Jökull - 01.12.1990, Qupperneq 168

It extends 150 km from the Torfajökull central
volcano to Trölladyngja.
2. Loki volcanic system. The volcanic centres on
the Loki Ridge form a volcanic system together
with the Fögrufjöll fissure swarm.
3. Grímsvötn volcanic system. The Grímsvötn vol-
cano is the centre of a system comprising the
Laki and Rauðhólar fissure swarms and extends
for an unknown distance to the NE of the volcano.
Þórðarhyma is probably a separate volcanic cen-
tre within this system.
4. Kverkfjöll volcanic system. The Kverkfjöll sys-
tem can be traced 10 km to the SW of the Kverk-
fjöll volcano, and seems to be 90 km long. It
appears to be separated from the Grímsvötn vol-
cano.
5. The Askja volcanic system. The fissure swarm
extending south from the Askja volcano contin-
ues beneath Dyngjujökull and possibly merges
with the Grímsvötn volcano.
In the area east of the main rift zone some evidence
is found for an active central volcano in Esjufjöll, but
no topographic indications are of a volcanic centre in
Breiðabunga.
The new knowledge on the topography of the sur-
face and the bottom of Vatnajökull makes it possible
to trace jökulhlaups in various rivers to their possible
volcanic sources. 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 Þórðarhyma. Núpsvötn may
receive meltwater from eruptions on the south flanks of
Þórðarhyrna. Djúpá drains meltwater from the west-
ern slopes of Þórðarhyrna, Geirvörtur and Hágöngur.
Jökulhlaups in Hverfisfljót may originate from erup-
tions in the Pálsfjall area. Skaftá drains the Loki Ridge
and the area west of Grímsvötn. Jökulhlaups in Þjórsá
(Kaldakvísl) must originate in the Bárðarbunga or the
Hamarinn - Loki systems. The only possible source for
volcanic jökulhlaups in Skjálfandafljót is the northem
part of the Bárðarbunga system. Jökulsá á Fjöllum
may receive meltwater from activity in all the vol-
canic systems in northem Vatnajökull, Bárðarbunga,
the Askja fissure swarm and Kverkfjöll. This includes
the voluminous, ice-filled calderas of Bárðarbunga
and Kverkfjöll. Activity in the northem part of the
Grímsvötn system can also produce floods in Jökulsá
á Fjöllum. Jökulsá á Brú may receive meltwater from
volcanic activity on the southeastem flank of a ridge
that strikes south from Kverkfjöll. Volcanic activity
further north on the eastern flanks of this ridge would
cause floods in Kreppa.
Volcanic activity in historical times has mainly
been associated with the Grímsvötn and Bárðarbunga
systems. A few volcanic events are known or pre-
sumed to have occurred on the Loki system and on
the Askja fissure swarm beneath Dyngjujökull, but
no event is known with certainty on the Kverkfjöll
system in historical times. Major rifting events af-
fecting large sections of the volcanic systems have
occurred on both the Bárðarbunga and Grímsvötn sys-
tem. Many smaller events affecting only the caldera
regions and adjacent parts of the fissure swarms are
also known. The activity of both systems is quite ir-
regular in time, and both have been remarkably quiet
for the last several decades. No substantial eruptions
have occurred in the Grímsvötn system since 1934-39
and in the Bárðarbunga system since 1862-64.
NOTE ADDED IN PROOF
Bursts of tremor were recorded on seismographs
nearVatnajökullon Aug. 12,1991,duringajökulhlaup
in Skaftá. The course of events is similar to that of
Nov. 30, 1986, and suggests that a small and short-
lived eruption may have occurred beneath the Eastern
Loki cauldron.
ACKNOWLEDGEMENTS
Continuing support of Landsvirkjun (National
Power Company) of both glaciological and seismo-
logical research in the Vatnajökull area is gratefully
acknowledged. The Icelandic Road Authority is ac-
knowledged for their support to studies of the Gríms-
vötn area. The Icelandic Science Fund supported the
radio echo soundings and the studies of the drainage of
ice and water. The Research Fund of the University of
Iceland and the Eggert V. Briem Research Fund have
supported the studies of the Grímsvötn area and the
164 JÖKULL, No. 40, 1990