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change it is not clear yet whether some of these
hazards are a normal part of glacier behaviour,
or whether they represent an evolving new threat
from a changing cryosphere.
the same time as an eruption this may rise by
another order of magnitude. Similarly, Maizels
(1997) distinguishes three types of proglacial
sedimentary (outwash) deposits in relation to
jökulhlaup type:
2.3.1.1 Fluvioglacial sedimentation and glacier
outburst floods
Type I - 'normal' braided river outwash
Type II - produced by sudden drainage of ice
dammed lakes
Type III - associated with drainage during
subglacial geothermal activity, with deposits
resulting from high sediment concentrations
and hyperconcentrated flows.
A jökulhlaup is an Icelandic term used to
describe a catastrophic flood caused by the
sudden drainage of a subglacial or ice-dammed
lake. Lakes may develop and drain seasonally
or may build up over many years before being
drained. Volcanic eruptions or geothermal heat-
ing under ice caps can often be the trigger for
such periodic drainage, leading to catastrophic
floods.The Grímsvötn area on Vatnajökull in
southern Iceland is particularly prone to such
activity, which is triggered by the Katla volcano
under the ice cap of Myrdalsjökull (Gerrard
1990). At Grímsvötn water is stored in a sub-
glacial lake located in a volcanic caldera of about
30 - 40 km 3 in the centre of Vatnajökull ice cap.
Geothermal heating causes internal drainage in
the ice cap, which causes the subglacial lake
to rise by about 100 m over a period of five or
six years. Eventually the ice dam is breached and
water flows under the ice cap to emerge at the
ice margin several tens of kilometres from the
source. Floods can have peak discharges of up
to 100,000 m 3 s −1 . Two main mechanisms lead
to the release of stored water. Drainage may begin
at basal water pressures less than the ice over-
burden pressure through the slow expansion of
glacial conduits by melting of ice walls through
frictional or sensible heat. Alternatively high
subglacial lake levels lift the glacier off the bed
along the flow path resulting in extremely sudden
discharges. As a consequence of the water release
river levels may rise by up to 10 m and millions
of cubic metres of sediment are deposited, often
raising sandur (alluvial plain) levels by several
metres. Sediments are transported from sub-
glacial sources and eroded from the proximal
zone of the moraine/sandur complex (Magilligan
et al. 2002). Björnsson (2003) estimates the
sediment load of a large jökulhlaup may be as
great at 10
Each type of activity results in a distinct set
of depositional landforms and sediments. In
historical times, the Skeidarsandur jökulhlaup
of November 1996 stands out as unprecedented
in its magnitude and duration (Magilligan et al.
2002). The event reached a peak discharge of
53,000 m 3 s −1 in 17 hours and was responsible
for widespread incision and aggradation with
3.8
10 7 m 3 of sediment being deposited in the
proglacial depression of the sandur. Deposits
of over 9 m depth were recorded. The total
volume of water released from the glacier was
estimated at 3.5 km 3 , and was the most rapid
jökulhlaup recorded for this area. The event
also eroded large stretches of the main high-
way around Iceland, destroyed two bridges, and
caused damage estimated at US$15,000,000
(Magilligan et al. 2002).
Because glacier outburst floods are sudden
discharges from water bodies dammed within
or at the margins of glaciers in steep glacierized
environments the downstream impacts of such
events can be devastating, leaving paths of total
destruction. At Mount Rainier (4364 m) in the
Cascade Range, Washington State, USA glacial
outburst floods occur on a relatively small scale.
Mount Rainier is a strato-volcano consisting
of overlapping layers of lava and tephra. The
mountain is topped by a summit ice cap, with
25 glaciers extending radially in all directions
from the summit (Fig. 2.11a). Most of the activ-
ity on Mount Rainier is restricted to the glaciers
on south-western slopes of the mountain due to
the local geographical-climatological conditions
×
10 6 t per event, but if this occurs at
×
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