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SUMMARY
The Þjórsá lava
- the largest Holocene
lava flow on Earth
by
Árni Hjartarson
Orkustofnun
(National Energy Authority)
Grensásvegur 9
Reykjavík, Iceland
The Þjórsá lava is the most voluminous lava in
Iceland and the largest Holocene lava flow on
Earth, both with respect to volume and area.
It was erupted from a crater row in the Veiði-
vötn volcanic area in Central Iceland. The
craters and the uppermost parts of the lava
seem to be mostly buried in younger erup-
tives, lavas and tephra. Some 75 km south-
west of the volcanic site the lava crops out and
covers an extensive area between the glacial
rivers Þjórsá and Hvítá/Ölfusá. These two,
the largest rivers of Iceland, have found their
way along the eastern and western edges of
the lava all the way to the sea. The lava forms
the reef-bound coast between the river
mouths and stretches several hundred meters
out from the shore below sea level. The sea
level seems to have been 5-10 m lower at the
time of the eruption than it is at present.
The total length of the lava is at least 140
km, making it the longest flow in Iceland. Its
thickness in the farming areas of South Ice-
land is fairly well known, being 15-20 m thick
in its distal parts in the Flói district. In Skeið
further north it is 20-25 m thick and in Land
the thickness is about 30 m. The average
thickness calculated from 40 boreholes along
most of its length is 22 m. The lava is estimat-
ed to be about 950 km2 in area. The total
volume is therefore 21 km3. All are minimal
numbers.
The Þjórsá lava is plagioclase porphyritic
tholeiite with 2-20% by volume of plagio-
clase macrophenocrysts. Phenocrysts of oli-
vine are also found and, more sparsely, au-
gites. The distribution of the phenocrysts is
rather even, but in a few places plagioclase-
rich bands are found. This phenomenon can
be explained by assuming that the amount of
phenocrysts in the lava did differ from one
crater to another along the fissure.
No ash layer is known to have accompanied
this eruption, but in deep ice cores from
Greenland there are indications of a violent
eruption somewhere in the northern he-
misphere around the date 6675±150 BC. A
new radiocarbon dating of the lava gives the
age of 7800±60 conventional radiocarbon
years. These two individual age measure-
ments fit fairly well and when they are con-
verted into calendar years the outcome from
both of them is close to 8600 B. P.
16