Jökull


Jökull - 01.12.1984, Page 29

Jökull - 01.12.1984, Page 29
jökulhlaups from Grímsvötn. The first report of a jökulhlaup dates back to A.D. 1332. From 1600 until 1934 there occurred about one jökulhlaup per decade with an estimated discharge of 6-7 km3 of water and a maximum discharge rate of approximately 40 000 m3/s (for example in 1903, 1913,1922,1934). But since 1934 there have been two, even three, bursts per decade with corres- pondingly smaller volumes (except for 1938), 1-3.5 km3, and maximum discharge rates of 5-8000 m3/s (1938,1939,1941,1945,1948,1954, 1960,1965,1972,1976,1982 and 1983; Thorarins- son 1974, Rist 1955,1973,1976, Björnsson 1983). In December 1983 the smallest jökulhlaup ever was observed with a total volume of 0.5 km3 (Sigurjón Rist, pers. comm.). The Grímsvötn lake owes its existence to the geothermal area. Melting due to the geothermal activity creates a depression in the surface of the Vatnajökull ice cap. Ice and water are diverted towards the depression from a 300 km2 drainage basin (Fig.l). The meltwater accumulates in a 25 km2 subglacial lake, which is covered by a 200 m thick floating ice shelf (Thorarinsson 1953b, 1974, Björnsson 1974). The lake is sealed and no water drains out of it between the jökulhlaups. Water accumulates in the lake and when the water level has risen to a critical level, water is forced out of the lake beneath a barrier east of the lake (Fig.2). The subglacial waterways are enlarged by frictional melting and the lake is drained in a fortnight by a catastrophic flood. Eventually, the ice overburden pressure is able to close the water tunnels and the flood stops abruptly before the lake is empty (Björnsson 1974, Nye 1976). The water-level change in the larger jökulhlaups has been estimated to be about 150 m, but is observed to be 80-100 m during the last three decades (Fig.3). The fre- quency and water volume are believed to depend on the thickness of the glacier. This model is considered to explain most of the jökulhlaups. The jökulhlaup in December 1983, however, occurred at a water level that was 20-30 m below the present critical level for triggering jökul- hlaups. Explanation of this will be discussed in the paper. The heat output of the subglacial geothermal area has been estimated by using the Grímsvötn lake as a natural calorimeter. A long term mass balance of the drainage basin has been given by Björnsson (1974). The average accumulation in Fig. 2. The Grímsvötn area. Contour map of the Icelandic Geodetic Survey from 1946 on which we have marked the position of craters in 1934 and 1983, the depression north of Svarti- bunki from 1938, and ice cauldrons. The cauldron that formed during the jökulhlaup in 1983 is marked on the map. The cauldron north- west of Grímsvötn drains to the river Skaftá. 2. mynd. Grímsvatnasvœðið. Kortið sýnir hœð- arlínur frá 1946, en á það eru merktir sigkatlar og gígarnir frá gosunum 1934 og 1983. Rennan norðan við Svartabunka myndaðist 1938 og sést enn árið 1946. Sigketillinn, sem myndaðist við hlaupið í desember 1983 er merktur á kortið. Sigketillinn norðvestan við Grímsvötn hleypir vatni í Skaftá. the form of ice is equivalent to 2200 mm /yr of water and the surface ablation amounts to 500 mm /yr. A long-term steady-state model for the drainage basin proves to be a valid approxima- tion (Björnsson 1974). The water added to the lake is 6.6T011 kg/yr. About 1.5T011 kg/yr are melted at the surface of the glacier by meteorological processes, but the difference, about 5T011 kg/yr is melted by the geothermal heat within the drainage basin. The heat flux required to melt this ice is about 5000 MW (ther- mal). In the present work we estimate the geoth- ermal mass fraction in the lake and present new estimates of the thermal power of the geothermal system. JÖKULL 34. ÁR 27
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