Jökull - 01.12.1957, Blaðsíða 19

r/?. j
r
/r/f. ^
At the end of the activity forming the Mull
dike-swarm the local temperature situation at
Glasgow may have been as given in Fig. 3.
It is assumed that the melting point of
basalt Tm is reached at the depth h. Below
this depth the temperature gradient is rela-
tively small whereas the gradient above the
reservoir is relatively large. From the above
picture we may subtract the normal tempera-
ture field and find an anomalous field as
shown in Fig. 4, where Tn is the normal
temperature at the depth h. Actually, the adia-
batic gradient should be applied below h but
this is unimportant.
The above model should maximize the tem-
perature anomalies. At the end of the volcanic
activity we may tentatively assume that the
reservoir has largely solidified and that con-
duction becomes the main mode of heat trans-
fer above as well as below the depth h. The
question arises how fast the anomaly given in
Fig. 4 decays.
The surface heat flow due to the anomaly
is easily computed on the basis of equation (30)
and is found to be
Q1= k(Tm~TAerf(h/2V^) (41)
where t is the time since the end of the
volcanic activity. By inserting h = 40 km, a =
0.007 cm2/sec and t = 50-106 years we find
that the anomaly of the surface heat flow
should have decayed to about 60% of the
initial value. Thus, if the total present ano-
maly at Glasgow amounting to 0.4 microcal/
cm2 sec is due to the above situation its initial
value should have been around 0.7.
To this figure we may have to add a small
contribution from the feeder dikes above the
reservoir. In fact, a part of this contribution
should be included in the observed anomaly.
However, the main influence of the dikes lies
at a more shallow depth and, therefore, decays
at a faster rate than the anomaly mentioned
above. In accordance with the data given above
on the relative volume of the dikes the total
surface heat flow anomaly due to feeder dikes
in Iceland as well as Scotland may at the end
of a long period of volcanism amount to as
much as 0.3 microcal / cm2 sec provided the
dikes extend down to the depth of 40 km.
I-Ience, by adding this to the above figure a
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