Jökull - 01.12.1990, Page 162

traced to this geothermal area have been reported and
the meltwater may drain away continuously.
KVERKFJÖLL
The Kverkfjöll mountain massif is located at the
glacier edge and is only partly covered by ice. It rises
to an elevation of more than 1900 m, i.e. more than
1000 m over its surroundings. To its north-east, a
fissure swarm can be traced for 60 km, marked by
hyaloclastite ridges, fissures and faults. The Kverk-
fjöll volcano contains two calderas of elliptical shape,
both 8 km long and 5 km wide (Þórarinsson et al.,
1973). The northem one is only about 100 m deep
and is rather indistinct. The southern one, on the other
hand, is a well defined structure, 400 - 500 m deep. A
mountain ridge extends 10 km SW from the southern
caldera but a 4-5 km wide, 850 m high pass separates
this ridge from the Grímsvötn mountain massif. An-
other ridge, 20 km long and more than 1200 m high,
strikes due south from the Kverkfjöll mountains. Vol-
canic activity on the E flanks of this ridge could cause
jökulhlaups in the rivers Kreppa and Jökulsá á Brú.
The Kverkfjöll volcanic complex displays consid-
erable geothermal activity (see Friedman etal., 1973).
Most of the thermal fields are associated with the
northem caldera. The volcano has, on the other hand,
not been particularly seismically active in recent years.
The majority of the few located earthquakes are asso-
ciated with the northem caldera.
All floods produced by eruptive activity in the
Kverkfjöll volcanic complex run into Jökulsá á
Fjöllum.
VOLCANOES BENEATH DYNGJUJÖKULL
The Askja fissure swarm disappears beneath the
central snout of Dyngjujökull. A few isolated peaks,
presumably made of hyaloclastite, are located on a
SW-NE trending line beneath Dyngjujökull. This
row of peaks can be traced for 30-35 km from the
edge of Dyngjujökull towards the northern part of the
Grímsvötn mountain massif. The highest peak which
reaches 1050 m is exceptionally steep and differs in
form from the other peaks. This may suggest that
it has not suffered much glacial erosion and is a rel-
atively recent feature. Two small earthquakes have
been recorded from this fissure swarm. Meltwater
from volcanic eruptions on this fissure swarm would
drain in jökulhlaups beneath Dyngjujökull into Jökulsá
á Fjöllum.
The map reveals a subglacial valley beneath
Dyngjujökull, trending NE along the western flank
of Kverkfjöll. This valley is likely to channel floods
from eruptions in Dyngjujökull, the eastem part and
the caldera of Bárðarbunga and the southern caldera of
Kverkfjöll. A narrow valley strikes SW from the low-
est pass on the western rim of the Kverkfjöll caldera.
This valley may have been eroded by floods pro-
duced by subglacial eruptions in this volcano. Be-
neath the central part of Dyngjujökullthe flood would
have tumed northward to emerge in Jökulsá á Fjöll-
um. Similarly, a narrow valley extending east from
Bárðarbunga may channel floods from that volcano
into the broad valley beneath Dyngjujökull. The area
to the NE of Dyngjujökullbears witness to large jökul-
hlaups (Þórarinsson, 1950, 1974; Tómasson, 1973;
Sigbjamarson, 1990).
OTHER VOLCANIC COMPLEXES
Two volcanic centres have been suggested beneath
Vatnajökull east of the axial rift zone, i.e. Esjufjöll
and Breiðabunga (Sæmundsson, 1980). A caldera
was suggested in Esjufjöll on the basis of a satellite
image (Þórarinsson et al., 1973), but the bedrock to-
pography has not been mapped sufficiently to confirm
this. Some minor seismicity has been recorded from
this area, and chunks of sulphur have been found in
Esjufjöll (Sigurður Bjömsson, pers. comm. 1990).
Both observations support the existence of an active
central volcano. No support can be found in the to-
pographic data for the existence of a central volcano
near Breiðabunga.
The Öræfajökull volcano is outside the ice field
of Vatnajökull and is therefore, strictly speaking, not
within our study area. It is a stratovolcano, more than
2100 m high and covered by a separate ice cap that
merges with Vatnajökull. Radio echo sounding lines
across the summit area reveal a 500 m deep summit
caldera (Björnsson, 1988). The seismicity of Öræfa-
jökull is quite low at the present time. A seismic
station at Kvísker near the mountain shows an occa-
158 JÖKULL, No. 40, 1990