Náttúrufræðingurinn

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Náttúrufræðingurinn - 1990, Qupperneq 26

Náttúrufræðingurinn - 1990, Qupperneq 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. 80

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