Jökull - 01.12.1957, Side 17

on the basis of equation (30) and a figure of
0.2 to 0.3 microcal/cm2 sec is found. No further
gravity anomalies are visible at this location.
The maximum undetectable gravity anomaly
may be estimated at 4 mgal and the minimum
age of the volcanic activity at i/2 million years.
Equation (37) gives then by a density contrast
of 0.25 a maximum surface heat flow of 0.6
microcal/cm2 sec.
The maximum total anomaly due to local
intrusives at the location of well (2) is, there-
fore, estimated at roughly 1 microcal/cm2 sec.
This gives a total surface heat flow of 3.6,
that is, about 0.8 less than the heat flow
observed at this location and reduced in ac-
cordance with hypothesis (A). On the other
hand, the figure is 0.2 above the heat flow
in accordance with hypothesis (B).
In view of the fact that the above estimate
of a total of 3.6 microcal/cm2 sec represents
in many ways a maximized figure we may infer
that hypothesis (A) is not likely to give a
correct picture of the situation at this loca-
tion.
There are no detectable gravity anomalies
at the location of well (3). The most recent
volcanic activity here dates back by several
millions of years. By inserting Ag = 4 mgal,
Ap = 0.25 and t = 2 * 106 years we find on the
basis of equation (37) a figure of Qmax = 0.2
microcal/cm2 sec. Thus, the total maximum
surface heat flow shoulcl amount to 2.8. This
figure is 0.5 less than the heat flow observed
at this location and reduced in accordance
with hypothesis (A), but equal to the heat
flow reduced on the basis of hypothesis (B).
Again hypothesis (A) turns out to be less likely.
As to the temperature data at well (1) it
may be remarked that they are quite similar
to the data from well (3). Probably this is an
indication that volcanic effects are small even
at the location of well (1).
Summing up the present results, the heat
flow observed in well (2) and (3) is found to
he larger than can be expected on the basis
of a relatively moderate erosion during the
Pleistocene as assumed in hypothesis (A). A
more rapid erosion as assumed in hypothesis
(B) or (C) appears to be more likely.
The results obtained so far have been based
on the assumption that the lignite deposits
are indicative of a certain maximum tempera-
ture at the bottom of the visible section of
the plateau basalts. As the quantitative evalua-
tion of this liypothesis is subject to uncertainty
we may inquire whether similar results may
be inferred on the basis of other data.
The question arises whether it is likely that,
in general, the average outward conduction
of heat in Iceland can in the absence of large
intrusives be of the order of 2.6 microcal/cm2
sec, or whether even higher figures can be
expected on the basis of the extensive volcanic
activity. This problem is, of course, beyond
complete resolution mainly because of the
limited knowledge about the physical processes
underlying volcanic activity in general. How-
ever, the following remarks may be made.
The thermal processes within the earth are
affected by the following factors:
(1) Heat release by radioactive disintegra-
tion
(2) Heat release by seismic foci
(3) Heat conduction
(4) Mass transport of heat by convection cur-
rents
(5) Mass transport of heat by magma
(6) Radiative transfer of heat
We may now enquire in what way these
processes could contribute to an anomalous
situation as discussed above. Three relevant
observations can be mentioned. Two of these
were discussed above.
Firstly, Einarsson’s conclusion as to the state
of the substratum of Iceland. Secondly, Ander-
son’s heat flow data from Scotland. Finally,
two recent observations on the depth of tlie
foci of volcanic shocks. Finch (26) reports that
the shocks preceding eruptions of the Kilauea
and Mauna Loa on Hawaii occur initially at
a depth of 30 to 50 km. The corresponding
clepth for the Paricutin in Mexico was found
to be 40 km (27).
The fact that the substratum of Iceland
appears solid, in general, indicates that the
temperature is almost everywhere below the
melting point of the material. Furthermore,
an upper limit to the temperature gradient
and the conduction flow of heat is indicated.
The normal conduction flow at tlie clepth of
100 km as computed by Jacobs (28) is 0.3 to
0.4 microcal/cm2 sec. It appears unlikely that
this flow can be raised very much without
15