Jökull - 01.12.1961, Blaðsíða 36

ectecl to the volcanic structures. Moreover,
subsurface temperature of more than 200° C
has been observed in shallow wells in these
areas. It is difficult to account for the high
temperatures without relying on the presence
of volcanic structures of a very recent age.
On the other hand, it is to be emphasized that
the heat output of the large high-teinperature
areas is of a considerable magnitude. The up-
flow of juvenile water from magmatic sources
can hardly account for a comparable heat
transport (Einarsson, 1942 and Bodvarsson,
1951). There are therefore reasons for assum-
ing that the circulating water has a direct cont-
act with the source rock. Moreover, recent iso-
topic studies of thermal waters are not indic-
ative of major juvenile components (see Craig,
Boato and White, 1956). For a broacl discussion
of relevant problems the reader is referred to
the paper by White and Brannock (1950).
Banwell (1958) is aware of a similar difficulty
in the case of large thermal areas of New
Zealand. He suggests the presence of large con-
vective magma chambers in order to explain
the necessary flow of juvenile water.
The heat supply of the low-temperature areas
appears in most cases to be closely related to
the physical conditions in the Tertiary districts.
These parts of the country have not been
subjected to volcanism through the Quaternary
period at least. Intrusives with elevated tem-
peratures are, therefore, not to be expected in
these districts.
On the other hand, the temperature in the
lower parts of the flood basalts in the Tertiary
districts appears to be relatively high. Tempera-
ture measurements in both shallow and deep
holes (Bodvarsson and Palmason, 1961) indi-
cate a temperature of 100° to 150° C at the
bottom of the flood basalts. The lower figúre
applies to the conditions in the northern
districts, where the temperature gradient is
relatively low but the flood basalts relatively
thick.
Some non-thermal boreholes in the western
regions indicate higher temperature gradients.
This may be due to local anomalies, partially a
very rapid erosion during the Pleistocene (Bod-
varsson, 1957).
The temperature conditions in the Tertiary
districts imply that water circulating to the
lower parts of the flood basalts can be heated to
a temperature of 100° C or more. Water per-
colating down through dikes and faults can
be heated by a contact with the hot rock and
ascend again, either by hydrostatic head or by
convective movement.
This heat supply is probably of a transient
nature. The rock adjacent to the channels of
flow is gradually cooled and the head supply
decreases.
In fact, it appears difficult to account
for the heat supply of the largest low-tempera-
ture springs on the basis of a stationary supply.
For example, the large spring at Deildartunga
in Borgarfjordur issues about 250 litres per
second of water at 100° C corresponding to a
heat output (above 4° C) of 2.4xl07 cal/sec.
By a stationary outflow of terrestial lieat of the
order of 2 to 3 microcal/cm2sec, the spring
would theoretically liave to drain the heat supp-
ly through an area of some 1,000 square kilo-
rneters. Actually, the heat take-up is incomplete
and the area drained has to be considerably
greater. This appears unreasonable, especially
in view of the fact that there is a number of
other springs in the vicinity of the Deildartunga
spring.
(d) The base temperature.
Theoretical investigations (Boclvarsson, 1951)
and temperature measurements in boreholes
(Bodvarsson and Palmason, 1961) indicate that
a fairly uniform temperature generally prevails
through a greater part of the upstream zones
of the hydrothermal systems. This tempera-
ture thould approximately be equal to the
temperature which the water has acquired at
the circulation base, that is, equal to the base
temperature. A decrease in temperature is
appreciable only in the uppermost 1,000 metres.
The base temperature is consequently a fig-
ure of fundamental importance. It affects both
the physical and the chemical conditions in
the thermal areas. The technique of measuring
or inferring the base temperature is discussed
elsewhere. (Bodvarsson and Palmason, 1961).
The above classification of the thermal areas
into two groups, that is, the low-temperature
and high-temperature groups, respectively, is
preferably carried out on the basis of the base
temperature. The present author has adopted a
base temperature of 150° C as the upper limit
for the low-temperature group.
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