Jökull - 01.12.1984, Page 27

The Grímsvötn Geothermal Area, Vatnajökull, Iceland
HELGI BJORNSSON
Science Institute, University of Iceland
Dunhagi 3, 107 Reykjavík.
HREFNA KRISTMANNSDÓTTIR
National Energy Authority, Geothermal Division.
Grensásvegur 9, 108 Reykjavík.
ABSTRACT
Melting ofice at the Grímsvötn geothermal area
has created a depression in the surface of the ice
cap Vatnajökull and produced a subglacial lake
from which jökulhlaups drain to Skeidarársandur.
The geothermal activity is also expressed by small
cauldrons on the surface of the ice as well as by
fumaroles on two nunataks that rise 300 m above
the lake level. Vapour from the fumaroles yields
little information about the deep reservoir fluid.
The vapour seeps upwards from the water table
and repeatedly condenses and evaporates on the
way to the surface. The chemistry of the water in
jökulhlaups, however, provides information
about the fluid in the geothermal system. This
information is not easy to interpret because of
water-rock interaction in the lake. Silica solubility
data and assumptions about the likely reservoir
temperature, however, indicate that about 15% of
the total mass in the lake is fluid discharged from
the geothermal reservoir. This information about
the geothermal mass fraction together with mass
and energy balances for the lake enables one to
calculate the masses of water and steam discharged
from the geothermal reservoir as well as the mass
of ice melted in the lake. The steam mass fraction
is estimated to be 20-35% when thefluid enters the
lake. From this, new estimates of the thermal
power of the geothermal system are obtained. The
total thermal power of the system is 4700—4900
MW, of which 2100-3000 MW are transported by
steam and the rest by water.
Grímsvötn is one offew geothermal areas where
active volcanism is observed and where there is a
direct interaction between magma and geothermal
water. Evidence of volcanic activity was found in
the water chemistry ofthe jökulhlaup in December
1983. The high content of sulphate and the pre-
sence ofiron indicated eruption ofmagma into the
geothermal fluid.
Since the nineteen-fifties jökulhlaups have
occurred regularly at 4-6 year intervals when the
lake level has risen up to a critical level required
for draining water from the bottom of the lake.
However, jökulhlaups may occur at lower water
levels. In 1983 a jökulhlaup was triggered at a
water level 20-30 m lower than the critical level.
This jökulhlaup may have been triggered by the
opening of waterways into the lake along the
slopes of Grímsfjall, where increased geothermal
or volcanic activity has melted ice in places. An
odour of hydrogen sulphide was detected for two
months on Skeidarársandur before the jökulhlaup
commenced. Sulphurous odour for long periods
may warrant a forecast of such premature jökul-
hlaups.
INTRODUCTION
The Grímsvötn geothermal area is located in
the interior of the Vatnajökull ice cap at the most
active caldera volcano in Iceland. The volcano,
the thermal area and the caldera lake Grímsvötn
are almost completely covered with ice. Periodic
bursts of water (jökulhlaups) drain the lake sub-
glacially down to the rivers on the Skeidarársand-
ur outwash plain : Skeiðará, Sandgígjukvísl and
Súla. (Fig.l). The volcanological and geothermal
activity in the area has been studied by many
authors. This includes the history of eruptions
and jökulhlaups, the mass balance of the lake,
JÖKULL 34. ÁR 25