Volcanoes beneath Vatnajökull, Iceland: Evidence from radio echo-sounding, earthquakes and jökulhlaup

Integration of knowledge of subglacial topogra-Phy, earthquake activity and jökulhlaups has contributed to more precise locations ofthe volcanic systems beneath Vatnajökull.Bárðarbunga is the centre of a volcanic systern which includes the fissure swarms ofVeiðivötn to the SW and Dyngjuháls to the NE.The Grímsvötn volcano is the centre ofa volcanic system that comprises the Laki fissure swarm in the SIT and extends for an unknown distance to the NE °f the volcano.Þórðarhyrna is probably a separate central volcano within this system.The Kverkfjöll system can be traced 10 km to the SW ofthe Kverkfjöll rolcano.It appears to be separated from the Grímsvötn volcano.The Askja fissure swarm extends beneath Dyngjujökull and can possibly be traced to the Grímsvötn volcano.Hamarinn and the ridge striklng eastward from Hamarinn (Loki Ridge) are central volcanoes which together with the Fögrufjöll fissure swarm form a volcanic system.No support can be found in the topographic data for the existence of a central volcano near Breiðabunga.Volcanic activlty in historical times has mainly been limited to the Grímsvötn and Bárðarbunga systems.Major rifting events have affected large sections of these volcanic tystems but both have been remarkably quietfor the Past decades.A few volcanic events are presumed to have occurred on the Hamarinn -Loki system and on the Askjafissure swarm beneath Dyngjujökull, but no event is known with certainty on the Kverkfjöll system.The rivers Skeiðará and Súla have the most frequent eruption-related jökulhlaups.They receive meltwater from the Grímsvötn caldera and the area to the SW of it, including Þórðarhyrna.Núpsvötn, Djúpá and Hverfisfljót drain the southwestern part ofthe Grímsvötn system, an area bounded by Þórðarhyrna, Geirvörtur, Hágöngur andPálsfjall.Skaftá drains the Loki Ridge and the area west of Grímsvötn.Jökulhlaups in Kaldakvísl, and ultimately Þjórsá, must originate in the Bárðarbunga or the Hamarinn -Loki systems, and the only possible source for volcanic jökulhlaups in Skjálfandafljót is the northern part ofthe Bárðarbunga system.Jökulsá á Fjöllum may drain meltwater from activity in all the volcanic systems in northern Vatnajökull: Bárðarbunga, the Askja fissure swarm and Kverkfjöll.This includes the voluminous, ice-filled calderas of Bárðarbunga and Kverkfjöll.Activity in the northern part ofthe Grímsvötn system may cause floods inJökulsá á Fjöllum.Meltwaterfrom volcanic activity on the SE flank of a ridge that strikes south from Kverkfjöll may drain to Jökulsá á Brú.Volcanic activity farther north on the E flanks of this ridge would cause floods in Kreppa.