Hekla - Historical and Prehistoric Eruptions - 1970 - Initial Eruption

Initial Eruption

Before the eruption a greater than normal amount of snow melting had occurred, indicating the volcano was heating up. Earth tremors began at 8:48 pm on the evening of the eruption, the largest had a magnitude of 4. The eruption started weakly at 9:23 pm IMT ± 2 min before increasing in power.The first pumice fell on Búrfell power station, 15 km away, at 9:35 pm causing people to evacuate. The eruption seems to have started in 2 locations at the same time - to the Shoulder crater's SSW and below the Lava Crater. At 10:30 pm a crater at 780 m was producing a lava column which reached an altitude of around 1000 m. During the night a 700 m high lava fountain was thrown up from the main crater. A 500 m long fissure starting below the Lava Crater opened and lava fountains and other lava flows emanated from it. One hour into the eruption a new 400 m fissure opened to the NE producing two main lava fountains and shortly after another adjoining fissure opened producing lava fountains to a height of 500 m. At around midnight another fissure opened NW of the Lava Crater, later hurling an over 300 m long lava fountain, 200–300 m into the air. By midnight lava had already covered over 1 km2 and this extended to 7.5 km2 by next morning implying a flow rate of around 1500 m3s−1.

For the first two hours tephra was produced at the rate of 10,000 m3s−1. The cloud from the eruption, which had reached 53,000 feet (16,154 m) by 10:10 pm caused a noisy lightning storm. The tephra was transported northwards by the wind causing the sky to turn black in places - 190 km away at Blönduós tephra fell from midnight until 2 am and ash fell on a trawler 330 km away at 2 am. Icelanders sampled the tephra fall in their locality by putting a plate outside to capture everything that fell onto it. This, and other measurements, showed the area covered was long and narrow with the 1 mm contour (an equivalent of 8 tonnes per hectare) extending to the north coast.

Read more about this topic:  Hekla, Historical and Prehistoric Eruptions, 1970

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