Melting Rates and Jokulhlaup Potential in Flank Eruptions at Ice-Covered
Stratovolcanoes
[*Magnús T. Gudmundsson*] and Thórdís Högnadóttir
(Institute of Earth Sciences, University of Iceland, Sturlugata 7, 101 Reykjavík, Iceland; ph.
+354-525 5867; fax: +354-562 9767; email: mtg@raunvis.hi.is, disah@raunvis.hi.is)
Eruptions on radial fissures are common at stratovolcanoes and on concentric volcanic
fissures at caldera faults. When volcanoes are ice-covered, eruptions usually melt ice causing
jokulhlaups and lahars. The magnitude of such floods is of prime importance in hazard
assessment and insight into the processes operating may throw new light on magma-water
interaction in such settings. Two principal end member cases can be defined, “thick ice” and
“thin ice”. In the thick-ice case the glacier responds to melting and drainage predominantly in
a ductile manner, with inflow of ice being of major importance. This type of activity mainly
occurs within ice sheets and large glaciers. At stratovolcanoes ice thicknesses are limited and
the thin-ice case is more common and the ice predominantly behaves in a brittle manner since
confining pressures are small due to the limited thickness. In the observed thin-ice eruptions
(e.g. Deception Island 1969, Grimsvötn 1998, 2004) chasms with vertical ice walls have
formed around the volcanic fissures. The rate of widening is fast at first but then slows down
quickly. An order of magnitude estimate of melting rate can be determined from a simple
empirical model based on this characteristic behavior. The model predicts maximum melting
rates similar to 1000 m3/s for every kilometer of ice-covered volcanic fissure for 100 m thick
ice. The model can be used to assess the potential of flooding from flank eruptions. It seems
to perform reasonably well for ice thicknesses of 50-200 m and has been applied in hazard
assessment for glacier-covered volcanoes in South Iceland where it yielded estimates of
maximum meltwater discharge of 103-104 m3/s, with the values for individual ice-covered
flank segments depending on possible fissure length and ice thickness.
ORAL
CORRESPONDING AUTHOR: MAGNÚS T. GUDMUNDSSON