Jökull - 01.12.1992, Side 9

tion period during the recent Krafla rifting episode
(up to the last eruption) was over 4 years. The to-
tal lava production in two Krafla rifting episodes and
Askja-Sveinagjá is strikingly similar although various
aspects of eruption behavior are different. The Krafla-
Gjástykki lavafield (1975-1984) is about 35 km2 with
an estimated volume of 0.25 km3 which is similar to
the lava production during the Krafla rifting episode
in 1724-1729 (Sæmundsson, 1991). The Sveinagjá
lavafield (1875-1876) is 30 km2 with a volurne esti-
mate of 0.3 km3. The dense-rock volume of the Plinian
eruption within the Askja caldera has been estimated at
0.21 km3 (Thoroddsen, 1925; Sigurðsson and Sparks,
1978) and 0.43 km3 (Sigvaldason, 1979).
However, the subsidence volume estimates for
the Askja 1874-1876 and the recent Krafla rifting
episodes are markedly different. The volume of the
Oskjuvatn caldera, formed during the Sveinagjá erup-
tions is approximately 1.9 km3 according to Þórarins-
son (1963) but Sigvaldason (1979) estimates the total
subsidence 2-2.5 km3. The total subsidence volume
°f the Krafla magma chamber, since 1975, is esti-
mated approximately 0.68 km3 (Tryggvason, 1984;
1986). The greater subsidence volume during the
Askja-Sveinagjá rifting could be taken as an indicator
of greater intrusive activity.
The intrusion/extrusion ratio of each rifting
episode is govemed by accumulated extensional stress
along the divergent plate boundary and magma up-
welling from the mantle. The markedly higher in-
trusion/extrusion ratio of the Askja 1874-1876 rifting
episode compared to the recent Krafla rifting episode
reflects variations in the spreading capacity within
the Northern Volcanic Zone. The 1874-1876 Askja-
Sveinagjá activity took place after more than 400 years
of quiescence, during which tensile stress within the
Askja fault swarm increased to the point that when the
central volcano became active it could accommodate
most of the available magma.
CONCLUSIONS
1 • Reports of felt earthquakes during the Askja rift-
ing episode 1874-1876 indicate that most of the
intrusive activity took place during the first few
months.
2. The biggest earthquakes associated with intrusive
activity in the Askja fissure swarm attained inten-
sity V or even VI on the modified Mercalli scale,
at a distance of 35 km, which indicates that they
may have exceeded magnitude 5 on the Richter
scale.
3. The total lava production during the Sveinagjá
eruptions, the Askja eruptions 1921-1933 and
the Krafla eruptions in 1724-1729 and since 1975
are nearly equal in volume, between 0.25 km3 and
0.3 km3 whereas the Askja 1961 eruptionhas an
extrusion volume of about 0.1 km3.
4. The estimated subsidence volume during the
Sveinagjá eruptions and the ongoing Krafla rift-
ing episode is markedly different indicating a dif-
ferent intrusion/extrusion ratio for these events.
5. The last eruption in Askja, October-December
1961, was followed by two earthquake swarms in
January and June 1962, which are strikingly sim-
ilar to the swarms accompanying lateral magma
intrusions in Krafla. Most likely two brief intru-
sive events followed the 1961 eruption in Askja.
ACKNOWLEDGMENTS
Thanks are due to Haukur Jóhannesson and Guð-
mundur E. Sigvaldason for giving me access to their
copies of old newspapers and other accounts of the
Askja and Sveinagjá eruptions, 1874-1876. Useful
reviews by Páll Einarsson, Magnús T. Guðmundsson
and Haukur Jóhannesson improved the manuscript.
REFERENCES
Annertz, Kristian, Magnus Nilsson and Guðmundur
E. Sigvaldason. 1985. The postglacial history of
Dyngjufjöll. Nordic Volcanol. Inst. Rep. 85 03,
22 pp.
Anonymous. 1875a. A letter from Mývatn dated Jan.
8. Norðanfari 14, no. 5-6, pl2, Jan. 29 (most
likely a letter from Jón Sigurðsson, Gautlönd, Mý-
vatn). (In Icelandic).
Anonymous. 1875b. A letter from Mývatn dated Feb.
5. Norðanfari 14, no. 9-10, p20, Feb. 19. (In
Icelandic).
JÖKULL, No. 42, 1992 7