Jökull - 01.01.2012, Síða 9

Sigurður Þórarinsson (1912–1983)
of the 12th Century, in other parts of the country it had
dwindled much during the 13th and 14th Centuries, but
in SW-Iceland it had lingered till the end of the 16th.
Finally, Sigurður described a layer of charcoal, 1 to
2 cm thick, in his soil sections around ruins in Þjórs-
árdalur and by a ruin in Borgarfjörður. The layer was
taken to indicate singeing of birch-shrub around farms
to open grassland or make fields. At the time of settle-
ment, singeing was practiced in Sweden and Finland
but neither in Norway nor Britain, which Sigurður
thought might support ideas, then in vogue, that some
of the Iceland settlers had originated in eastern Scan-
dinavia. Thus his doctoral thesis dealt equally with
volcanology and the history of the Icelanders. In 1948
he published two articles (in Icelandic) on the his-
torical implications of his tephrochronological work,
his last on pollen analysis. Tephrochronology, which
originally had been intended as support for pollen-
analytical studies in Iceland under the influence of von
Post, now had become the very purpose.
Hekla 1947–1948
The Hekla eruption 1947 started on 29 March and
lasted for 13 months, giving scientists ample oppor-
tunity to study its various aspects. For Sigurður
Þórarinsson it came at a fortunate time. He had soon
realized that the key to Icelandic tephrochronology
lay with Hekla’s tephra layers and eruption history.
His early work with Hákon Bjarnason had mainly
dealt with the light-colored layers in North- and East-
Iceland, and the first soil sections he measured around
Hekla were dug in Þjórsárdalur in 1939. At that time,
little was known about Hekla’s behavior, e.g. neither
that the composition of the magma changes with time,
nor that the greatest part of the tephra is erupted in a
relatively very short time. For that reason the tephra-
sector produced in each eruption tends to be narrow
and having a definite direction governed by the pre-
vailing wind at its outbreak. Many of these prob-
lems came to light as Sigurður worked on his data
from 1939, pointing the way to further work after the
War. Already in 1945 he started work on tephra layers
around Hekla, but the eruption redoubled his efforts.
The eruption aroused tremendous interest in Ice-
land. By the end it was estimated that about half the
nation had witnessed it. Also, it was the first erup-
tion to be studied by Icelandic scientists whose efforts
were from the beginning well organized. The Ice-
land Science Society in cooperation with the Natural
History Museum launched a five-volume publication
of collected research papers, edited by T. Einarsson,
G. Kjartansson and S. Þórarinsson. Sigurður him-
self contributed four papers, about 300 pages alto-
gether. The first one (1950) described the approach
and beginning of the eruption based on twenty eye-
witness accounts. No earthquakes were found by hu-
mans prior to the eruption, but farm animals proba-
bly sensed some tremors 3–4 days before. Appar-
ently the tremor-intensity grew during the night be-
fore the eruption and may have been felt up to 100 km
away. The eruption broke out at 6:41 on 29 March,
a sharp earthquake occurred at 6:50, and at 7 o’clock
the volcanic cloud had reached 26 km height. Sig-
urður also collected photographs of the onset of the
eruption which were interpreted by T. Einarsson.
In his second paper (1954), Sigurður dealt with the
tephra-fall on the first day of the 1947-eruption. This
68-page treatise introduced many new approaches and
ideas. The results of 96 measured samples are sum-
marized in a map showing the distribution on land
of brownish-grey and brownish-black tephra, respec-
tively, and grain-size measurements are summarized
in a graph showing linear relationship between the log
of the mean of the tephra grain size and distance from
the volcano (Figure 2). The course of the tephra from
Hekla via Scotland to Finland is traced and explained
with the help of six meteorological maps. Finally, one
of Sigurður’s best-known diagrams appeared for the
first time, the relation between SiO2-content of the
initial products of an Hekla eruption and the preced-
ing interval of quiescence (Figure 3).
The third and most monumental of Sigurður’s
Hekla papers was a book entitled The eruptions of
Hekla in historical times. A tephrochronological study
(1967). In the book he recounted everything known
to have been written about Hekla’s eruptions up to
and including the one in 1693, and for later erup-
tions summed up the ever-increasing wealth of doc-
uments. Based on numerous soil sections he con-
structed isopach maps of seven Hekla tephra layers
from 1104 to 1947 and summarized all 14 tephra lay-
JÖKULL No. 62, 2012 7