Jökull - 01.12.1974, Qupperneq 3

JOKULL
Á R S R I T
J Ö KLARAN N SÓ K N AFÉLAGS ÍSLANDS
24. ÁR REYKJAVÍK 1974
Explanation of Jökulhlaups from Grímsvötn,
Vatnajökull, Iceland
HELGI BJÖRNSSON,
H. H. WILLS PHYSICS LABORATORY, UNIVERSITY OF BRISTOL, ENGLAND*
ABSTRACT
The cause ancl process of jökulhlaups from
the subglacial lake Grimsvötn are explained. in
t.his paper. Possible future changes in the
phenomenon are discussed.
A topographical moclel of Skeidarárjökull
ancl the Grímsvötn water basin, based on seis-
mic, gravity and geodetic surveys is presented.
Although the present knowledge of the sub-
glacial topography is not detailed, the model
is accurate enough to explain the jökulhlaup
phenomenon. The glacier surface map, which
is toell known, proves to be of dominating im-
portance.
In a periocl of five or six years the water
level in the lake rises about 100 m. The glacier
is lifted off a subglacial ridge east of the lalie
and water is forced subglacially through a 50
km long route beneath Skeiclarárjökull, causing
vast floods. When the water level has fallen
about 100 m the ivaterway is sealed by rapicl
þlastic deformation of t.he ice at the easfqm.
'i j>.
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edge of the Grimsvötn clepression. The water
level is not lowered to the subglacial rim.
The mass and energy balances of the basin
cre discussed. A water basin of 300 km2 collects
water into a subglacial lake of about 30—40
km2. About 3/i of this water has been melted
by a subglacial geothermal area. The geothermal
area is probably the most powerful one in Ice-
land and its extent is about the same as that
of the Torfajökull area (energy flux 1.5 ■ iO17
]lyr, energy flux clensity 50 Wjm2, area 100
km2). The flow of water estimated from the
model agrees well with the estimated discharge
of glacial rivers on Skeidarársandur.
The primary cause of jökulhlaups from
Grimsvötn is the melting of ice in the geo-
thermal area. The slope of the glacier surface
*) Present address: Science Institute, Uni-
versity of Iceland, .Dunhagi 3, Reykjavík, Ice-
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JÖKULL 24. ÁR 1