When glaciers were carried away with themselves: Iceland's cataclysmic jokulhlaups
Abstract
Unprecedented scenes of floodwater bursting from Skeidararjokull, Iceland in 1996 have prompted a revised understanding of dynamic glacier response to sudden, massive influxes of floodwater. Using evidence from the 1996 jokulhlaup, we present an overview of glacier response and jokulhlaup characteristics resulting from igneous activity beneath the Myrdalsjokull and Oræfajokull ice caps, Iceland. The need to improve understanding of cataclysmic jokulhlaup processes is motivated by findings from recent geophysical surveys at Myrdalsjokull, which reveal that magma is accumulating at depth beneath the ice cap. Specifically, we seek to explain: (i) glaciohydraulic processes responsible for rapid, unstable floodwater release; (ii) consequent sub-glacier hydrodynamics; (iii) glacier response to the kinematic movement of basal floodwater; (iv) the propensity for intraglacial floodwater routing; (v) hydromechanical processes at the glacier terminus; and (vi) resulting ice-proximal to ice-distal sedimentary and glacier ice deposits. Glaciological and sedimentological evidence is presented from jokulhlaups that accompanied volcanic eruptions beneath Myrdalsjokull in 1721 and 1918, and Oraefajokull in 1362 and 1727. In summary, phreato-magmatic eruptions within the ice-filled calderas of both ice caps caused tremendous ice-melt that generated hydraulic pressures much greater than glacier yield strength, thereby facilitating unstable cavity growth and localised glacier flotation. Imposition of negative effective-pressure inhibited the formation of classical tunnelled drainage, despite ice-melt-widening effects due to thermal advection. Instead, floodwater moved swiftly both down-glacier and laterally as an unregulated kinematic wave. The hydrodynamic effects of this wave caused supraglacial outbursts of basal floodwater, widespread upheaval of the glacier surface and attendant ice shearing, and enhanced (albeit localised) glacier sliding. Massive sections of glacier ice were fractured and incorporated simultaneously in flows laden with juvenile eruptives, which on reaching the North Atlantic, perpetuated as hyperpycnal flows. Glacier ice removed during these jokulhlaups lay stranded in ice-proximal locations for up to 200 years.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA.....7717R