Jökull - 01.06.2000, Side 30

Surface and bedrock topography of the Mýrdalsjökull ice cap,
Iceland: The Katla caldera, eruption sites and routes of jökulhlaups
Helgi Björnsson, Finnur Pálsson and Magnús T. Guðmundsson
Science Institute, University of Iceland, Hagi, Hofsvallagata 53, IS-107 Reykjavík, Iceland
Abstract – Radio echo soundings have revealed a large caldera beneath the Mýrdalsjökull ice cap. The caldera
encircles an area of 100 km

, is 600 to 750 m deep and its highest rims reach 1380 m a.s.l. Three major outlet
glaciers have eroded 300 to 600 m deep breaches into the caldera rim. The northern part of the caldera floor,
where an area of 25 km

is below 800 m, is smoother and lies deeper than the southern part, which is character-
ized by subglacial ridges and individual mounts rising from 750 m a.s.l. to about 1100 m. A number of ridges
radiate out from the caldera, however, none toward south. One ridge strikes west toward the neighbouring
volcano Eyjafjallajökull, and a second ridge strikes E from the eastern rim of the caldera. Ridges also radiate
from the caldera rim toward NW, N, and NE. A linear depression, bounded by steep slopes, 200-250 m deep and
1.5 km wide, strikes NE toward the volcanic fissure Eldgjá. Twenty eruptions have been documented from the
caldera during the last 1100 years, all causing catastrophic jökulhlaups. Over the last four centuries eruptions
have occurred on single vents and volcanic fissures trending both E-W and S-N. We suggest that the largest
eruption of the last millennium (1755 A.D.) took place on a several kilometers long fissure trending east from
Goðabunga; and the eruptions of 1823 and 1918 on a northerly striking ridge from the eastern rim of Háa-
bunga. At present, meltwater drains southeastward, down to Mýrdalssandur, from an area of 60 km

within the
caldera. All but two of the 20 recorded jökulhlaups in historical times have taken this path. An area of about
20 km

within the caldera drains to the southwest, down to Sólheimasandur. Two jökulhlaups are known to
have taken this route to the river Jökulsá á Sólheimasandi in historical times. A third route, northwestward into
Fremri Emstruá and the Markarfljót river, was taken by a prehistoric jökulhlaup in 1600 B.P. Presently, geot-
hermal activity is displayed by several small cauldrons, 0.5-1 km in diameter on the glacier surface. Meltwater
accumulates beneath two or three of these cauldrons and is frequently drained in small jökulhlaups.
INTRODUCTION
The Mýrdalsjökull ice cap is the fourth largest
glacier in Iceland, with a total area of 600 km

. The
glacier rises 1300-1500 m above the surrounding low-
land (Figure 1), receives heavy winter precipitation,
has high rates of summer melting and is thus drained
by many rivers. Located at the southern tip of the
propagating Eastern Volcanic Zone, the ice cap is
underlain by a highly active central volcano contain-
ing a large caldera. The caldera, together with a 80
km northeast-trending fissure swarm, comprise the
Katla volcanic system (Jakobsson, 1979; Jóhannes-
son et al., 1990). The volcanic system has been acti-
ve over several hundred thousand years and the basal
mountains consist of hyaloclastites from glacial per-
iods (Robson, 1957; Jakobsson, 1979). The caldera
subsidence may date from the Pleistocene (Sæmunds-
son, 1982) and ash layers originating from the volcano
have been identified in the GRIP ice core dating
from 75,400 and 77,500 years B.P. (Grönvold et al.,
1995). An eruption producing more than 10 km


of
tephra in 12,000 years B.P. may have contributed to
its formation (Lacasse et al., 1995; Sigurðsson et al.,
1995). During the Holocene 150 to 200 eruptions
may have taken place in the volcanic system both
on short volcanic fissures and single vents (Larsen,
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