Náttúrufræðingurinn - 1983, Blaðsíða 202

rent distance from the fissure - ie. some-
where on Næfurholtshraun — that it must
be due to the collection of boiling water
from the volcano’s flanks and not an erup-
ting vent.
The most striking thing I was to witness
during the eruption was a glacier burst
down the northwest flank of Hekla, north-
east of Litla-Hekla. The water broke
through the clouds, which were now as low
as the 1000 m level, and literally ‘cart-
wheeled’ down the 20% slope covering a
distance of 1 km in about 10 seconds; a
speed of 360 km/h (225 mph.). The water/
ice formed a hollow cylinder and from its
size relative to Litla-Hekla, I estimate its
diameter to have been around 50 m. This
penomenon was presumably the product
of viscosity and basal friction effects simi-
lar to those found in waves breaking onto
a beach. The cylinder had collapsed before
it passed out of sight behind Hringlan-
dahraun. The volume of material involved
was considerable.
About ten minutes after the start of the
eruption I saw the first bomb arc through
thc overlying cloud and strike the ground
some 700 m west of the 931 m summit in
Rauðkembingar. It was large — in the
order of two metres — and angular, more
like a piece of volcanic breccia ripped off
the side of the fissure than juvenile, mol-
ten rock.
Within seconds, I was being showered
with black, crusty tephra. The whole
bombs were ovoid in shape and up to 300
mrn long. The majority of the tephra were
fragments of bombs about 50 mm in size.
Because of the extremely high vesicularity,
the tephra was lighter than it looked. The
vesicles were upp to 20 mm diameter in the
centre of the bombs but their size
decreased towards the edges which were
smooth but fractured as if the insides had
continued to swell after the outer skin had
formed. No phenocrysts were visible in the
tephra. They were falling at a rate of 35
impacts per square metre per minute.
At this point 1 thought it prudent to
withdraw from the scene and set off across
the plateau at a healthy trot in the direc-
tion of Rauðaská! with my mapcase pro-
tecting my head.
Photograph (fig 3) was taken from the
rim of Rauðaskál at about 13:50 hours and
by then a black curtain seemed to stretch
the length of Heklugjá. The ash fall
became much greater and with the erup-
tion column directly overhead it was
nearly as black as night. At this point I
considered shedding some excess weight
but as this included four weeks-worth of
field map and notes as well as my photo-
graphs I decided against it.
Even though I was now nearer to my
goal, the trip across Skjólkvíahraun was
the worst 1 was to experience. The dark-
ness and low visibility left me stumbling
blindly over the hummocks in the old lava
flow. 1 was fast losing my sense of direc-
tion and staying on my feet was becoming
difficult because of the unconsolidated
ash. At the top of each rise I could just see
the black outline of the 1970 lava and I
knew that when I reached its northeast tip
the camp lay only 300 m to the west.
When I arrived at the camp about 14:20
hours, Andrew as already there with the
vehicle. Tephra had ripped the nylon
flysheet of the stores tent to shreads and
severely damaged the cotton of the large
tent. Whilst inside, collecting a few things
together for the evacuation, a large piece
of tephra broke the ridge pole and we
realised that both tents were probably
writeoffs. Outside, the noise from Hekla
changed. The steady roar became more of
a rumble with detonations every few
seconds. Red ‘fires’ could be seen through
the blackness and lightning flashed low
over our heads.
Before we could leave, we had to refuel
the motorbike. One speck of dirt or badly
mixed petrol and oil in the tank would
leave us stranded. I never had much faith
in its mechanical reliability but after it
started on the tenth desperate kick 1 for-
gave its previous idiosyncrasies.
By the time we set off, so much ash had
fallen that it became near impossible to
follow the fast disappearing track back to
the Landmannalaugar road. The explo-
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