Jökull - 01.01.2021, Blaðsíða 43

Bedrock and tephra layer topography within the Katla caldera
Figure 1. a) Mýrdalsjökull ice cap shown as a shaded relief image and a contour map (100 m elevation interval
between contours) using a surface DEM obtained in 2010 (Jóhannesson et al., 2013). This study focuses on
the area within the red square. Names of outlet glaciers, glacier peaks and cauldrons are shown. Inserted map
indicates the geographic location of Mýrdalsjökull (blue square) along with the neo-volcanic zones (grey) of
Iceland and active central volcanoes (red). b) Schematic image showing the instruments setup for the RES-
survey on Mýrdalsjökull. Red lines indicate transmitter and receiver antennae. c) The first author in the field
checking the receiver unit (photograph by Þorsteinn Cameron). – a) Mýrdalsjökull og svæðið sem hér er til
skoðunar (rauður kassi) ásamt helstu örnefnum og staðsetningum sigkatla. Gráu svæðin á innfelldu myndinni
sýna legu gosbeltanna og rauðu svæðin megineldstöðvar. b) Skýringarmynd af uppsetningu íssjármælibúnaðar.
c) Fyrsti höfundur greinar að athuga íssjármóttakara (myndina tók Þorsteinn Cameron).
Jökulhlaups originating in Mýrdalsjökull are fre-
quently observed on rivers flowing from the ice cap.
The largest of these have been caused by eruptions
in Katla; the peak flow of the jökulhlaup in 1918
has been estimated at ∼300,000 m3 s−1 (Tómas-
son, 1996). Most of the jökulhlaups are, however,
known to originate from the geothermal areas at the
glacier bed. The geothermal areas typically form and
maintain circular or semi-circular dips in the glacier
surface, generally referred to as ice cauldrons (e.g.
Björnsson, 1975); 20 of them have been denoted with
the names K1 to K20 (Figure 1a). Beneath some of
these cauldrons, water can collect and subsequently
be released in jökulhlaups, resulting in lowering of the
cauldron surface. Jökulhlaups originating from be-
neath these cauldrons are of variable magnitude with
peak drainage between a few m3s−1 to a few thousand
m3s−1. The surface elevation of the ice cauldrons
has been monitored by means of radar altimetry or
ground Differential Global Navigation Satellite Sys-
tem (DGNSS) profiling since 1999 (Gudmundsson
et al., 2007; Gudmundsson and Högnadóttir, 1999–
2020).
The three best known jökulhlaups since the 1918
eruption, all with a peak discharge that probably ex-
ceeded 1000 m3s−1 (Larsen et al., 2013), occurred
in Múlakvísl and Skálm, 25 June 1955 (Rist and
Þórarinsson, 1955; Thorarinsson, 1957; Rist, 1967b;
Tryggvason, 1960), destroying bridges over both
rivers; in Jökulsá á Sólheimasandi, 18 July 1999 (Sig-
urðsson et al., 2000; Guðmundsson et al., 2007), cut-
ting the power line across Sólheimasandur outwash
JÖKULL No. 71, 2021 41