Náttúrufræðingurinn - 1995, Page 104

and the fissure system associated with the
Grímsvötn central volcano may also extend
into the mapped area as Holuhraun lava.
Figure 2 shows a general geological map of'
the research area where the lavas from each
fissure swarm are as a rule grouped together as
a unit. No attempt was made in separating the
different lava flows originating in the same
fissure swarm, with the exception of some
lavas from craters or crater rows situated
somewhat outside the main fissure swarms.
Still the lavas from the Dyngjufjöll fissure
swarm crossing Gígöldur area are devided into
two groups, Kverktjallahraun and Kreppu-
tunguhraun, based on the great difference in
appearance and texture of the lavas and their
craters and also due to their different age. The
main lava fields introduced on the map are
Kverkfjallahraun, Trölladyngjuhraun, Kreppu-
tunguhraun, Dyngjufjallahraun, and some
lavas surrounding Herðubreið and Holuhraun.
Kverkfjallahraun lavas originate in at least
five volcanic fissures in the western part of
Kverkfjallarani ridge, but on its eastern slope
there are two crater rows that have produced
the two lavafields, Kreppuhraun and Linda-
hraun. No attempt was made to differentiate
lavafields originating in the Trölladyngja
shield volcano from lavas from the Bárðar-
bunga fissure swarm on Dyngjuháls which at
least six eruptive fissures. Krepputunguhraun
originates in two or more old volcanic fissures
in Gígöldur, which in both texture and flow
structure are very similar to the Þjórsárhraun
lavas in southern Iceland. They can also be es-
timated to be of the same age, about 6000-
8000 years old, since they are everywhere the
oldest unit in the lava sequence. The lavafield
ot Krepputunguhraun can be traced toward the
east as far as the end of Krepputunga at
Herðubreið, and its western branch can even
be traced as Bárðardalshraun almost all the
way down to the coast. Dyngjufjallahraun
lavas originate in Dyngjufjöll central volcano
and its fissure swarms. They have flowed from
a great number of craters on the southern
slopes of Dyngjufjöll and in Gígöldur as well
as from craters in the Askja caldera. Mount
Herðubreið is surrounded by lavas from two
small shield volcanoes, Flatadyngja and
Svartadyngja, as well as lava streams from NE
trending fissure swarm of Dyngjufjöll central
volcano. Holuhraun at the northern edge of
Dyngjujökull is most likely a historical lava
and can possibly be related to the fissure
swarm of Grímsvötn central volcano.
The tectonic features of the research area are
very distinct and obvious in the field and are
closely connected with the evolution of the
central volcanoes and their associated fissure
swarms. The main SW-NE tectonic trend of
the rift-zone of southern Iceland here gradu-
ally shifts to the N-S direction of northern Ice-
land and it is commonly characterized by
transverse volcano-tectonic features such as
eruptive fissures and faulting. The most
prominent faulting is that trending to the NE
from Kverkfjallarani, where single faults in a
postglacial lava have been measured up to 20
m. Krepputunguhraun lava was erupted 6000-
8000 years ago and by studying its flow struc-
ture between Gígöldur and the river Kreppa, it
is clear that the downthrow of the block be-
tween the fissure swarms of Dyngjufjöll and
Kverktjöll can be estimated to be at least 60-
70 m, whereof at ieast 20 m date from the last
prehistoric, catastrophic flooding of river
Jökulsá occurring about 2000 years ago. That
is in good agreement with the average subsid-
ence of the main rift-zone.
In the next article of this series the cata-
strophic floods of the river Jökulsá and related
volcanism will be discussed.
PÓSTFANG HÖFUNDAR/AUTHOR'S ADDRESS
Guttormur Sigbjarnarson
Skaftahlíð 8
IS-105 Reykjavík
Iceland
212