Jökull - 01.12.1957, Side 7

The average outward conduction of heat in
normal areas is estimated at 1.2 microcal/cm2
sec (9). The figures given in Table (I) are,
therefore, exceptionally high.
Thus, it may be concluded, in spite of the
scarcity of the data, that both the thermal
activity and the well data indicate a highly
abnormal subsurface temperature field in Ice-
land.
An attempt may now be made to arrive at
a quantitative evaluation of the above data in
terms of the past geological history. The pro-
blem consists of an evaluation of the possible
influence of the various geological processes
on the temperature field and the heat
flow in the near surface layers. Following a
correction for these shallow effects it may be
possible to infer the reduced heat flow, that is,
the heat flow as unaffected by the surface
conditions.
Various assumptions about the progress and
magnitude of the geological processes lead to
different figures for the reduced heat flow. The
likelihood of the assumptions can then be dis-
cussed on tlie basis of the results. The pos-
sibilities are that certain assumptions can be
ruled out on the basis that they lead to
improbable results for the reduced heat flow.
The first step consists of a brief discussion of
the theory of the shallow temperature field.
THEORY OF THE SHALLOW
TEMPERATURE FIELD
Important factors on which the shallow tem-
perature field may depend are listed as fol-
lows:—
(1) The reduced heat flow
(2) Thermal properties of the rock
(3) Erosion
(4) Sedimentation
(5) Seasonal and climatic changes of the
surface temperature
(6) Intrusion of magma
(7) Movement of fluids and gases
(8) Topography
(9) Dissipation of mechanical energy
(10) Tectonic movements
(11) Chemical processes, metamorphism.
In the present case the reduced heat flow is
the rnain unknown factor. The thermal con-
stants of the rock have already been discussed
briefly.
Erosion, on a large scale has no doubt been
of importance in Iceland. The influence of this
factor depends on the geological history and
can be estimated as will be shown below.
The question of the possible influence of
local volcanism will be discussed below.
The influence of percolating ground water
on the general temperature field in Iceland
depends on the permeability of the flood basalts.
The thermal areas show that water is an
important carrier of heat and that the upward
movement of thermal water may completely
alter the local temperature field. These pheno-
rnena are, however, confined to the immediate
vicinity of certain structures as permeable faults
and dikes. The effect is in general very local.
The unfractured basalts appear, on the other
hand, to have a negligible vertical permeability.
This is clearly indicated by the presence of
many lakes at mountain tops and at the edges
of the plateau. The very low permeability of
the Tertiary basalts has also been observed in
the negligible yield of water wells drilled into
these basalts.
The wells in table (I) were actually selected
with regard to the minimum influence of per-
colating water. Furthermore, it may be re-
iterated that the conditions on the north-
western penisula can hardly be understood on
the basis of ground water percolating at steady
state ancl normal conditions. It will, therefore,
be assumed that the influence of percolating
ground water may be disregarded in the case
of the temperature observed in the above wells.
Topography is of little importance in the
present case. Dissipation of mechanical energy
may be disregarded at the first instance as there
are no seismic foci and no signs of recent
tectonic movements in the vicinity of the wells.
Finally, neither metamorphism nor other
chemical processes can be of any importance
in the present case.
Tlie main task in the following will conse-
quently consist of a correcting for the possible
effect of erosion, sedimentation and climatic
changes. Various assumptions will be made
about these processes and the likelihood of the
results will be discussed in general terms.
The theory will be based on the simplest
5