Jökull - 01.12.1975, Page 3

JOKULL
Á R S R I T
JÖKLARAN NSÓKNAFÉLAGS ÍSLANDS
25. ÁR REYKJAVÍK 1975
Subglacial Water Reservoirs,
Jökulhlaups and Volcanic Eruptions
HELGI BJÖRNSSON,
SCIENCE INSTITUTE, UNIVERSITY OF ICELAND,
REYKJAVIK, ICELAND
ABSTRACT
Water may accumulate in a reservoir that
forms beneath a depression in a glacier surface.
The water reservoir will grow unstable. The
accumulation will cause a jökulhlaup from the
reservoir. Water may also accumulate beneath
a slightly inclined or a convex glacier surface.
The reservoir will remain stable. A jökulhlaup
will not result under these conditions.
A depression in the glacier surface may be
created by melting above a permanent geo-
thermal area. The depression at Grímsvötn in
Vatnajökull is a well known example. Jökul-
hlaups at Skeidarársandur originate at Gríms-
vötn. An ice cauldron which is situatecl 10 km
north-west of Grímsvötn is an other example.
]ökulhlaups in the river Skaftá drain from a
reservoir wliich is situated beneatli the ice caul-
dron.
A depression may also be created by a sub-
glacial volcanic eruption. The eruption will
cause considerable subglacial melting. A depres-
sion is formed in the glacier surface if the
meltwater drains toivards the glacier rivers. The
subglacial waterways around the depression may
become sealed. Meltwater would then be trapp-
ecl beneath the depression. A dome-shaped sub-
glacial ivater reservoir will be formed at the
bed of the glacier. Jökulhlaups will occur from
the reservoir. Pillow lava and hyaloclastic ma-
terials are piled up within such a reservoir
during subglacial volcanic eruptions.
SYMBOLS
g acceleration of gravity
z vertical coordinate for glacier elevation
zb glacier bed-rock elevation
H[ glacier thickness
z0 datum level for glacier elevation
zs glacier surface elevation
zw — zb -f- Hw, elevation of the top surface of
a subglacial water layer
JÖKULL 25. ÁR 1
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