Rit (Vísindafélag Íslendinga) - 01.06.1967, Blaðsíða 120

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tions undisturbed by water movement are given in Table
II. Heat flow values are computed on the assumption that
the thermal conductivity is 0.0045 cal/cm sec°C. The high
values are from southwestern and western Iceland, but those
from other parts of the country are all between one and two
times the global average of 1.5X10-6 cal/cm2 sec.
The Vestman Islands borehole is of particular importance
because of its depth, 1565 meters, and distance from thermal
areas. It is located on the seaward continuation of the east-
ern branch of the neovolcanic zone in southem Iceland.
In the upper 900 meters, the hole encounters mainly sedi-
ments, but below that it reaches basalts, which are consi-
dered to be of Tertiary age. The temperature-depth curve
(Fig. 3) in the upper part of the hole indicates slow convec-
tion in the sediments, but below about 1000 meters the gra-
dient reaches a fairly constant value of 65°C/km. Using the
conductivity of 0.0045 cal/cm sec°C, which has been mea-
sured on samples from the hole (Table I), one obtains a heat
flow of 2.9X10‘c cal/cm2 sec. Although rapid sedimentation
may have had some effect on this value, it is probably the
best available value on regional heat flow in Iceland.
More boreholes have been drilled in southwest Iceland
than in other parts of the country. Fig. 2 shows the loca-
tion of those which are of interest for the present discus-
sion. Surface temperature gradient at 0—100 meters depth
is also given. Very high gradient is found west and north-
west of Reykjavík as well as in the Hlídardalur borehole.
The Reykjavík thermal area, from which about 5X106 m3 of
water are pumped annually by the Reykjavík Municipal
District Heating Service, is shown dotted. In this low-tem-
perature area, surface gradients as high as 500°C/km
have been found to be due to water ascending to 200
—300 meters depth from deeper formations. To the east of
Reykjavik is the western boundary of the neovolcanic zone,
which is characterized by recent faulting and open fissures
running in SW-NE direction. Many boreholes which have