Holocene volcanic activity at Grímsvötn, Bárdarbunga and Kverkfjöll subglacial centres beneath Vatnajökull, Iceland

Abstract

Assessment of potential future eruptive behaviour of volcanoes relies strongly on detailed knowledge of their activity in the past, such as eruption frequency, magnitude and repose time. The eruption history of three partly subglacial volcanic systems, Grimsvotn, Bardarbunga and Kverkfjoll, was studied by analysing tephra from soil profiles around the Vatnajokull ice-cap, which extend back to ~7.6 ka. Well known regional Holocene marker tephra (e.g. H3, H4, H5) were utilized to correlate profiles. Stratigraphic positions and geochemical compositions were used for fine-scale correlation of basaltic tephra. Around Vatnajokull ice-cap 345 tephra layers were identified, of which 70% originated from Grimsvotn, Bardarbunga or Kverkfjoll. The eruption frequency of each volcanic system was estimated; Grimsvotn has been the most active with an average of ~7 eruptions/100 years (range 4–14) during prehistoric time (before ~870 AD); Bardarbunga has been the second most active with ~5 eruptions/100 years (range 1–8); and Kverkfjoll has remained essentially calm with 0–3 eruptions/100 years but showing periodic activity with repose times of >1000 years. All three volcanic systems experienced lulls in activity from 5 ka to 2 ka, referred to as the “Mid-Holocene low”. This reduced eruption frequency appears to have resulted from a decrease in magma generation and delivery from the mantle plume rather than from changes in ice-load/glacier thickness. In prehistoric time, there was a time lag of 1000–3000 years between a peak of activity at volcanoes directly above the mantle plume versus at volcanoes located in the non-rifting part of the Eastern Volcanic Zone, closer to the periphery of the island. This time-space relationship suggests that a significant future increase in volcanism can be expected there, following increased levels of volcanism above the plume.