Jökull - 01.12.1989, Page 17

South
North
Fig. 8. Schematic cross-section of the infill of the main caldera. Figure is not to scale. Lava flows and sills
are believed to constitute the major part of the infill in the southem part of the caldera but sediments have
accumulated in the northem part.
Explanations / Skýringar:
1 Hyaloclastite rocks/ Móberg
2 Lava flows and shallow intrusions/ Hraunlög og grunn innskot
3 Sediments/ Setlög
4 Water/ Vatn
5 Ice/ Is
Mynd 8. Einfaldað líkan af meginöskju Grímsvatna.
volume of 0.1 km3 of magma is erupted onto the cal-
dera fioor and intruded at shallow depths. This
amounts to 107 m3/year of magma. According to
the calculations of Bjömsson (1988, p. 104), the
heat released by magma which solidifies and cools
from 1300 °C down to 200 °C is 4.2-109 J/m3. If the
volume of magma is completely solidified and
cooled down over the 10 year period the contribu-
tion to the power of the geothermal area is
1300 MW. Evidence from the Heimaey eruption
shows that lava is cooled very rapidly when in direct
contact with water (Bjömsson, 1987). It is therefore
likely that lavaflows on the lakefloor are solidified
and cooled down within few years from the time of
eruption.
Heat release from intrusions is probably more
gradual, as access of water is not as free as on the
lakefloor. However, it is unclear how much role
penetration of water into an intrusion plays in
extracting its heat, which makes it difficult to quan-
tify the process. A likely upper limit on the time it
takes a sill-like intrusion to cool down can however
be estimated by assuming that conduction is the
only means of heat transport within the intrusion
and below it. In the following model the upper sur-
face of the intrusion is kept at a constant tempera-
ture which equals that of the surroundings before the
magma was intruded. This corresponds to the situa-
tion where all heat that is conducted through the
upper surface is carried away by hydrothermal con-
vection. No convection is supposed to take place
below the intrusion. Buntebarth (1980) presents a
JÖKULL, No. 39, 1989 15