The Icelandic „icy flood”
the Icelandic „icy flood”, the jökulhlaup is a severe natural disaster and at the same time special natural phenomenon, moreover, a tourist attraction. In most cases the cycle of events is started by an under-ice volcanic eruption and for the explanation of the process and the forecast of its process severe physical reflections are needed. This special phenomenon happened in the autumn of 1996 in the depth of Vatnajökull glacier. The effluent lava melts ice around itself. The volume of the water created this way is less than that of the melted ice since it is a well-known quality of ice that its density is less than that of water. So a hollow would be created above the lava if the layer of ice above the hollow would not sink (because of its huge weight) filling the difference of density and a dent is created on the surface. We are used to observing a kind of positive movement of material (emerging of mountains and islands, outflow of smoke and soot, outfllying of stones, lava flow), not negative change of the surface this is why we might feel surprising the huge dip on the ice field.
The Icelandic glaciers are only partly similar to those on the continent: they are not only streams of ice in a steep valley but cover full ranges of mountains as well (on Iceland volcanic mountains of course). Vatnajökull is a huge mountain lying in south-east part of Iceland covered fully by smooth ice cap. The ice field of an average 100 km diameter is varied with irregularly shaped, slashed, steep peaks and deep valleys and the thickness of ice varies between 300 and 1000 m depending on the height of the geological formations under it. On 30th September shortly before midnight shaking on earth indicating the beginning of volcanic activity was observed. It has revealed that a dip has developed on the surface of the ice cap and its diameter and depth grow continuously. The dip finally had a depth of 100 m and a diameter of 1 km. At the place of the dip practically ice moved vertically, that is why clefts developed around the sunk area.
The power of the eruption broke the ice layer above the crater the next day, 2nd October. The black, hot, 500 m high cloud of ashes was lifted into a height of 3000 m by the uplift of the cold air. The lava flow lasted until mid-October along a straight fault line of volcanic origin further dips developed, the colour of the cloud above the crater became whiter and by this time it reached the height of about 100 kilometres.
According to later counts in the first two weeks of the eruption the volcano gave so much energy to its environment s if a bomb of Hiroshima would have exploded in the crater in every 4 minutes! Most part of it melted the ice of the glacier, so huge amounts of water came into being (3 thousand million tons). (the volume of Lake Balaton is about 2 km2!)
this huge amount of water collected in the great caldera developing around the volcanic eruption: a lake under the ice of the glacier. The level of the ice covering the water rose 10-15 m by 1st October and this went on the following days as well. The glacier ice namely keeps the water in trap or least until the pressure of the water grows high enough to get out of its ice prison. It can be known in advance: this breakthrough causes great flows. In Icelandic language there is a separate word these glaicer flows: jökulhlaup. It is a frequent phenomenon in the world of ice. It can not only be caused by volcanic eruptions but by geothermal hot water that is usual in Iceland. That is why glacier floods are frequent on the island, 2-3 jökulhlaups develop in 10 years. But the flood in 1996 was outstandingly great and destructive, the biggest since 1938, than 4.5 cubic metres of water broke loose. The breaking out of the huge amount of water was waited for earlier this case than as it actually happened. The glacier closed inside the water for some weeks after the water’s level had exceeded the critical limit according to counting based on static powers. At 9.30 p.m. on 4th November strong quake (ice quake) was observed from the direction of the glacier lake. The breaking of the ice dam happened and the lake almost totally emptied within 3 days and its level decreased with 165 m!
The glacier flow first happened on the plain in the morning on 5th November in the form of a quick water-wave, than the amount of the destructive water increased step by step. Its temperature was around 3-5 Celsius degrees and its colour was black because the huge amount of volcanic dust and ash. It carried along rocks and blocks of ice coloured dark, some of the blocks of ice reached the height of 10-15 m and the weight of 1000 tons. The flood dispersed these rocks and blocks of ice on the 50 km wide plain. Approximately 100 million tons of volcanic material got into the sea.
Honyek Gyula
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| A Vatnajökull a levegőből | És a földről | Vulkáni tevékenység a gleccser mélyén | Vulkáni tevékenység a gleccser mélyén |
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| A hő hatására kialakult beszakadások | És repedések |
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Honyek Gyula