Jökull - 01.01.2012, Síða 10

S. Steinþórsson
ers in two different pictures, one recasting them as
arrows on a single map (Figure 4), the other show-
ing cross sections taken 15 km away from the volcano
(Figure 5).
Figure 2. Hekla 1947–1948. Log of median size of
tephra grains plotted against distance from the vol-
cano. – Hekla 1947–1948. Meðalstærð korna (log-
skali) dregin móti fjarlægð frá eldstöð.
His fourth and final article in The Eruption of
Hekla 1947–1948 series gives a day-to-day account of
the eruption (1976), summarized in a single graph in
which tephra production, lava flow, earthquakes, ex-
plosions and activity in different craters, are plotted
against time. All in all the research reported in the
Hekla series was a milestone in Icelandic geoscience
and some of the articles, not least Sigurður’s descrip-
tions, are still quoted by volcanologists.
After the 1947-eruption it was generally believed
that Hekla would remain dormant for a century or so.
That was not to be, and during Sigurður’s life time
Hekla was to erupt twice, in 1970 and 1980–1981,
(and after that in 1991 and 2000). Sigurður, now uni-
versally recognized as Iceland’s foremost volcanolo-
gist, took active part in the study of both eruptions.
The power of tephrochronology
In the summer of 1948 Hans Ahlmann, Sigurður’s old
teacher and colleague, brought a group of Swedish
geographers to Iceland and Sigurður guided them
around the South. Following this brief excursion he
wrote an article in Geografiska Annaler (1949) about
tephrochronology and its use for dating in glaciology
and volcanology. On the trip he had demonstrated to
the Swedes the power of his method by dating on the
spot moraines at Hagafellsjökull, the Great Geysir,
and the Helgafell lava flow in the Vestman Islands.
In view of this success, Ahlmann suggested that Sig-
urður try his tephrochronological hand at the study
of frost phenomena in Iceland, which was to lead to
further such studies. This adventure was followed up
by annual Nordic geological and geographical expe-
ditions to Iceland, guided for many years by Sigurður
Þórarinsson, and indirectly led to the foundation of the
Nordic Volcanological Institute in Reykjavik in 1974.
Around 1950 Sigurður did reconnaissance geo-
logical studies in preparation of various hydropower
projects, including the rivers Laxá and Jökulsá. This
led him to the study of the volcanic history of the Mý-
vatn area, about which he was to write many articles.
Likewise he studied the geology of the Jökulsá gorge,
using tephrochronology to trace changes in the course
of the river and the evolution of the gorge.
Prior to the advent of radiocarbon dating in 1949,
the only "absolute" ways to date postglacial geologi-
cal phenomena were the counting of varves and tree-
rings. To this arsenal Sigurður added tephrochronol-
ogy, built on written documents. In 1954 he wrote an
article about dating in geology, and a year later he had
obtained radiocarbon dates on Hekla’s three prehis-
toric marker layers H3, H4, and H5, thereby extend-
ing Iceland’s absolute tephrochronological time scale
back to 7000 years.
A jökulhlaup in Múlakvísl in 1955 prompted Sig-
urður to start studying the eruption history of Katla,
the numerous black tephra layers of which appeared
to be indistinguishable from each other. Twenty years
later, after having finally found the 1362-Öræfajök-
ull layer within the Katla sequence, and hence tied
the latter with the tephrochronological time scale, he
could publish Katla and its eruption annals (1975).
8 JÖKULL No. 62, 2012