Náttúrufræðingurinn - 1993, Síða 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.
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