Náttúrufræðingurinn - 1990, Blaðsíða 26

SUMMARY
The Látrar Poltergeist, a case of
frost-cracking
by
Páll Einarsson
Science Institute
University of Iceland
Dunhaga 5
IS-107 REYKJAVÍK
lceland
In the exceptionally cold winter of 1918
people in Látrar in Aðalvík (W-Iceland)
experienced an unfamiliar phenomenon,
thought at the time to be of supernatural
origin. A vivid and thorough description
by V. Jónsson (1985) convinced the pre-
sent writer that the events were due to
frost-cracking. Frost-cracking events are
commoniy recorded by seismographs in
Iceland, especially by instruments located
in the higher parts of the country. Hun-
dreds and even thousands of events may
be recorded per day during a frost-crack-
ing swarm (Fig. 1 and 2). Following condi-
tions seem to be favorable for frost-crack-
ing:
1. Temperature is - 5° to - 10° C and falls
rapidly.
2. The weather is calm and the sky is
clear.
3. Little or no snow is on the ground.
4. The ground is frozen following a peri-
od of thaw.
Frost-cracking is caused by thermal con-
traction of frozen soil. Falling tempera-
ture leads to tensile stress in the surface
layer that may exceed the tensile strength
of the frozen soil. The fracturing of the
soil is accompanied by a small seismic
event that may be heard and even felt by
nearby people. The cracks fill with loose
material and do not close when the tem-
perature rises again. The next cooling pe-
riod leads to fracturing on the same crack,
which thus becomes a permanent struc-
ture. Frost cracks form four-, five- or six-
sided polygons with a diameter of the or-
der of a few tens of meters (Fig. 3). Crack
polygons are found in numerous places in
the highland of Iceland (Þórarinsson,
1954, 1964, Scheving Thorsteinsson, 1956,
Friedman et al., 1971). The seismic obser-
vations support the conclusion of Fried-
man et al. (1971) that most of the polygons
in Iceland are the result of frost-cracking.
They are therefore not proper ice-wedge
polygons of the type found in permafrost
areas of the world.
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