Jökull - 01.12.1957, Blaðsíða 13

that wood may be transformed in the labora-
tory into substances resembling low-rank coal
if it is heated to 300° C in closed tubes.
Furthermore, bituminous coals decompose and
yield hydrocarbons at temperatures from 300° C
to 350° C.
Great caution is needed in the application
of such data in the study of geochemical pro-
cesses. However, as time is an important factor
it would appear likely that the natural pro-
cesses could proceed at temperatures below the
laboratory values. With regard to this it
appears unlikely that the lignites could have
been subjected to 400° C for any appreciable
length of time, and certainly less likely that
the temperature could have exceeded 500° C
during millions of years.
Subsurface temperatures in Iceland as in-
dicated by the post-Glacial upwarping. Einars-
son (16) concludes on the basis of studies of
Áskelsson (17) and Bárdarson (18) that the
post-Glacial upwarping of Iceland lagged a
few thousand years after the deglaciation.
Marine shells found in ancient elevated shore
lines indicate a relatively high temperature of
the ocean at a time before the beginning of
the upwarping. The conclusion appears inevit-
able that the deglaciation had taken place be-
fore the main upwarping started. The average
viscosity of the substratum may be estimated
on this basis. Einarsson (16) finds figures which
appear to preclude the existence of any larger
molten horizons in the substratum. In general,
the temperature of the substratum must, there-
fore, be below the melting point.
Volume of instrusions in the Tertiary basalts.
The Tertiary basalt plateau of Iceland appears
to ltave been built up mainly by fissure erup-
tions. Typical historic events of this type are
the large eruptions of the Laki in 1783 and
the Eldgjá around 950. The former fissure
erupted a total of 12 km3 of lava frorn a
length of 25 km. The Eldgjá erupted 9 km3
from a length of 30 km. The average output
of both fissures is 0.4 km3 of lava per km of
length. These two eruptions are responsible
for a considerable part of the lava formecl
during historic time in Iceland.
The erupting fissures are connected to feeder
dikes which extend down to the sources of
the magma. These dikes are very numerous in
the basalt plateau. Walker (14) has studied
the distribution of dikes in the Reydarfjörd-
ur area and concluded that 4% of the 43 km
long section consists of dikes which have an
average width of 3 metres. There are indiaca-
tions that the ratio may be as high as 6% in
the lowest part of the section.
This compares well with the data furnished
by the Laki and the Eldgjá. If the feeder
dikes have a width of 3 metres, the ratio
between the volume output per km of the
fissure and the volume of 1 km of depth of
the dikes is 133. Estimating the minimum total
thickness of the Tertiary lavas at 8 km we
find that the lowest sections should contain
6% by volume of dikes.
These figures include the dikes only. There
are other forms of instrusions as sills and
volcanic necks. Their integrated volume ap-
pears less tlian that of the dikes. We may,
therefore, conclude that the integrated volume
of intrusives in the lower basalt plateau is
probably not in excess of 10% of the country
rock and that most of it is occupied by thin
dikes which are scattered around.
Einarsson (16) has carried out a fairly com-
prehensive survey of gravity in Iceland. Mainly
the south-western parts have been studied in
detail. The average density of a section of the
basalt plateau is found to be 2.7. Intrusives,
on the other hand, may have a density of 2.9
to 3.0. The density contrast may be larger in
locations of heavy alteration. Large intrusives
should, therefore, lead to substantial gravity
anomalies or about 10 mgal per km in thick-
ness.
A nurnber of intrusives are revealed by the
gravity survey. What is of interest in the
present study is that there are apparently no
major intrusives at the locations of well (1)
and (3). On the other hand, well (2) is located
at the northern end of an intrusive which
appears to have the dimensions of 12 by 3 km
and a thickness less than one km. Parts of the
intrusive outcrop.
Local volcanism in the vicinity of the ivells.
All three wells listed in Table (I) have been
drilled into Tertiary basalts. There are no
signs of post-Glacial or late Quaternary volcan-
ism in the close vicinity of the wells. The
distance to the nearest centra of post-Glacial
activity are 25, 25 and 50 km in the case of
well (1), (2) ancl (3) respectively.
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