Jökull - 01.12.1979, Síða 34

reached where the fissure swarm through it is due
for rupture. This would also explain that extensive
rifting may occur without any volcanic activity
taking place if the magma supply from below is
small. The fissure swarm is in that case due for
rupture several times before an eruption
occasionally takes place. Rifting of the Theista-
reykir fissure swarm in northern Iceland in 1618
and of the Hengill swarm in southwestern Iceland
in 1789 are examples of the latter type of “abortive
fissure eruptions”. The largest fissure eruptions of
historical age have occurred on fissure swarms
southwest of Vatnajökull. Two of them, Eldgjá
(Katla system) around 930, and Lakagígar
(Grimsvötn system?) in 1783, are an order of mag-
nitude larger than all other historical volcanic
eruptions. Each of them produced around 10 km3
of lava. During the latter enormous amounts of
gases polluted the environment causing the worst
famine in the history of Iceland. About 70% of the
livestock perished and as a result 22% of the popu-
lation died.
The total production of lava and tephra in Ice-
land during the 11 centuries of settlement is
approximately the following, both lava and tephra
calculated as lava with spec. weight 2.5:
Basic
Intermediate
Acid
Total
lava km3
25
7
0.3
32
tephra km3
6
1
2.8
10
This makes altogether 42 km3, whereof 10% are
intermediate and 7% acid. The abnormally high
percentage of basaltic tephra is due to some of the
most productive basalt volcanoes being ice covered.
The production rate of the active volcanic areas
in historical time, about 4 km3/century, is roughly
the same as in Postglacial time as a whole and the
production rate has been similar through the last
~ 16 m.y. The percentage of acid and intermediate
eruption products in historical time is about double
the average of the entire Postglacial time as well as
for the last ~ 16 m.y.
SELECTED REFERENCES
Björnsson, A., K. Saemundsson, P. Einarsson, E
Tryggvason and K. Grönvold, 1977: Current
rifting episode in' North Iceland. Nature 266:
318—323.
Björnsson, H., 1974: Explanation of jökulhlaups
from Grímsvötn, Vatnajökull, Iceland. Jökull
24: 1—26.
Einarsson, T., G. Kjartansson, and S. Thorarins-
son, (Eds.), 1951 —1976: The eruption of Hekla
1947—1948. 1—4. Soc. Sci. Islandica. Reykja-
vík.
Jakobsson, S. P., 1979: Petrology of Recent basalts
of the eastern volcanic zone, Iceland. Acta Nat.
Islandica 26: (in press).
Larsen, G., 1979: Um aldur Eldgjárhrauna
(Tephrochronological dating of the Eldgjá
lavas in South Iceland). Náttúrufrædingurinn
49: 1—26.
Lock, W. G., 1881: Askja. Iceland’s largest volcano.
Published by the author at 16, Kingston Ter-
race, Charlton, Kent: 106 pp.
Thorarinsson, S., 1958: The Öræfajökull eruption
of 1362. Acta Naturalia Islandica II, 2: 100 pp.
Thorarinsson, S., 1967: Some problems of volcan-
ism in Iceland. Geol. Rundschau 57: 1 — 20.
Thorarinsson, S., 1969: The Lakagígar eruption of
1783. Bull. volcanologique 33—3: 910—929.
Thorarinsson, S., 1974: Vötnin stríd [The swift
flowing rivers]: 254 pp. Bókaútgáfa Menning-
arsjóds. Reykjavík.
Thoroddsen, Th., 1925: Die Geschichte der islán-
dischen Vulkane. Det Kgl. Danske Vidensk.
Selsk. Skrifter. Naturv. og Mathem. Afd. 8,
Række IX: 458 pp.
32 JÖKULL 29. ÁR