Jökull - 01.12.1957, Side 18

violating the temperature condition. Hence,
we may expect that larger anomalies of the
surface heat flow either originate above the
depth of 100 km or that the anomalous flow
is transported through this level by some other
process than condution.
According to the data given by Anderson
the surface heat flow in the vicinity of Glasgow
should be around 1.8 microcal/cm2 sec, that is,
about 0.4 above the average for England. The
wells at Glasgow used by Anderson are located
very close to the Mtdl dike-swarm which is
a marginal part of the Brito-Arctic basalt
province. In applying Anderson’s data it is to
be remarked that the heat flow has not been
corrected for glacial erosion.
Nevertheless, there appears to be a relatively
small anomaly at Glasgow which can be due
to the fact that the location belongs to the
Tertiary basalt province. We may now attempt
to trace the history of this anomaly back to
the early Tertiary when the volcanism was in
vigour at this location.
It is clear that the radioactive processes and
convection currents in the mantle are quasi-
stationary phenomena that do not change ap-
preciably during the Tertiary. Hence, if the
anomaly is due to these processes it would
have remained almost constant through the
Tertiary and the total heat flow at Glasgow
would be very much the same as now, that
is, 1.8 microcal/cm2 sec.
The present data on the seismicity of Ice-
land are not indicative of a noticeable contri-
bution by the seismic foci to the heat flow in
general. However, it is to be underlined that
the present heat flow is unrelated to the pre-
sent seismicity. This fact is clearly shown by
equation (34) which gives the time required
for the outflow of heat released at a given
depth. For example, if one cal is released
at the depth of 20 km, only about \/2 cal
has been conducted out of the surface in
the course of the first 20 million years after
the release. Hence, a contribution to the pre-
sent heat flow by seismic foci would have to
be the result of the seismicity during the early
and middle Tertiary. Furthermore, in order
to amount to a noticeable contribution the
seismicity must then have remained at a much
higher level than present for tens of millions
of years.
There are good reasons to expect that the
thermal processes within the earth are the
cause of the release of mechanical energy in
the form of seismic waves, orogeny etc. The
conversion of the heat to the mechanical energy
is necessarily a process of very low efficiency.
On the other hand, the present average energy
release in the form of seismic waves as given
by Gutenberg and Richter (19) is not less than
1/700 part of the present estimates of the
total outward flow of terrestrial heat. This ratio
appears quite high when it is taken into account
that the energy of the seismic waves consti-
tutes only a part, probably a small part, of the
total mechanical energy released in the earth.
Thus, there appear reasons to suspect that the
present seismicity is above the long time aver-
age. The possibility that the seismicity of Ice-
land was much larger than present during a
substantial part of the Tertiary is, therefore,
not likely.
Clark (29) has discussed the radiative transfer
of heat within the earth. He comes to the con-
clusion that the radiative transfer becomes
noticeable at temperatures above 1,000° C and
may contribute substantially to the heat trans-
port in the upper 600 km of the mantle. The
radiative effect is negligible in the zones near
to the surface where the temperature is below
1,000° C.
Thus, an increase of the temperature at the
depth of 100 to 200 km may enhance the effec-
tive conductivity and lead to an anomalous
flow of heat. However, the relaxation time for
anomalies at this depth is very large, probably
very much larger than the length of the Tertiary
period. Therefore, if the anomalous heat flow
at Glasgow is due to an enhanced conductivity
at depth it would have remained largely un-
changed during the Tertiary period.
Mass transport of heat by upflowing magma
is, therefore, the main process requiring atten-
tion. The data obtained at the Kilauea, Mauna
Loa and the Paríchutin indicate that the aver-
age depth of the magma reservoirs may be
around 40 km. Below this depth there may be
large local reservoirs of fluid magma where
convection may be the main mode of heat
transport. Above the reservoir heat is mainly
transported by conduction, and a relatively
smaller part by magma flowing up through the
feeder dikes of the erupting fissures and vents.
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