Subglacial explosive basaltic eruptions during the last 8 ka in Iceland revealed by tephrostratigraphy. A review.

Tephra is formed in 80% of eruptions in Iceland making tephrostratigraphy an ideal tool to reconstruct eruptive history of a given volcanic area from written documentation (reach back 900 yr), glaciers ( 800 yr), terrestrial ( 8000 yr), lake ( 13000 yr) and marine environments (>50000 yr). The main producers of basaltic tephra are volcanic systems on the Eastern Volcanic Zone, Grímsvötn, Bárdarbunga and Katla, partly covered by the ice caps Vatnajökull (covers Grímsvötn and Bárdarbunga volcanoes) and Mýrdalsjökull (covers Katla volcano). Their subglacial central volcanoes have frequently erupted explosively throughout the Holocene forming basaltic tephra. The frequency can be assessed through correlations of well-known marker layers, such as the widespread silicic Hekla layers, and both direct and relative dating of tephra layers. Terrestrial tephra around the ice caps Vatnajökull and Mýrdalsjökull show periodic volcanic activity during the last 8000 years. Volcanoes beneath Vatnajökull are located close to the center of the Iceland mantle plume and show maximum activity at 7-5 ka and 2-1 ka with a lull in activity from 5-2 ka. This lull has been referred to as the Mid-Holocene low and is most likely caused by decrease in volcanic activity rather than environmental factors. The Katla volcano, farther away from the mantle plume, shows activity peaks from 8-7 ka and 4-2 ka with a lull from 7-4 ka. Eruption frequency estimated for Grímsvötn and Bárdarbunga volcanoes was highest 2-1 ka ago, with 140 and 80 basaltic eruptions, respectively, whereas Katla volcano had two peaks of estimated 60 eruptions in the periods 4-3 ka and 8-7 ka. The reason for different timing of maximum activity at volcanoes above or beside the mantle plume center is still an open question but may be related to delay in volcanic response to magma generation at depth. Magnitudes of subglacial explosive basaltic eruptions have been estimated for part of the historical eruptions and a few prehistoric Katla eruptions (largest one VEI 5). The aim is to compare magma fluxes at a given volcano during periods of low and high eruption frequency.