Náttúrufræðingurinn - 1992, Síða 48

SUMMARY
Hekla, a volcano with a long
history
by
Haukur Jóharmesson
Icelandic Museum of Natural History
Hlemmur 3
IS-105 REYKJAVÍK
Iceland
and
Sigmundur Einarsson
Birtingakvísl 19
IS-110 REYKJAVÍK
Iceland
Hekla is the most famous volcano in
Iceland and in the past it was believed in
Europe that it was one of two entrances to
hell, the other being Mt. Etna. Volcanic
eruptions in the Hekla volcano have
caused destruction in the past, mainly by
tephra fall.
It seems that the volcano itself has
caused fear in the past and it was climbed
with certainty for the first time in the year
1750. The Hekla has interested many for-
eign travellers and scientists.
The late Sigurdur Thorarinsson devel-
oped the tephrochronology during the
fourth decade of the twentieth century
and later applied that technique to estab-
lish the volcanic history of the Hekla vol-
cano since the settlement of Iceland (900
AD).
The great Hekla eruption in 1947^18
aroused interest, and many papers on the
volcano were published in the following
decades.
The volcanic zones in Iceland are of two
types. Firstly, a rift zone which marks the
boundary between the North American
and Eurasian plates, respectively, and sec-
ondly, the flank zones which are offaxis
phenomena (Fig. 2). The volcanic activity
within the volcanic zones is restricted to
volcanic systems, each of which consists of
a central volcano and an integrated fissure
swarm. The central volcanoes within the
rift zone seem to be different from the
central volcanoes of the flank zones. The
former are usually lower, due to constant
rifting and relatively thin underlying crust,
but the Iatter are usually majestic strato
volcanoes which rest on thicker underly-
ing crust. The Hekla volcano is Iocated
where the rift zone is gradually changing
into a flank zone. It thus bears the charac-
ter of both types. It is high as the flank-
zone volcanoes, but elongated, and erup-
tive activity is mainly along a fissure
swarm as in rift-zone volcanoes.
The Hekla volcano is a part of a larger
volcanic system, i.e. Hekla volcanic^sys-
tem. The system is about 50-60 km long
and 20 km wide. The Hekla mountain is in
the centre of the volcanic system, but else-
where, crater rows and individual craters
are common.
The oldest formations believed to be-
long to the Hekla volcanic system, are
subglacially formed hyaloclastite ridges
(Fig. 6) from the last glacial period (Wei-
chelian glaciation).
As the glaciers retreated about 10.000
years ago, the volcanic activity changed
from subglacial to subaerial volcanism.
During the Holocene time, the volcanic
activity seems to have been more or less
continuous. During that time, numerous
lava flows have formed, and most erup-
tions have produced tephra which can be
identified as tephra layers in soil sections.
The lava flows can be classified into
three groups, based on their age (Fig. 4).
1. Older Hekla lavas. In this group are
mainly basaltic lava flows which are older
than 4500 years.
2. Vatnafjöll lavas. In this group are
mainly basaltic lava flows which were
formed during thc time interval from 4500
years ago until about 2000 years ago. They
originate from crater rows in the Vatna-
fjöll area east of Hekla.
3. Younger Hekla lavas. In this group
are mainly intermediate lava flows but al-
so a number of basaltic lava flows. These
flows were formed after 500-600 AD.
It is a well-known fact that mature cen-
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