Jökull


Jökull - 01.06.2000, Page 32

Jökull - 01.06.2000, Page 32
Surface and bedrock topography of Mýrdalsjökull ted flood waves in coastal waters. During 18 of the 20 documented eruptions the associated jökulhlaups flowed southeast down to the Mýrdalssandur outwash plain (Figure 1), but in two cases jökulhlaups flowed southwest to the Sólheimasandur outwash plain; the third route, northwestwards into the river Markarfljót, was taken by a jökulhaup in 1600 B.P. (Haraldsson, 1981). During the jökulhlaups, a mixture of water, ice blocks, volcanic products and sediment, frequently hyperconcentrated, surges over the outwash plain. Velocities of 5-15 m/s, peak discharge of 100-300,000 m  /s reached in a few hours, and total volumes of 1-8 km  have been suggested (Jóhannsson, 1919; Þórar- insson, 1957, 1959, 1975; Hannesson, 1934; Maizels, 1993; Tómasson, 1996; Karlsson, 1994). These jök- ulhlaups, along with heavy fallout of tephra, make the Mýrdalsjökull volcano the most hazardous one in Iceland. In this paper we present detailed ice surface and bedrock topography data from Mýrdalsjökull, and describe the morphology, size and shape of the Mýrdalsjökull volcano with reference to its erupti- on history. Furthermore, we present new informati- on regarding the location of eruptive vents, ice and water drainage basins and subglacial flowpaths of jök- ulhlaups during eruptions. Previous mapping of the glacier surface and bedrock topography The first maps outlining with some accuracy the co- verage of Mýrdalsjökull were surveyed by the Dan- ish Geodetic Institute in 1904-1907 (the southernmost margins) and during 1937-1938 (the main ice cap). The maps were published in a scale of 1:100,000 (Nørlund, 1944). However, the indicated ice-surface elevation on these maps was not based on the survey- ing data, except along the glacier edge. Instead, the contour lines were drawn with reference to oblique air photos and show, according to Nørlund (1944), the shape of the ice surface rather than its elevation. In later editions of these maps, the position of the glacier edge has been revised using aerial photographs. The first triangulation surveying on the ice cap was carried out in 1943 by Steinþór Sigurðsson, who compiled a surface map, later published by Rist (1967a). The next maps of the ice cap were produced by the U.S. Army Map Service in a scale of 1:50,000 on the basis of aerial photographs taken in 1945-46, and the triangulation system previously surveyed by the Danish Geodetic Institute. The ice- surface contours along the marginal areas of the ice caps (some few km up from the edge) were compiled from aerial photographs; but higher up, the contours were identical with those of the Danish Geodetic Survey maps. The surface maps showed the gener- al shape of the ice cap, the outlet glaciers and the caldera depression surrounded by the higher domes of Háabunga and Goðabunga (bunga means dome in Icelandic). Exploration of the ice thickness of Mýrdalsjök- ull and its subglacial topography began in 1955 when seismic reflection soundings were carried out at 9 locations on the ice cap, showing an ice thickness of 300-400 m (Rist, 1967a). In 1977 a few radio echo- sounding profiles on Mýrdalsjökull showed consi- derable variations in bedrock topography. An ice thickness of 500-600 m was observed in the central part of the ice cap (Björnsson, 1978) confirming the presence of a deep depression (caldera) beneath the central part of Mýrdalsjökull. ERTS Landsat-images from the early 1970’s also revealed surface forms which strongly suggested that the Mýrdalsjökull ice cap covered a prominent volcanic caldera (Sigbjarna- son, 1973; Sæmundsson,1982). SURFACE AND BEDROCK TOPOGRAPHY OF THE ICE CAP AS MAPPED BY RADIO ECHO SOUNDING In May in 1991 the ice surface and bedrock elevations of Mýrdalsjökull were mapped in detail. Ice thickness profiling was carried out by continuous radio echo sounding (Figures 2 and 3). Navigation on the ice cap employed GPS and Loran-C and position of the sounding equipment was logged at 50 m intervals with an accuracy of 50-100 m. Most of the sound- ing lines run along longitudes or latitudes, but some were placed perpendicular to the trend of the buried subglacial structures in order to minimize lateral ref- lection. Crevasses prevented sounding on the steepest outlet glaciers, flowing to the east, south and north- JÖKULL No. 49 31
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Jökull

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