Jökull - 01.12.1957, Blaðsíða 20

total surface heat flow anomaly of 1 microcal/
cm2 sec is found. This could be the situation
at Glasgow at the end of the Tertiary volcan-
ism. Assuming the normal heat flow to be 1.4
the total heat flow could possibly have amount-
ed to 2.4 microcal/cm2 sec.
This figure may tentatively be accepted as
an estimate of the possible surface heat flow
during the formation of other parts of the
basalts of the Brito-Arctic province, provided
there are no local perturbations by large single
intrusives above the depth h. As a matter of
fact, the result turns out to be much the same
as the estimate given above on the basis of the
presence of the lignites.
Thus, the present result again lends some
support to the conclusion that the heat flow
observed at well (2) and (3) is larger than is
probable on the basis of hypothesis (A). It
appears reasonable to infer that the erosion
has been more rapid and has contributed more
to the surface heat flow than indicated on the
basis of this hypothesis.
The evidence presented, therefore, indicates
that the present landscape forms of Iceland
are of a relatively recent age, that is, possibly
formed in the second half of the Pleistocene.
This may be taken as an indication that the
basalt plateau was lifted to its present level as
late as in the middle of the Pleistocene.
However, it shoulcl be underlined that
further studies are necessary before final con-
clusions can be reached. There is mainly a
need for further temperature data from suit-
ably located wells, above all data from loca-
tions of different glacial erosion. Furthermore,
data on the heat conductivity of the plateau
basalts are needed. It is hoped that these data
can be secured in the coming future.
At this juncture it should be mentioned that
the above results as to the possible surface heat
flow in Iceland do, of course, not apply to the
regions of very late Quaternary or post-Glacial
volcanism. Small intrusives of very recent age
may lead to very large but brief heat flow
transients as is indicated by equation (33).
CONCLUSIONS
Evidence has been presented that rapid ero-
sion during the Pleistocene as well as Tertiary
and Quaternary volcanism are the main factors
that may modify the general subsurface tem-
perature field and outward conduction of heat
in Iceland. Results of a study of the tempera-
ture conditions in two wells lend support to
the inference that the erosion has had a great
local influence on the conditions. Further-
more, the erosion appears to have been very
rapid and the present landscape forms of Ice-
lancl can hardly be the results of a time as
long as the entire Pleistocene.
It appears more likely that the erosion of
the present forms has taken place mainly in
the seconcl half of the Pleistocene. This situa-
tion may be understood on the basis that
the plateau was lifted to its present position
as late as in the middle of the period.
In this way the thermal data appear to
supplement the geological and paleomagnetic
results.
A corollary to the above results is that a
substantial part of the low-temperature thermal
activity in Iceland appears to be largely a
short time transient phenomenon. The circu-
lating water appears to derive the bulk of the
heat content by a contact with hot rock which
has been brought near to the surface by the
combined effect of erosion and uplift. The
phenomenon lasts until the heat storage in the
rock has been depleted.
This conclusion is not unreasonable. The
deglaciation of Iceland some 12,000 years ago
led to a general upwarping of the country.
The vertical movement may have amounted
to 30 to 40 metres in coastal areas and up to
150 metres in central areas. The difference in
the upwarping has no doubt caused a sliding
of blocks along existing faults and possibly
also the formation of new faults. The results
can very well have been the opening of sub-
surface channels for water and the formation
of subsurface circulating systems. As the water
came into contact with relatively hot rock a
substantial heating and the formation of hot
springs may have resulted.
Thus, we are arriving at the peculiar result
that the Pleistocene glaciation may have been
one of the main promoters of the widespread
low-temperature thermal activity in Iceland.
The phenomenon appears to result from a com-
bined effect of abnormal subsurface tempera-
tures due to volcanism, rapid glacial erosion
and the readjustment of the isostasy following
the deglaciation.
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