Greinar (Vísindafélag Íslendinga) - 01.01.1976, Blaðsíða 125

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vatn, one finds rounded basalt blocks up to 30-A0 cm in diam.
This was almost certainly a temporary shore of the open ocean
during the eruption. At present the exposure is 7 km from the
southem coast of the peninsula. Another example is at about 480 m
height on the NE flanlc of Vífilsfell. Here we find fine gravel and
clean sand (i.e. free of glacial clay) intercalated between the two
lowest lava flows just above the transition zone (9, p. 61). This
material, formed at a temporary shore during the eruption, was
most likely deposited by big waves upon the surface of the lowest
lava flow — in just the same way as occurred during the Surtsey
eruption, as the author and others observed. Here again we seem
to recognize a shore of the open ocean, not just that of a lake. The
transition zone is found in individual blocks and ridges on the
peninsula up to nearly 700 m in Bláfjöll. In these cases we can
then infer uplifts up to such heights.
Moreover, this is not the only way of demonstrating the uplift.
It can be proved directly by tracing the stepwise uplift of blocks
of the “Reykjavík dolerite”, to be described later, from Lækjar-
botnar (100 m) — across Rjúpnadalahraun (200-250 m) — up to
the base of Rauduhnúkar (350-400 m) — to the block (500-550 m)
connecting the Vífilsfell- and Bláfjöll-ridges (this block is the
underlying one in Fig. 2). Finally, there is an uplift of these
dolerite lavas up to the top cover of the Bláfjöll ridge at 650-
660 m just around the latitude 64° N. The Reykjavík dolerite is
much older than the visible activity on the peninsula; its surface,
e.g. in the most uplifted blocks, is heavily eroded whereas the rocks
resting on it are not. (It is also practically certain that the Hafnar-
fjördur dolerites cover the uplifted Langahlíd block.)
These older dolerites are now in a subsided position below the
Thingvellir lake and the subaquatic volcanic tuff-breccia of the
Vífilsfell-ridge extends 200-300 m below the surface of the older
dolerites in the underlying block, cf. Fig. 2. This is clear evidence
of the tectonic formation of basins, and formation of the younger
subaquatic volcanics in such basins. Uplift of blocks and ridges out
of the previously subsided areas followed later.
This is intended to be a very short account of the data collected
by the author on the Reykjanes peninsula. The fundamental active
force, producing the tectonic and volcanic phenomena in the pre-