Jökull - 01.12.1957, Page 14

There is doubt about the most recent activity
at well (1). The formation of the above men-
tioned intrusive at well (2) probably constitutes
the most recent activity at this location. Ac-
cording to Einarsson (personal communica-
tion) the intrusive has a reverse magnetization
and its minimum age is, therefore, about 1%
million years. The most recent volcanic activity
at well (3) is probably contemporaneous with
the formation of the grey section, possibly
sometimes during the Pliocene.
Release of seismic energy in Iceland. Accord-
ing to E. Trygguason (personal communica-
tion) the total annual release of seismic energy
in Iceland ancl the surrounding shelf is in
average about 1022 erg, that is, about 1/1,000
of the annual average for the earth as given
by Gutenberg and Richter (19). No foci deeper
than 60 km are recorded. The distribution of
the foci is clearly related to the centra of post-
Glacial volcanic activity. No foci have been
recorded in the eastern and the western parts
of the Tertiary basalts.
The mechanical processes at the sesimic foci
release heat in an amount that may be con-
siderably in excess of the energy radiatecl in
the form of seismic waves. Benioff (20) finds in
the case of shocks in Southern California that
the heat released is about twice the energy
of tlie waves in the main shocks. Moreover,
other mechanical processes in the crust may
lead to a local release of heat.
However, the application of Benioff’s results
to the case of Iceland gives at steady state
conditions an average outflow of seismic heat
of 0.01 microcal/cm2 sec only. Moreover, it is
doubtful whether the steady state figure can
be applied. The anomalous heat flow due to
a more transient seismicity would be smaller.
Radioactive elements in the lcelandic effu-
sives. Jeffreys (21) has studied the concentra-
tion of radioactive elements in the silicic and
basic rocks of Iceland. He reports normal
values.
Terrestrial heat flow in Great Britain. Great
Britain is the only part of Europe where reli-
able data on the heat flow have been obtained.
These data are of course of special interest in
the present case.
Birch (8) lists 2 results from Yorkshire and
8 from Nottinghamshire. Chadwick (22) reports
one recent result from Cambridge. The results
from Nottinghamshire include 4 results ob-
tained by Bullard and Nibblet (23) from 4
wells at Eakring. The wells are quite close to
each other and the data can, therefore, not be
regarded as independent. Furthermore, the
average heat flow obtained at Eakring is sub-
stantially larger than the results obtained at
Caunton and Kelham Hills only 6 resp. 10 km
south of Eakring. Bidlard and Nibblet (23)
suggest that the conditions at Eakring are
influenced by a local circulation of ground
water. The data from this location are, tliere-
fore, questionable.
The data from Eakring excluded, the average
of the results given by Birch (8) and Chadwick
(22) is 1.17 microcal/cm2 sec and the range
0.68 to 1.67. If Eakring is included as a single
result the average is raised to 1.35 ancl the
range becomes 0.68 to 2.60. The figures re-
present the unreduced heat flow.
According to Chadwick (22) a correction of
0.2 microcal/cm2 sec should be applied in
orcler to correct for the climatic change at
the end of the Pleistocene. This raises the
above averages to 1.37 and 1.55 microcal/cm2
sec resp.
Results from Scotland by Anderson (24) may
be mentioned in addition to the above data.
Anderson finds in two wells in Fife in eastern
Scotland a heat flow of 0.95 and 0.89 micro-
cal/cm2sec. Furthermore, in two wells in the
vicinity of Glasgow the figures of 1.73 ancl
1.48 are found. Taking again into account a
correction for climatic effects of 0.2 microcal/
cm2 sec the average for the wells in Eife is
found to be 1.12 microcal/cm2 sec and the
average for the wells at Glasgow is 1.81. It is
of interest to note that Glasgow is in the
vicinity of the Mull dike-swarm.
Anderson’s data may be regarded as two
independent results, that is, one for Fife and
one for Glasgow. The average of all data on
the unreduced heat flow in Great Britain as
listed by Birch, Chadwick and Anderson is
then 1.33 and 1.19 microcal/cm2 sec depending
on whether the data from Eakring are inclucl-
ed or not. Correction for climatic effects raises
the average to 1.53 and 1.39 resp.
The figure including the data from Eakring
is about 25% in excess of the global average.
It is, however, somewhat uncertain whether
this excess is entirely real. None of the data
12