Jökull - 01.12.1975, Page 13

those at Öræfajökull, Kverkfjöll, and Mýrdals-
jökull.
Jökulhlaups can be caused by subglacial vol-
canic eruptions. Ice is melted by magmatic
heat. If the meltwater drains away, a depres-
sion is formed in the glacier surface. Subglacial
waterways may become sealed and a water
cupola would cap the volcanic vent. Volcanic
materials are piled up within the water cupola,
and a jökulhlaup results. However, volcanic
eruptions which burst out within large water
reservoirs do not neccessarily cause jökulhlaups.
Eruptions within Grímsvötn will not cause un-
expected jökulhlaups.
Volcanic eruptions at Katla trigger the most
catastrophic jökulhlaups. The high discharge
of the jökulhlaups indicates that the burst ori-
ginates at a water reservoir. A water cupola
may be formed due to melting by magmatic
heat some time before the eruption breaks out.
In that case, geodetic surveys of the glacier
surface might predict eruptions from Katla.
Jökulhlaups may occur from all glaciers which
are situated in the active volcanic zones, Fig. I.
The postglacial volcanism has been most active
in the eastern and the northern zone. The east-
ern zone includes Mýrdalsjökull and the west-
ern part of Vatnajökull. Jökulhlaups which are
caused by volcanic eruptions may occur in the
following rivers which drain from Vatnajökull:
Skeidará, Súla, Djúpá, Hverfisfljót, Skaftá,
Tungnaá, Kaldakvísl, Skjálfandafljót, and Jök-
ulsá á Fjöllum. Langjökull and Hofsjökull are
situated in the less active western zone. How-
ever, one should not exclude the possibility of
jökulhlaups in the rivers Thjórsá, Jökulfall,
Blanda, Héradsvötn, and Nordlingafljót, Fig. 1.
ACKNOWLEDGEMENTS
The author is indebted to Sveinbjörn Björns-
son for his review of the manuscript. Professors
Sigurdur Thorarinsson, and Thorbjörn Sigur-
geirsson, and Dr. Sigurdur Steinthórsson read
a draft of the paper and made several helpful
suggestions. The author is grateful to Sigurjón
Rist for liaving the figures drawn at the Na-
tional Energy Authority.
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