Jökull - 01.12.1961, Síða 35

garded as hydrothermal systems, each related to
certain relatively large geological structures.
The geographical distribution of the major
low-temperature systems is given in FIGURE II
and some data are given in TABLE II. The
magnitude applies in each case to the system as
a whole. Moreover, the table contains informat-
ion on temperature conditions and drilling per-
formed on the individual systems. The subsur-
face temperature is characterized by data on the
base temperature in the individual thermal
areas. This concept will be discussed below.
(d) The high-temperature aclivity.
There are 13 high-temperature areas, all in the
Neo-Volcanic Zone, as shown in FIGURE II.
These areas are quite extensive and can prob-
ably be regarded as indvidual hydrothermal
systems. They are characterized by a great numb-
er of steam holes, large areas of hot ground and
a very high degree of thermal metamorphism.
The total heat ouput of all high-temperature
areas has been very roughly estimated at the
order of 10° cal/sec. Data on the individual high-
temperature areas are given in TABLE III.
4. THE MAIN CHARACTERISTICS OF
THE HYDROTHERMAL SYSTEMS.
(a) Basis concepts.
The circulation and heating of water in geo-
logic bodies depends on the structural control and
the nature of the heat source. Further itemsofin-
terest are the depth of the circulation base and
the temperature of the water as it enters the up-
ward movement. These two concepts will be
called the base depth and the base temperature,
respectively.
Moreover, the total amount of water and heat
transported through individual systems is of in-
terest. Also, the total amount of surplus heat
accumulated in the upstream and discharge zone.
A brief discussion of these concepts and topics
follows.
(b) Structural control.
The seismic results given in FIGURE 1 indi-
cate a relatively sharp velocity centrast between
the flood basalt A and the underlying formation
D. In the South-West the P-velocity jumps from
4 to 6 km/sec. The lower formation appears to
be a dense and probably an impermeable for
mation. It is therefore probable that the circu-
lation of the water is confined to the flood ba-
salts and that the base depth in the South-West
in general does not exceed 2 km.
Conditions in the North appear to be com-
plicated by the presence of the layer C which
is of an unknown composition and permeabili-
ty. The maximum base depth may here exceed
3 km.
In general, flood basalts may be permeable
due to (1) tubes and openings at the contacts of
lava beds, (2) columnar structure in and fissures
along the walls of instrusive bodies, and (3) re-
cent faults.
In the literature a considerable emphasis has
been placed on the importance of faults. In the
case of the low-temperature activity in Iceland
the evidence appears to be somewhat on the
contrary. Contacts between lava flows as well as
dikes and sills appear to furnish the main per-
meability. The hot springs in the North are in
a great number of cases controlled by dikes. In
the South-West the low-temperature activity ap-
pears to be largely controllecl by outcropping
contacts.
The high-temperature areas, on the other
hand, appear to be controlled by faults and
fissures of relatively recent age. Many of the
structures may have been formed or reopened
in very recent times.
It is possible that the isostatic upwarping
of Iceland at the end of the Pleistocene may
have influenced the general permeability condi-
tions in the floocl basalts. The stresses which
were induced by differential movement of the
individual blocks may have formed new faults,
and also reopened old passages in faults and
along dikes.
(c) The heat source.
The extensive volcanic activity in Iceland
immediately suggests a connection between the
thermal activity and the volcanism. It is evi-
dent that recent volcanic processes can in-
fluence the subsurface temperature field and
build up the heat sources of thermal activity.
This appears to be the case as far as. the high-
temperature activity is concerned. All areas of
this type are located in regions of post-Glacial
and in some cases very recent volcanism. The
fissures controlling the areas are closely conn-
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