Náttúrufræðingurinn - 1983, Side 201

við ásamt félögum okkar til að bjarga
því sem eftir var af blautum og skít-
ugum tjöldunum og við að sjá tætlurn-
ar af þeim urðum við sælustu flótta-
menn sem um getur. Við fólkið í Búr-
fellsvirkjun stöndum við í ævarandi
þakkarskuld fyrir gestrisnina.
The Eruption of Hekla 1980
lan P. Hutchinson,
Department of Geology,
University of Witwatersrand,
1 Jan Smuts Avenue,
Johannesburg 2001,
South Africa
In common with most universities, the
Oxford geology degree course requires the
submission of a field mapping report. For
this, one has to individually map the geo-
logy of an area, 15 km2. With help from
Drs. I. B. Fridleifsson and H. Jóhannes-
son suitable areas northeast of Hekla were
chosen for myself and my colleague
Andrew McKenzie. My area lay between
the end of Heklugjá and Hestalda,
incorporating Rauðkembingar and Skjól-
kvíahraun with the 1970 volcanics.
Up to the August Bank Holiday we
were lucky with the weather and on a
clear, sunny day at the end of July we
climbed to Hekla’s summit. All was quiet
on the mountain, and snow filled the fis-
sures of the previous eruption in 1947. The
simmering power within the volcano was
only betrayed by wisps of steam rising
from the rubble walls of the summit crater.
During my stay in the area the snow on
Hekla’s northwest flank had shown signs
of pregressive melting and crevassing. This
can be seen on my photographs of the
mountain. Comparison of these with aerial
photographs of the area taken in August
1979 could say whether this melting was
seasonal or exceptional, ie. due to a pre-
eruption internal temperature rise.
The four weeks prior to the eventful day
had seen most of the work completed and
only a few areas such as the plateau
between Rauðkembingar and Skjólkvía-
hraun remained to be mapped. This was
the task for the 17th August 1980.
There was a contiuous low cloud cover
at an altitude of about 2000 m with
intermittent periods of fine rain. At 13:00
hours a slight NNE breeze carried a small
shower from over Hekla to where I was
standing on the plateau (map reference:
6930 0070), 5.4 km from the summit. I
could still see the summit despite the
weather. This rain delayed the reloading
of my camera until after the start of the
eruption — a serious error in retrospect.
The first I knew of the eruption was a
loud bang from the mountain. When I
looked up I saw a column of white stearn
rise from slightly southwest of the summit,
but from my angle of sight it was hard to
judge where the summit was. This coluntn
of steam quickly penitrated the cloud
cover. I had not consciously observed for
possibly 20 minutes prior to the explosion
so I can not say whether there was any
quiet effusive activity before it. I timed the
start of the eruption at 13:30 hours on my
watch which turned out to be one minute
slow when compared with what was said to
be a correct electronic watch at the Búrfell
Power Station Dam. This rnakes my star-
ting time 13:31 hours - somewhat diffe-
rent to the official one (13:27 hours).
The noise that followed was an
increasingly loud roaring sound similar in
quality to a jet aeroplane. There were no
individual detonations and none were
heard until about an hour later. Detona-
tions occur only with the production of
lava. This first hour must have produced
only tephra.
Minutes later the steam turned a dirty
brown colour and then black. Soon the
entire summit was enveloped in billowing
clouds of all shades. Meanwhile, the erup-
tion was spreading away from me along
the fissure and isolated white mushroom
clouds appeared from behind Litla-Hekla.
On the photographs (fig 1 and 2) one
mushroom cloud can be seen to rise due
west of my position. So great is its appa-
181