsvötn (1998, 2004 and 2011) and Eyjafjallajökull (2010) occurred where ice was 0-200 m thick, forming ice cauldrons with vertical walls and ice flow played a very minor role, and explosive activity, mostly phreatomagmatic, was dominant. The third type of activity was observed above the NE-wards propagating dyke from the subsiding Bardarbunga caldera, formed in the days prior to the onset of the large Holuhraun eruption in 2014.  These minor leaks of magma caused small, fully subglacial eruptions where the ice was 300-500 m thick.  Ice melting was of the order of 1-10 million m³ in the smallest events (2014), while 3 km³ melted during the Gjálp 1996 eruption, with another 1 km³ melted in the following months.  That eruption formed a 6 km long, up to 500 m high ridge under the glacier. Ice melting caused jökulhlaups in some of the eruptions.  The one following the Gjálp 1996 eruption was by far the largest. It had a peak discharge of 40,000-50,000 m³/s as 3.5 km³ of meltwater were released from the subglacial Grímsvötn caldera lake, where it had accumulated over five weeks.  The jökulhlaups observed had some impact on the glaciers above the meltwater path.  However, this change was relatively minor and did not cause major disruption.  For the largest events some breaking up of the glacier snout occurred, resulting in large ice blocks being carried by the floodwater.   Considerably larger events have occurred in the recent past, notably the eruption of Katla in 1918.  The very powerful phreatomagmatic early part of that eruption, starting under initially 300-400 m thick ice, produced over 100,000 m³/s of meltwater and deposited several hundred million m³ of water-transported tephra on the Mýrdalssandur outwash plain.