Náttúrufræðingurinn - 1985, Side 36

Snið 14 Stóri-Klofi er í efnisnámu Vegagerð-
arinnar austan við Stóra-Klofa á Landi. Á þess-
um stað sést, að áll úr Selsundsvikurshlaupinu
hefur komist inn á vatnasvið Pjórsár. Áll úr H4
hlaupinu hefur einnig farið hér yfir. H3 lagið er
hinsvegar loftborið. Hin dæmigerða ásýnd þess
á þessum slóðum og víða um Suðurlandsundir-
lendi er sem hér sést. Lagið sjálft er dökkt og lítt
áberandi inn á milli þykku svörtu fylgilaganna
tveggja, en gleggsta einkenni þess er hinn ljósi
fokríki jarðvegur yfir því.
SUMMARY
Pumice Flows
During Hekla Eruptions
by
Elsa G. Vilmundardóttir and
Arni Hjartarson
National Energy Authority
Grensásvegur 9
108 Reykjavík
The Hekla eruptions are often accompanied
by mud- or pumiceflows and floods in rivers in
the neighbourhood of the volcano. These flows
and floods may have three main causes. The
first is the melting of snow and small glaciers on
the flanks of the mountain. The second is con-
densation of vapour from the volatiles of the
eruption. The third is damming of rivers due to
the enormous pumice production and ashflows
at the initial stage of the eruptions. These floods
are sometimes mentioned in historical littera-
ture and Annals. They have also been geolo-
gically explored in the eruptions of 1947 and
1845. From prehistorical times geological evi-
dence shows that vast pumice- and mudflows
have taken place during the great Plinian erup-
tions of Hekla. These eruptions produced the
large tephra layers H3 2800 years BP., HSv -
Selsund pumice - approximately 3400 years BP
and H4 4000 years BP, (C,4 age, Tl/2 = 5570)
Thick layers of fluviatiles, mostly made of acid
Hekla tephra have been found along the large
rivers in South Iceland, Ytri-Rangá, Þjórsá,
Stóra-Laxá and Hvítá. The distribution of the
tephra and the floodchannels are shown in fig.
4. In the H3 and H4 eruptions, ashflows have
rushed down the watercourses of Ytri-Rangá
and Þjórsá. Branches of the floods in Þjórsá
have overflowed the river channel near Árnes
and flowed westwards to Stóra-Laxá (fig. 4, 5
and 6). Thick layers of fluviatile H3 pumice are
found in the district of Skeið, some 50 km
southwest of Hekla. The eruption producing the
Selsund pumice has been somewhat different.
For some unknown reasons, a great amount of
the tephra did not become airborne but instead
rushed down from the vent or vents and rushed
over the birchwoods to the southwest of Hekla
and along the channel of Ytri-Rangá and pro-
bably all the way down to the coast.
HEIMILDIR
Guðmundur Kjartansson. 1945. Hekla. -
Árbók F.f. 1945: 167 bls.
Guðmundur Kjartansson. 1951. The Er-
uption of Hekla 1947—1948, 11,4. Wat-
er Flood and Mud Flows. - Vísindafélag
íslendinga: 60 bls.
Guðmundur Kjartansson, Sigurður Pórar-
insson og Þorleifur Einarsson. 1964. C-
14 aldursákvarðanir á sýnishornum
varðandi íslenska kvarterjarðfrði. —
Náttúrufræðingurinn 34: 97—149.
Guðrún Larsen. 1978. Gjóskulög í ná-
grenni Kötlu. - 4. árs ritgerð við Há-
skóla íslands: 60 bls.
Guðrún Larsen og Sigurður Þórarinsson.
1977. H4 and Other Acid Hekla Tep-
hra Layers. - Jökull 27: 28—46.
Islandske Annaler indtil 1578, 1888. -
Udg. ved G. Storm. Christiania: 485.
bls.
Oddur Erlendsson. 1847. Dagbók, óprent-
uð, varðveitt á Landsbókasafni.
Sigurður Þórarinsson. 1968. Heklueldar. -
Sögufélagið, Reykjavík: 202 bls.
Sigurður Þórarinsson. 1976. Gjóskulög. -
Samvinnan 70, 1: 4—9.
Sigurður Þórarinsson, Trausti Einarsson
og Guðmundur Kjartansson. 1959. On
the geology and geomorphology of Ice-
land. - Geografiska Annaler. Stock-
holm 41: 135-169.
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