Jökull - 01.06.2000, Síða 41

Helgi Björnsson et al.
the passes in the caldera rim. Kötlujökull would col-
lect water from the entire eastern part of the caldera.
The watersheds, however, coincide with the ice divi-
des on the sharpest rims of the caldera at Háabunga
and Austmannsbunga. Outside the caldera the water
divide between Mýrdalssandur and Markarfljótsaurar
lies down the central part of Sléttjökull.
M arkarfljó t
Sólheim asandur
M ýrdalssandur
0 5 10 km
Figure 11. Estimated water pressure potential (
23
) at
the ice-bed interface (kPa). Water divides are shown
as blue lines. The location of ice cauldrons are marked
by red circles and the caldera rim by a red hachured
line. – Mat á mætti sem rekur vatn eftir jökulbotni.
Skipting Mýrdalsjökuls í nokkur helstu vatnasvæði.
Hringir sýna legu sigkatla á jarðhitasvæðum og slitr-
ótt lína afmarkar öskjubarmana.
Meltwater is known to have accumulated in sub-
glacial lakes beneath the cauldron west of Kötlu-
kollar and frequently drain in small jökulhlaups
under Kötlujökull. Frequent observations of sulp-
hurous smell from Múlakvísl indicates that meltwater
may continuously drain from geothermal areas insi-
de the drainage basin of Kötlujökull. The cauld-
rons east of Goðabunga drain to Fremri-Emstruá.
Continuous smell of hydrogen sulphide from Jökulsá
á Sólheimasandi indicates drainage of geothermal
fluid from beneath cauldrons at the saddle between
Goðabunga and Háabunga.
GEOTHERMAL AREAS, ERUPTION
SITES AND ROUTES OF
JÖKULHLAUPS
Geothermal activity under Mýrdalsjökull is mani-
fested by several ice cauldrons in the glacier surface.
The current activity is located just inside the caldera
rims, where faults allow rapid vertical transport of
geothermal fluid. Accurate and reliable descriptions
of eruptions over the last four centuries indicate that
they have occurred both on single vents and short
volcanic fissures. According to Þorsteinn Magnússon
at Þykkvabær in Álftaver (1626, Safn til Sögu Íslands,
IV, p. 208) many small vents were observed east of
the main crater six days after the start of the eruption.
Due to melting in the eastern part of the ice cap during
the 1721 eruption, black cliffs or a mountain became
visible that had been covered by the ice for more than
100 years (p. 228).
In 1755, Jón Sigurðsson (1755, Safn til Sögu Ís-
lands, IV, p. 236) reported two vents almost due
north from the farm Holt in Mýrdalur. He counted
up to five vents two months after the start of the
eruption. Three distinct columns of fire side by side
were depicted towards the end of the eruption (Eggert
Ólafsson, 1772). Based on this description we sug-
gest that this eruption took place on a several kilo-
meters long fissure trending east from Goðabunga
(Figure 12). The fissure extended far into the draina-
ge basin of Kötlujökull and therefore meltwater was
directed eastward to Mýrdalssandur. This was the lar-
gest eruption of the millennium in Mýrdalsjökull after
the settlement, producing 1.5 km


of tephra (Þórar-
insson, 1975). The proximity of the eruption site to
Sólheimajökull may have caused the peculiar surge-
like behavior of this glacier outlet. Eggert Ólafsson
(1772) reported that “during the eruption the glacier
went up and down in an undulating motion and finally
settled so puffed-up that it became twice as high as
before ”. Moreover, the neighbouring Eyjafjallajökull
“subsided due to the eruption so two ice free peaks
rise above the glacier with a black cliff between them,
which nobody has seen as far back as people remem-
ber” (Ólafsson, 1772).
Jón Austmann (1845, Safn til Sögu Íslands, IV,
40 JÖKULL No. 49