Náttúrufræðingurinn

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Náttúrufræðingurinn - 1993, Side 45

Náttúrufræðingurinn - 1993, Side 45
SUMMARY Pseudocraters and lava castles By Jón Jónsson Smáraflöt 42 lS-210 GARÐABÆR Icelatul and Dagur Jónsson Skrifstofa bcejarverkfræðings Strandgata 6 IS-220 HAFNARFJÖRÐUR Iceland In the summer of 1991, one of us (Jónsson, D.) observed pseudocraters and a peculiar lava flow east of Helgafell, near Hafnarfjörður, which we call Litluborgir. Since then, we have visited the place several times. It is a kipuka surrounded by younger lava flows, quite small, or approximately 70.000 m2. At the north side of the area, there are pseudocraters built up of scoriaceous lava, lava slag and diatomaceous sediment, evidently deriving from underneath the craters and thus proving the existence of a former lake. Presumably, the eruption occurred in early postglacial lime. The lava is tholeitic and of a pahohoe type. The lava flow strongly reminds of Dimmuborgir at Mývatn, Northern Iceland, previously described by Barth (1942). There, a former lava lake has been drained and has a “considerable number of lava pillars standing up as ragged peaks”. The area has later been mentioned by Thor- arinsson (1951), van Bemmelen and Rutten (1955), and most recently by Kristján Sæmundsson (1991). None of the above- mentioned authors seems to have found an explanation of the formations of the lava pillars (Borgir), nor of the fact why they “remained standing when the crust around them sank because the liquid lava ran out below it” (Barth op. cit. p. 66). From our investigations at Litluborgir, we have come to following conclusion: At Ihe bottom of the lava Iake underneath lava, the powerful steam and gas accumulated and eventually penetrated the liquid lava, forming a sort of chimneys through it. This caused a rapid cooling and solidification of the lava around the gas channels. Consequently, the lava pillars (borg) could remain standing when the lava lake drained. For other peculiar secondary formations in lava flows, first observed and described by Karl Sapper (1908) front the crater row east of Laki, the name sappi (sing. sappi, plur. sappar) has been proposed (Jónsson 1983) in honour of the discoverer. At the above-mentioned place, last summer we observed traces of diatomaceous sediment in and around some of the sappar, which indicate that a lake existed there before the eruption 1783. Therefore, sappar and lava castles (lava pillars) seem to be formed principally in the same way. 155
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