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as a block an important feature of geological development of Ice-
land during the Neogene and Quaternary was the concentration of
volcanic and tectonic activity in more and more narrow bands.'
Starting in the Paleocene on a vast area, much larger than the
limits of the present island, the endogenous activity finally con-
centrated in bands several tens of kilometers wide.
This peculiarity of developments is similar to the events observed
in the zones of continental rifts. For example, in East Africa in the
region of the future Gregory Rift volcanic effusions initially (in
the Miocene) covered very wide areas, while afterwards with the
formation of the major graben (in the Pliocene and Pleistocene)
they were concentrated within its limits. Similarly, manifestations
of tectonic block dislocations also occurred with time in still nar-
rower zones. Likewise, the character of volcanism in East Africa
also changed: areal effusions on large territories were replaced in
later periods by eruptions of the central type also along separate
narrow fissures (25).
An important turning-point in the development of Iceland was
the fracturing and beginning of block tectonics in the Inner zone
in the Pleistocene.
In the early Holocene, in connection with the lifting of the gla-
cial load, a very rapid glacio-isostatic uplifting of the whole island
took place, which now thus forms a gentle dome in the relief.
The role of tension in the tectonic developmenl of Iceland
There are quite a number of structures in Iceland which provide
evidence on the tension stresses in the Earth’s crust. Some of these
structures are connected with local conditions. For example, the
faults on the crest of the Borgarnes anticline are obviously con-
nected with the tension of layers in the axial zone of the fold, the
formation of which took place in conditions of local bending of the
crust by the vertical pressure from beneath. The same is fhe origin
of faults on the roofs of some other anticlines.
But there are structures in Iceland whioh may refer to tensions
on a larger scale and not of local but regional importance. For
example, the total manifestations of normal faults in the rift zones
indicate general tension.