Jökull - 01.12.1957, Blaðsíða 11

total erosion is the result of the second half
of the Pleistocene. Furthermore, that the remn-
ants of the peneplaine have been lowered,
that is, eroded by a certain amount.
In hypothesis (C) it is assumed that the
total erosion assumed in hypothesis (B) is the
result of the fourth glacial stage only. More-
over, that no erosion occurred during the first
three stages. This possibility is by no means
excluded in the case of the locations of wells
(2) and (3). In the case of the location of well
(1) it is probably a somewhat extreme hypo-
thesis.
The peneplaination during the Pliocene will
have to be taken into account in addition to
the Quaternary events.
Data on the total amount of erosion can be
obtained from the topographical map, that is,
by comparing the elevation of the well loca-
tions to the elevation of the remnants of the
peneplaine. The estimates selected are given
in Tables (II), (III) and (IV). These data are
of course only approximate.
The dating of the various glacial stages
during the Pleistocene is still rather uncertain.
A new approach to this problem has been
presented by Emiliani (11). His results will be
applied as the general basis in the following.
The total length of the Pleistocene is estimat-
ed at 600,000 years. The end of the glaciation
of Iceland is assumed at 12,000 y. b.p. in ac-
cordance with recent radiocarbon dating (Thor-
arinsson, 12).
The Pleistocene is currently believed to in-
clude four major glacial stages. The first stage
covers the entire first half of the Pleistocene
and is probably not a single stage but a series
of substages. The dating of the individual sub-
stages is impossible and it will therefore be as-
sumed that they are approximately equal to a
single uninterrupted stage. The middle of this
stage will be assumed to have occurred at 450,000
y. b. p. Furthermore, in hypothesis (A) it will be
assumed that the erosion during the first stage
equals the half of the total erosion during the
Pleistocene.
In hypothesis (B), on the other hand, it is
assumed that the peneplaine was at a low
level during the first half of the Pleistocene.
No major erosion should thus have taken place
during the first stage.
The second and thircl glacial stages were
according to Emiliani (11) rather short. They
will therefore be combined in one stage the
middle of which will be dated back to 150,000
y. b. p. Hypotheses (A) and (B) assume that the
erosion during this combined stage equals the
half of the total erosion during the second
half of Pleistocene.
The fourth glacial stage is assumed to have
lasted from 80,000 to 12,000 y. b.p. The erosion
during this stage is in hypotheses (A) and (B)
assumed to equal the other half of the total
erosion during the second lialf of the Plei-
stocene.
The peneplaination during the latter part
of the Pliocene will have to be taken into
account. The data on this process are of
course very uncertain. However, the influence
of the peneplaination on the present tempera-
ture field is probably small and we may conse-
quently rely on rough estimates. It will be
assumed that an average of 1,500 metres were
eroded away during the period from 5 ■ 10°
to 10e y. b.p.
A relatively unimportant correction will be
made for the climatic change at the end of the
Pleistocene 12,000 y. b.p. In accordance with
the results of Birch (13) this correction will be
assumed to amount to the addition of 3°C/km
to the observed temperature gradient.
Finally, the post-Glacial sedimentation at
well (I) will have to be taken into account.
The piling up of 50 metres of sediments
during 12,000 years induces a lowering of the
observed temperature gradient. This can be
computed on the basis of equation (9) where
r — — 50 metres is to be inserted.
The data on the erosion and the results of
the final computation of its influence on the
temperature conditions in the three wells are
given in the following Tables (II), (III) and
(IV).
The correction for the climatic change and
sedimentation raise the average heat flow given
in Table (I) from 4.7 to 5.0 microcal/cm2 sec.
Erosion according to the hypotheses (A), (B)
and (C) accounts for 28%, 40% and 48% re-
spectively of the latter figure.
The results given in Table (II), (III) and
(IV) indicate that the outflow of heat by con-
duction is considerably larger in Iceland than
in normal areas, even after corrections for
various surface effects. The reduced lieat flow
9