Jökull


Jökull - 01.12.1960, Blaðsíða 23

Jökull - 01.12.1960, Blaðsíða 23
duction of the eruption on June 25th 1955 has been some three million m3. An eruption of similar intensity as the initial phase of the Hekla eruption 1947 could, ac- cording to these assumptions, produce a jökul- hlaup more than ten times larger than the hlaup of 1955, or of similar intensitv as a normal Kötluhlaup (jökulhlaup from Mýrdals- jökull associated with eruption of the volcano Katla). It may be difficult to give a physical ex- planation of such a rapid melting of the glacier as is herc suggested (some 104 m3/sec of ice), if the volcanic material is brought to the subglacial surface through one vent, but this problem will not be discussed in details here. The behaviour of subglacial volcanic erup- tion is not known in details. These eruptions seem usually to be highly explosive and pro- duce only tephra (volcanic ash). Such eruptions must be very active in melting the ice, as long as they have not broken through the glacier. The tephra will be suspended in the meltwater and freely give its heat energy to the water. The melting speed of ice in direct contact with water depends on several factors, such as the temperature and turbulence of the water, the porosity of the ice, the pressure etc. The importance of each of these factors is not. known to the writer, but the temperature and turbulence of the water are probably the most important. If the contact melting is not suffi- ciently fast, the tempeature of the water will rise untill the vapour pressure exceeds the static pressure of the glacier. This high pressure will Iift the glacier ice from the earth and allow the overheated water to flow under it, to in- crease the melting area sufficiently to keep the water below its boiling point. Fig. 1 illustrates the writer’s opinion of the possible evolution of such an explosive subglacial eruption. If a subglacial eruption is not explosive, the process will be different. The lava will be cooled very fast at the surface by direct contact with the ice or. meltwater. Its flow will be very slow, but it will grow in thickness to form a dome shaped mountain. The surface layer of this mountain will be characterized by the con- tact with the ice (móberg), but the interior will be compact and coarse grained, as it cools very slowly. An evidence of this structure is Bedrock Tuffbreccia and pillowlava Subaerial lava Magma-gabbro, Glacier ice Fig. 2. Three stages in the evolution of sub- glacial volcano of non-explosive eruption. The third stage shows the volcano after having reached through the glacier. If the glacier melts away, the mountain will get the form indica- ted by dashed line (table mountain). — Þrjú stig flœðigoss undir jökli. Punktalinan á mynd III sýnir útlínur fjallsins eftir að jökullinn er bráðnaður. 21

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Jökull

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