Greinar (Vísindafélag Íslendinga) - 01.01.1976, Page 123

121
vík. Approximately the same line has earlier, and for other reasons,
been indicated as a probable SE-NW fault (6), cf. Fig. 1.
The Reykjanes part of the Median Zone is only ahout 10 km
broad and presents mainly very recent activity, whereas the Thing-
vellir section is about 45 km broad in the south, and activity in it
can he traced much farther hack into Pleistocene time, at least
about one million years, not to mention the flanks of Lower Pleisto-
cene and Tertiary plateau basalts exposed in the area.
This marked difference between the two Sections most probably
means that in the southem one the older rocks have largely sub-
sided, and are buried under younger ones; recent drilling by the
National Energy Authority proves this in case of the outer parts of
the peninsula (8).
A) The Reykjanes peninsula is largely covered by postglacial
lavas hut these are in the main outside our present scope. In addi-
tion, there are nxunerous late-Pleistocene eruption centers and-lines,
mainly seen in elevated, i.e. uplifted blocks and ridges; the latter
consist of successions of hlocks. The youthfulness of the volcanoes
is evidenced by the fact that the craters are mostly still intact. Such
craters form, for instance, the summit of Fagradalsfjall, Geitahlíd,
the SW end of the high (703 m) Bláfjöll ridge, and several craters
cover the ridge SW of Vífilsfell. Erosion of these volcanoes is on
the whole insignificant. Their lavas form caps of the blocks but not
flows down their slopes.
These volcanoes clearly started to form under water. Their lower
parts, up to about 200 m visible thickness, consist of tuff-hreccias
(pillows and block-jointed basalt patches included) up to a clear
horizontal level where the tuff-breccias change within few metres
or less into horizontal lavas which certainly were formed in the
same emption. Often the tuff-breccias have dipping stratification
(25-30°), suggesting foreset heds. In this thin transition zone one
finds at various places clearly erosionally worn material which, in
keeping with the foreset bedding, shows that these exposures were
temporary shores during the eruption (For a more general treat-
ment of the quite similarly built table mountains in Iceland, see
ref. 9, pp. 58-63, and 104—115).
In such a transition zone at 230 m above sea-level, as seen in an
E-going gully 1 km south of Gullbringa at the SE-corner of Kleifar-