Náttúrufræðingurinn

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Náttúrufræðingurinn - 1992, Side 48

Náttúrufræðingurinn - 1992, Side 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- 190
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