Náttúrufræðingurinn

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Náttúrufræðingurinn - 1995, Side 85

Náttúrufræðingurinn - 1995, Side 85
Reverend Jón Steingrímsson also tells of ruins of houses and fences in the area which now is covered by the lava flow from the Laki eruption cf 1783. Although this information is frag- mentary, the author favors that the sources men- tioned refer to the same event, the damage of Skál and destruction of Tólfahringur. Only a small part of the volcanic structure itself is now visible, most of it being covered by the lava flow from the Laki eruption. The center of the eruption seems to be a relatively short fissure, direction northeast-southwest. Evidently the activity started with heavy fall of juvenile clasts, pumice and ash and pene- contemporaneously a pyroclastic flow or rock and ash flow. The entire erupted material is characterized by round forrn, particles 1 mm or less in diameter up to boulders. The occur- rence of accretionary lava balls is striking. The uppermost 0.5-1 m of the pyroclastic flow is loosely consolidated and breaks up in vertical co- lumnar form indicating high temperature at the moment of consolidation. Here and there on the flat surface of the pyroclastic flow there are fun- nel shaped holes, 8-12 m across at the top and 3- 4 m deep. They are considered to be degassing pipes of the hot flow. At the bottom of each hole there is as a rule one big accretionary boulder formed of a 5-8 cm thick solid lava twisted around fragmentary lava evidently from the same eruption. The destruction of Tólfahringur should, ac- cording to Reverend Jón, have occurred about the year 1112 A.D. Remains of vegetation col- lected from underneath the erupted material were subjected to l4C age determination. This material, partly carbonized, was collected at three different places in the same area and equally many determinations were made. All the deteminations gave approximately ihe sarne 14C age, or about 300 years. Well known geological facts, however, indicate that this is entirely out of the question and ntuch to low. So far, no explanation for this has been found. The next step was to turn to tephrochronology. In this area an approximately 1 mm thick layer is added to the surface of the soil every year. By measuring several places the distance be- tween two tephra layers with known age and so down to the layer from the eruption in ques- tion, it turned out that the eruption must have occurred close to the year mentioned by Rev- erend Jón Steingrímsson, i.e. in 1112. PÓSTFANG HÖFUNDAR/AUHOR'S ADDRESS Jón Jónsson Smáraflöt 42 210 Garðabær Iceland 193
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