Jökull - 01.01.2019, Side 47

Einarsson
Krafla
The Krafla volcanic system in the Northern Volcanic
Zone (Figure 1) extends along more than 100 km of
the plate boundary and a part of it has been populated
since the settlement of Iceland. Its activity appears to
occur in episodes at a few hundred years interval. The
magma-tectonic episode of 1724–1746 is the only his-
torically documented activity of Krafla prior to its re-
cent episode of 1974–1989. The two episodes appear
to have been quite similar, with repeated intrusions
and several lava eruptions, particularly towards the
end of the episodes (Björnsson et al., 1977; Sæmunds-
son, 1991; Einarsson, 1991b). The main difference is
that the early episode began with an explosive phase
from the Víti crater, apparently triggered by an intru-
sion of basalt into a more silicic batch of magma. The
contemporary description is by Sæmundsson (1726)
as reported by Thoroddsen (1925).
1724: Frequent earthquakes were felt in the district
around Krafla during the night between May 16 and
17. An explosive eruption occurred around 9 in the
morning of May 17 from the crater Víti within the
Krafla caldera. Earthquakes and eruptive activity con-
tinued for some time after the initial explosion, and
large surface deformation was documented, probably
indicating a dike intrusion into the southern fissure
swarm (Hjartardóttir et al., 2012).
1725: Earthquakes were felt in the Krafla area begin-
ning after the New Year. An eruption broke out in the
middle of the caldera on January 11.
1725: An eruption broke out in Bjarnarflag, 6 km
south of the caldera, on April 19 following terrible
earthquakes and rifting over 30 km long segment of
the southern fissure swarm.
1727: A lava eruption began in the center of the
caldera on August 21 producing a large lava field, but
earthquakes are not mentioned.
1728: An eruption began in the central part of the
caldera on April 18 at 2–3h in the morning, and a
few hours later also 3 km south of the caldera. These
eruptions were preceded by felt earthquakes during
the whole night.
1728–1729: Further eruptions occurred during these
years but earthquakes are not mentioned: December
18, January 30, July-August.
1746: Eruption occurred in the middle of the caldera
accompanied by earthquakes.
Grímsvötn, Laki
One of the largest volcanic events in Icelandic his-
tory was the Laki eruption (Figure 1) that occurred on
the fissure swarm of the Grímsvötn volcanic system in
1783–1785. It was a fissure eruption of 30 km length
but was accompanied by eruptive activity of the cen-
tral volcano of Grímsvötn (e.g., Thordarson and Self,
1993). Several of the phases of the eruption were ac-
companied by felt earthquakes. The closest inhabited
areas were at about 25 km distance from the active
fissure swarm and 50 km from the Grímsvötn central
volcano (Figure 1).
1783: The eruption began on June 8 with a tall erup-
tive column that could be seen from neighboring dis-
tricts. Earthquakes had been felt there for ten days be-
fore this. Earthquakes were felt off and on during the
following months, sometimes in association with in-
creasing intensity of the lava eruption. The lava erup-
tion from the Laki fissure ended in February 1784, but
discontinuous activity was seen in the Grímsvötn vol-
cano until May 26, 1785 (Thordarson, 1991).
1934: The eruption in the Grímsvötn caldera that be-
gan on March 30 was apparently triggered by a jök-
ulhlaup from the caldera lake that began on March
22 (Thórarinsson, 1974). Similar triggering was doc-
umented for the 2004 eruption when the pressure
drop in the caldera lake apparently triggered an erup-
tion from an inflated magma chamber of the volcano
(Vogfjörð et al., 2005). In 1934 earthquakes were
recorded by the seismic station in Reykjavík at a dis-
tance of 225 km from Grímsvötn on March 30 but
their temporal relation to the beginning of the erup-
tion is not clear (Brandsdóttir, 1984).
Askja
1874–1876: Nothing is known about earthquakes in
the remote Askja volcanic system (Figure 1) prior
to the rifting episode of 1874–1876 (Brandsdóttir,
1992). The first signs of activity were steam clouds
over Askja, seen from afar. Eruptive activity was
verified in January 1875 and was preceded by earth-
quakes felt for two weeks in the adjacent inhabited
46 JÖKULL No. 69, 2019