Náttúrufræðingurinn

Årgang

Náttúrufræðingurinn - 1983, Side 202

Náttúrufræðingurinn - 1983, Side 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- 182
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