Jökull - 01.12.1973, Page 49

Fig. 3.
View from eastern
Dyngjiifjöll towards NW
across Öskjuop. The fault
scarp of the Öskjuop
downthrow is seen in
the foreground, cutting
through the lavafilled
oldest caldera in Dyngju-
fjöll, the fault scarp of
which can be seen along
the hill row in the back-
ground.
MyncL 3.
Horft frá eystri Dyngju-
fjöllum til norðvesturs
yfir Oskjuop. í forgrunni
sest öskjuops-misgengi, sem sker elzta ketilsigið, en það er nú fyllt hraunum.
younger lavas and lava has even begun to flow
mto the Askja Lake, although it is less than
100 years old.
The morphology of the eastern part of
Dyngjufjöll, the eruption fissure of Mývetn-
tngahraun lava together with the Öskjuop
faults, indicate that the Askja Lake caldera is
surrounded by a circular fracture system, alter-
nately appearing as eruption fissures or tectonic
fractures of considerably greater age than the
Askja Lake itself. The móberg formation of SE
Dyngjufjöll points to it having been active
during the last glaciation. Öskjuop has ac-
cordingly been formed by subsidence of the
wedge at the intersection of the oldest nortli-
ernmost caldera and the circular fracture system
surrounding the youngest one (Fig. 2).
The Askja eruption of 1961 showed that both
tFe oldest calderas are still active, as the sol-
fataras and fumaroles followed the circular
fracture of Askja caldera (N—S), while the
eruption itself followed the fracture (E—W)
demarcating the oldest one. Thus, all three
calderas can be assumed to be still active to
some extent.
Fig. 2 also illustrates the major eruption
fissures in Dyngjufjöll. The eruption centres
are approximately drawn from aerial photos
with the result that many may be left out.
Strange to say, the eruption centres of Dyngju-
fjöll have never been mapped; as a result the
map (Fig. 2) is both incomplete and unsatis-
factory in tliat respect. Yet, it shows more than
40 eruption fissures and single vents. Probably
a number of eruption centres of recent age are
hidden below younger volcanic formations. The
eruption fissures either follow the circular frac-
ture systems of the calderas or the trend of the
neovolcanic zone crossing the Dyngjufjöll massif
from SW to NE, along the western part of
Askja caldera.
According to the existing data all the erup-
tions in the Dyngjufjöll area have been basic
except the Askja eruption of 1875 which pro-
duced rhyolitic pumice. Sigvaldason (1964) also
notes some acidic and intermediate rocks at the
southern caldera rim of the Askja Lake. The
rhyolite eruption of 1875 together with a great
amount of xenoliths of plutonic origin, found
in the rhyolitic pumice, the high temperature
activity and the caldera indicate strongly that
Dyngjufjöll are a central volcano, in spite of
the volcanic ejecta being predominantly basic
for several ten thousands of years.
THE SUBSURFACE
OF MÝRDALSJÖKULL
Fig. 4 is a map of Mýrdalsjökull, but how is
its subsurface? Thickness measurements of the
Mýrdalsjökull ice cap were carried out in 9
places in the year 1955. The thickness of the
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