Jökull - 01.12.1975, Blaðsíða 11

level z0. In a period of five or six years the
water level rises about 100 m and a jökulhlaup
results. An ice cauldron of 30 km2 is formed,
the so called Grímsvötn-depression.
Jökulhlaups due to subglacial
volcanic eruptions
The main ice caps in Iceland are all located
within the active volcanic zones. Subglacial
eruptions can cause sudden jökulhlaups.
Consider the effect of a fissure eruption be-
neath a sloping ice cap. Magmatic heat will
cause considerable local melting at the bed of
the glacier. One may expect the meltwater to
flow away from the site of the eruption towards
the edge of the glacier. The flow in the water-
ways is limited by the production of meltwater.
The rate of opening of the waterways by melt-
ing may be balanced by the rate of closing by
plastic deformation (cf. Nye in press). So far,
the model agrees with Tryggvason (1960). A
depression will be formed in the glacier sur-
face above the eruption site. The ice over-
burden pressure at the bed surrounding the
depression will exceed the water pressure at the
subglacial water reservoir. The waterways which
run out from the reservoir may therefore be-
come sealed by plastic closure. Meltwater would
then be trapped beneath the depression. A
water cupola will become located as a cap on
top of the volcanic vent. Lava erupts directly
into water. A rapid heat transfer takes place by
boiling of water (at ice depths less than 2400 m
as the critical pressure for water is 220 bar).
Water and volcanic products are then accumu-
lated in a closed system. In a rnodel of perfect
heat transfer one volume unit of lava melts
thirteen volume units of ice, which equals
twelve volume units of water. The lava would
take up the free space created by melting. In a
more realistic model of imperfect heat transfer
the volume of lava would exceed the free space.
Therefore, the water cupola will rise beneath
the depression and jökulhlaups may be expect-
ed from the reservoir. After a jökulhlaup the
subglacial waterways may become sealed again.
A few jökulhlaups might occur during a long
eruption rather than one catastrophic jökul-
hlaup. — After an eruption water will ac-
cumulate beneath a depression in the glacier
surface. Ice and water would flow into the de-
pression and cause a jökulhlaup. If the geo-
thermal activity decays, ice will gradually fill
the depression and the jökulhlaups cease in the
end. (Observations do not indicate such a de-
velopment at the ice cauldron north-west of
Grímsvötn, Björnsson in preparation).
Table-mountains and hyaloclastic ridges are
considerecl to be formed by subglacial eruptions
(Kjartansson 1943, 1964, 1966, Matheivs 1947,
van Bemmelen and Rutten 1956, Jones 1969).
Studies of the shape and structure of these
mountains indicate a certain growth sequence.
During the first phase lava appears to erupt
directly into a water-filled vault. The structure
of subglacial and submarine volcanoes were
expected to be similar. The birth of the island
Surtsey in 1963 provided a virtual example of
the creation of a submarine volcano. The pre-
sent paper explains how the water-filled vault
may be formed during a subglacial eruption.
Fig. 6 shows a model for the generally accepted
growth sequence of a table-mountain. The sub-
glacial volcano is built within a water cupola
which would explain why the base length of the
volcanoes is multiplum of its height (cf. Einars-
son 1972, p. 111). The shape of the water cupola
is determined by the shape of the glacier sur-
face depression according to Equation (8). This
explains why a subglacial volcano can be ex-
pected to be more confined and more steepsid-
ed than a submarine volcano. Further, sub-
glacial volcanoes are built up inside ice-locked
water whereas the submarine volcanoes are con-
stantly dashed by oceanic waves.
Grímsvötn and Katla are the most active sub-
glacial volcanoes in Iceland. Both volcanoes are
situated at geothermal areas. Jökulhlaups are
frequent from both sites. A strong geothermal
area has created a large water reservoir at
Grímsvötn, say 3 km3 in volume. In a period
of five or six years the water level rises about
100 m and a jökulhlaup occurs. Volcanic erup-
tions within this reservoir cannot trigger un-
expected jökulhlaups. The accumulation of vol-
canic materials can only raise the water level
by a few meters. An input of, say 0.3 km3 of
volcanic materials would raise the 30 km2 water
surface by 10 m. Melting of the floating ice
cover does not raise the water level, and the
inflow of the surrounding ice is a relatively slow
process. However, a volcanic eruption within
the water cupola 10 km north-west of Gríms-
vötn might at any time trigger jökulhlaups.
JÖKULL 25. ÁR 9