In the beginning a fissure opened at right angles to the ice flow which was from the southeast to the northwest. It was about 15 km from the glacier snout. There is a big terminal moraine at the coast at Álftanes. Rapid ice melting was the consequence of explosive eruption and therefore magma fragmentation from the beginning on. The authors refer to their experience from watching and researching the Gjálp eruption from 1996 which has had many similarities to the Helgafell formation. At Helgafell next an edifice was formed about 300 m high in an ice vault, meltwater drained away very fast through subglacial channels so that the explosive activity continued to the end, which seems to have been after some days. This explains the fact, that Helgafell is next to overall made from mafic hyaloclastite, whereas most other researched smaller subglacial volcanoes showed a basis of pillow lavas. Of these and some small intrusions, the researchers just found minimal quantities.