Jökull - 01.01.2012, Blaðsíða 8

S. Steinþórsson
duced. Also it was less silicic than the light-colored
Hekla layers found in soil sections in N-Iceland and
in Þjórsárdalur, in 1947 the first tephra contained 62%
SiO2 as compared to 67% in the Þjórsárdalur pumice.
This suggested that the voluminous silicic tephra lay-
ers mark the inset of a new eruptive cycle in Hekla, in
which the succeeding eruptions are both smaller and
less silicic. It also became clear that the largest part
of the tephra emitted in each eruption is produced in
a short time at the beginning, usually forming a nar-
row tephra sector next to the volcano. This first tephra
tends to be lighter-colored and more silicic than the
fine-grained ash spreading in all directions later in
the eruption. For that reason, tephra profiles must be
taken quite closely-spaced around the volcano for cor-
rectly ascribing tephra layers to given eruptions. With
this in mind, Sigurður undertook the project of mea-
suring tephra layers around Hekla, a task that with
other work lasted many summers. It emerged that
a dark ash layer which he in his initial Þjórsárdal-
ur study had marked as H 1693 was in fact H 1300,
the main part of which having fallen to the east of
Stöng. In view of its unusual thickness and chemical
composition, the light pumice in Þjórsárdalur and N-
Iceland could be related to a succinct description for
the year 1104 in some annals: "The first eruption in
Mt. Hekla." In a newspaper article in 1949 Sigurður
declared 1104 A.D. as the correct age of the Þjórsár-
dalur tephra and the beginning of the present eruptive
cycle in Hekla.
The riddle of the 66 skeletons was first solved
15 years later when Sigurður learned that in 1935
an Icelander studying anthropology in Germany had
removed about 30 skeletons from the cemetery in
Skeljastaðir and transported them abroad.
Tephrochronological Studies in Iceland
The outbreak of the war in 1939 thwarted Sigurður’s
plans to extend his Þjórsárdalur research with addi-
tional two summers’ field work in Iceland. The in-
tended project had been to map the geographical evo-
lution of Iceland in postglacial time, with emphasis
on variation in climate and vegetation during the first
centuries of settlement in the country. Instead, be-
ing unable to gather physical data he turned to the
study of written documents, the fruits of which were
to become an important part of his doctoral thesis in
1944. Luckily for him the greatest part of old Ice-
landic documents were kept in Copenhagen, and from
the philologists studying them Sigurður learned to use
them and assess their veracity; his early training in
Latin (and some classical Greek) also came to good
use. In Aristotle’s Meteorologica he found the word
tephra for volcanic ash, in a description of an eruption
in the island Vulcano.
Sigurður’s doctoral thesis Tephrochronological
studies in Iceland, which he defended at the Univer-
sity of Stockholm in 1944, consists of two main parts.
The first relates tephra studies in Iceland up to 1939
with special emphasis on the Þjórsárdalur work. Soil
sections are described and discussed with references
to written sources, chemical analyses of tephra lay-
ers are given and compared with the refractive index
of the glass, and detailed description is given of the
1875-Askja eruption, the tephra layer, wind directions
etc., as a well-documented model to compare with
older ash layers. In this study Sigurður may be said to
have employed all methods used today except grain-
size and microprobe analysis. The latter, of course,
didn’t exist at that time, but in the thesis he suggests
that estimates of tephra grain size be standardized in
the way sediments are. He also introduced standard-
ized graphical symbols to denote the different types
and grain sizes of tephra.
The second part of the thesis was more akin with
his initial plans of studying the environmental effects
of settlement in Iceland. Pollen profiles of soil sec-
tions by farm ruins Stöng and Skallakot in Þjórsár-
dalur are described, and at both locations Sigurður
found a very marked change in vegetation above the
tephra layer denoted VIIa+b (now known as "Settle-
ment layer" and dated at 871±2 A.D.). Birch pollen
decrease suddenly to be replaced by pollen of plants
associated with human activity: grasses, medicinal
plants, spices, ale making. At the time this work
was so avant garde that some of the cited articles had
just been published whereas others were still being
printed. Following this he studied the history of cereal
growing in Iceland using trade files and place names
indicating such activity; the result was that cereal
growing had ceased in the North and East by the end
6 JÖKULL No. 62, 2012