Jökull - 01.12.1986, Page 19

Fig. 7. Lava shields. The olivine-
tholeiite craters are arranged on
strips like the eruptive fissures (Sig-
urðsson 1985) Mynd 7. Dyngjur. Óli-
vín-þóleiít-gosstöðvarnar eru á rein-
um eins og gosprungurnar (Frey-
steinn Sigurðsson 1985).
Explanations/Skýringar:
E Tablemountains / móbergsstapi.
2- Strip with lava shields / Dyngjurein.
3. Postglaciallavashieds/Nútímadyngja.
4. Older lava shields / Eldri dyngja.
5. Small recent olivine-tholeiite crater / Lítil ól.-þól.
gosstöð frá nútíma.
6. Older olivine-tholeiite crater / eldri ól.-þól. gos-
stöð.
7. Postglacial picrite lava shields / Pikrit gosstöð frá
nútíma.
8. Older picrite volcano / eldri píkrít gosstöð.
degree of confidence to infer the distribution of the
older rocks from the recent, systematic volcanological
Pattern, especially in the western part of the penin-
sula. It should be noted that the hyaloclastite ridges lie
generally west of the corresponding volcanic strips, at
least west of lake Kleifarvatn. The mean distance
between strips and ridges in 0.7 km. If one-sided
sPreading and a spreading rate of 0.02 m/y is as-
sumed, this would correspond to a mean age of
35,000 years for the hyaloclastite ridges. Most of them
should then be from the last glacial period.
There are indeed many signs of glaciation on the
Reykjanes peninsula: glacial striae, glacially scoured
surfaces, tillites and subglacial hyaloclastites. Signs of
■ce-free conditions are not 'lacking either: subaerial
lava flows, sedimentary bects, weathered and irregu-
larly eroded surfaces. A correlation of these features
from one place to another should be possible, al-
though in some cases difficult or even questionable.
The results of such an attempt should indeed be re-
garded with some caution (fig. 8).
There seems to have been a long icefree interval
between the forming of the most eroded hyalo-
clastite ridges and tablemountains and the underlying
basement rocks This probably represents the last
interglacial period. In the younger rocks signs are
found of at least two major glacial events, perhaps re-
presenting stadials of the last glacial period. In these
younger rocks occurrences of reverse magnetic
polarity are known (Kristjánsson & Guðmundsson
1980), that have been tentatively dated as near to
40,000 years old i.e. probably from the last glacial
period. At present it therefore seems most probable
that at least the highest and most prominent hyalo-
clastite ridges are from the last glacial. The underlying
basement seems to be somewhat older, probably from
the second or even third last major glacial period.
The elevation of the basalt caps on the mountains
gives some indication of the elevation of the surface of
the glaciers, at the time of the eruption. Tuff layers,
hanging as a cloak from the top of a mountain down
its slopes, were probably erupted in shallow water or
even subaerially and indicate the elevation of the gla-
cier surface. From such signs the elevation of the gla-
cier surface has been reconstructed for the Fagradals-
fjall-area, or at least a surface related to it (locations,
see fig. 7). One result of this reconstruction is that the
mountains reached only up to 150—200 m asl. (pre-
sent elevation) before the second last glacial period, or
some of its stadials. At that time the glaciers from
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