Greinar (Vísindafélag Íslendinga) - 01.01.1976, Side 110

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ably, at that time the middle part of Iceland subsided somewhat
and the plateau-basalts acquired a slightly sloping synclinal dis-
position. During the same period the plateau-basalts on the north-
westem and eastem margins of the island were lifted and subject-
ed to denudation. A peneplain has been developed on their surface.
The third stage began about 5 million years ago, i.e. in the be-
ginning of the Pliocene. The concentration of volcanic activity into
a smaller and smaller area continued. Now it accumulated in the
south of the island in the band about 250 km wide, while in the
north this band divided into two branches; between them was
formed a more stable and uplifted massif of the central-northem
Iceland. The quiet subsidence of the cmst has changed to frag-
mentation and vertical block movements.
Judging by the coastal sea sediments on the Tjörnes peninsula in
the north of Iceland, apparently, the beginning of the Pliocene
should be considered as the time of separation of Iceland as an
island and of formation of sea basins dividing it from Greenland
and the Faroes. That was, obviously the time when the Icelandic
plateau subsided, which according to the results of the marine part
of our Expedition was land before that time or very shallow sea.
Iceland became a fragment, which remained above sea level, of an
earlier much larger land mass.
Finally, about 700 thousand years ago the grabens of the modem
rift zones were formed and volcanic activity concentrated in them.
It completely subordinated to local stmctural conditions. A con-
siderable role now belonged to the effusions of the central type.
The history of rift zones is, apparently, complicated. It is still
not known completely. Perhaps volcanic and tectonic activity in the
Eastern rift zone occurred on a large scale, which now distinguishes
this zone, only after the “dying out” of the northem part of the
Westem rift zone in the Pleistocene. It is also possible that the
Eastern rift zone first originated in the north of Iceland and then
gradually spread out to the south.
During the Pleistocene the whole Inner Zone was broken into
blocks, the vertical displacements of which divided this zone into
grabens and horsts of different orders.
Thus, besides the irregular subsidence of the Earth’s crast of a
previously large land, which resulted in the formation of Iceland