Jökull - 01.12.1957, Side 16

pared with the radius. The maximum surface
heat flow is then measured vertically above the
centre of the sphere.
DISCUSSION
The occurrence of lignites in the lower parts
of the basalt plateau in eastern Iceland ap-
pears to provide the most direct clue to the
present problem. Provided that the above inter-
pretation of the low rank of these carbonaceous
deposits is correct the temperature in the
lignite horizons should never have exceeded
500° C for any appreciable length of tirne.
Thus, the temperature at the depth of 8 km
in the basalts of eastern Iceland should
practically never have exceeded 500° C.
In the steady state the equation for the
temperature in the region above the lignites is
kTxx + q = 0, (38)
where q is the amount of heat liberated in
the rock per unit time and unit volume. If
q is constant the solution at the boundary
conditions
x = 0, T = 0
x = h T = Th
is
and the surface heat flow is
Q0=kTh/h + H/2 (40)
where H = qh, that is, the heat liberated per
unit time and unit area of the column
0 < x < h.
In the present case heat may be liberated
by (1) radioactive disintegration, (2) intrusion
of magma and (3) seismic foci.
According to Walker (14) the above plateau
section in eastern Iceland consists of 83% of
basalt, 7% of rhyolite, 5% of andesite and
5% of sediments. Applying the figures given
by Birch (9) we find the average heat libera-
tion by radioactive processes of 1.6xl0 —13
cal/cm3sec, that is, a total production in the
upper 8 km of 0.13 microcal/cm2 sec.
The heat liberated by the intrusions may
be estimated on the basis of the figures given
above. We will assume that at most 10% af
the basalt plateau consists of small intrusive
bodies, mainly dikes, which have been formed
at an approximately uniform rate during the
building up of the eastern plateau in the
period from the Eocene to the middle or late
Tertiary. Assuming this time to be 40 • 10°
years and the average sensible heat content of
the magma to be 103 cal/cm3 we find an
average liberation of heat of 0.8-10 —13 cal/
cm3sec.The total heat flow due to the intrusives
in the upper 8 km is, therefore, 0.06 microcal/
cm2 sec.
At present the seismic foci may liberate an
average of 0.01 microcal/cm2 sec. Most of the
heat is released in the zone of post-Glacial
volcanism where the average may be 0.04. We
may tentatively assume that all foci are above
the depth of 8 km and apply this figure as an
average for the period during the building up
of the eastern plateau.
Thus, the quantity H is estimated at 0.2
microcal/cm2 sec and the second term in equa-
tion (40) at 0.1. The surface heat flow is,
therefore, dominated by the first term. By in-
serting Th = 500° C, h = 8 km and k = 0.004
cal/cm °C sec we find Q0 = 2.6 microcal/cm2sec.
This should be the maximum surface heat
flow, provided the temperature of the lignites
has not exceeded 500° C for any appreciable
length of time.
The Reydarfjördur area can no doubt be
regarded as representative for the general
conditions in the basalt plateau. Therefore,
the average temperature conditions there can
hardly be much different from those in the
western parts of the plateau. Important devia-
tions of the present surface heat flow in the
western parts can only be expected in locations
where intrusives have been formed in the
course of relatively recent volcanism. These
may be large single intrusives of late Pliocene
or Quaternary age, or a multiple intrusion of
dikes of late Quaternary or post-Glacial age.
In discussing the various possibilities we may
apply equation (37) in combination with the
gravity survey of Einarsson (16).
The uncertainty as to the dating of the
most recent volcanism at well (1) renders the
data from this well inapplicable for an estima-
tion of the volcanic effects.
The intrusive at well (2) has a thickness of
less than 1 km and a minimum age of J/2
million years. The present surface heat flow
due to this intrusive can be estimatecl roughly
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