Jökull - 01.12.1990, Qupperneq 151

Volcanoes beneath Vatnajökull, Iceland:
Evidence from radio echo-sounding,
earthquakes and jökulhlaups
HELGI BJÖRNSSON and PÁLL EINARSSON,
Science Institute, University oflceland,
Dunhaga 5,107 Reykjavík.
abstract
Integration of knowledge of subglacial topogra-
Phy, earthquake activity and jökulhlaups has con-
tributed to more precise locations ofthe volcanic sys-
tems beneath Vatnajökull. Bárðarbunga is the cen-
tre 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 sys-
tem can be traced 10 km to the SW ofthe Kverkfjöll
rolcano. It appears to be separated from the Gríms-
vötn volcano. The Askja fissure swarm extends be-
neath Dyngjujökull and can possibly be traced to the
Grímsvötn volcano. Hamarinn and the ridge strik-
lng 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 activ-
lty 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 Gríms-
vötn system, an area bounded by Þórðarhyrna, Geir-
vörtur, 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 Vatna-
jökull: 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.
INTRODUCTION
The volcanic rift zones of Iceland radiate from the
centre of the Iceland hot spot, considered to be located
in the eastem part of the central highland. The volcanic
production rate has a maximum in this area (Jakobs-
son, 1980) and here the volcanic products display the
JÖKULL, No. 40, 1990 147