Jökull - 01.12.1966, Síða 46

recharge area
Fig. I
Sketch of a hydrothermal system
contact with hot rock. Thus, the geothermal
sources are parts of ground-water circulation
systems, often referred to as hydrothermal sys-
tems.
It must be stated that our knowledge of
hydrothermal systems in general is quite limit-
ed. Even systems which have been exploited
for decades are only poorly known. But on
the basis of common sense and some observa-
tional material it appears that these systems
consist of (1) recharge areas, (2) zones of sub-
surface flow, (3) primary source rock at elevat-
ed temperatures, and (4) zones of ascend and
outflow. The source rock is the energy reservoir
of the system. The conjectured flow diagram
of a hydrothermal system is sketched in Fig. 1.
The temperature of the thermal water as it
leaves the primary source rock is a rather im-
portant quantity generallv refered to as the
base temperature of the system. Moreover, the
depth of the circulation base is called the base
depth.
The base temperature of hydrothermal sys-
tems varies from a few tens up to around 300
degrees C. There are good reasons for dividing
the systems into two classes according to the
base temperature. Systems having base tempera-
tures below 150 degrees C are called low-
temperature systems: those exceeding this figure
are called high-temperature systems. A great
200 JÖKULL
number of important high-temperature hydro-
thennal systems have base temperatures rang-
ing from 230 to 260 degrees C. These include
the great systems (1) in Tuscany, Italy, (2) at
Wairakei, New Zealand, ancl (3) at Hengill
some 45 km east of Reykjavik. The Reykjavik
district heating system draws on two low-temp-
erature areas having base temperatures of 140
— 147 and 87 degrees C, respectively.
As yet there is no general agreement on the
ultimate heat source of the primary source rock.
However, available data indicate that shallow
magmatic intrusions must play a major role
in most high-temperature areas. The low-tem-
perature areas, on the other hand, will, in
most. cases, be maintained by heat which has
been supplied by the conduction flow through
the earth’s crust.
The thermal water leaving the primary source
rock transports heat to the country rock sur-
rounding the channels of upflow. This effect
is apparent mainly in the zone of ascent below
the surface thermal areas. The ascending water
heats up a great volume of rock just below the
thermal areas and creates a secondary source
rock which, in some cases, is the heat source
of main practical importance. As a matter of
course, the distinction between a primary and
a secondary source rock is by no means clear.
There are no doubt cases where these are