Jökull - 01.12.1961, Side 33

metamorphism. Various invgstigations have
indicated that the two types differ mainly in
the subsurface temperature and that most other
factors can probably be regarded as functions of
this quantity.
The hot-water areas are characterized by a rela-
tively low subsurface temperature, in the upper
flood basalts at least. Afost of the areas in the
North-West and North appear to have a tem-
perature below 100° C. Some of the large hot-
water areas in the South-West and the West
have temperatures between 100° C and 150° C.
The large natural-steam areas, on the other
hand, are characterized by subsurface tempera-
tures above 200° C. The maximum measured is
about 230° C.
The thermal areas are preferabiy classified on
the basis of the temperature conditions. The
hot-water areas are classified as the low-tempera-
ture activity and the natural-steam areas as the
high-temperature activity.
As a matter of course, there are no weli-de-
fined limits between the two groups. There is a
number of important borderline cases, as can be
expected. The classification can be made some-
what more quantitative on the basis of the con-
cept of the base temperature, which will be dis-
cussed below.
(b) Magnitude of the therrnal areas.
The heat flow to the surface, that is, the heat
output of the thermal areas, varies within wide
limits. The output of some of the smaliest hot-
water areas is only of the order of 105 cal/sec,
whereas the largest naturai-steam areas have an
output of the order of 10a cal/sec. This great
variation suggests the introduction of a scale of
magnitude in order to classify the areas with re-
garcl to the heat output. On the other hand.
the heat output is a figure which in most cases
is known with little accuracy and is even sorne-
what uncertain by definition. This situation
warrants only a scale with relatively wide steps.
A scale with three steps appears appropriate.
TABLE I.
Magnitude.
I.
II.
III.
Total Heat Output.
5 — 25 x 10° cal/sec
25 - 125
125 - 750
This scale will be applied mainly to systems
of thermal areas, as will be discussed below,
where also the various components of the heat
output will be discussed in some detail.
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