Greinar (Vísindafélag Íslendinga) - 01.01.1976, Side 131

129
a cross section of Botnsdalur through Hvalfell (H:Lr-l). Behind
Hvalfell formed the lake Hvalvatn.
After this consideration of the distances of the volcanoes from
the Thingvellir axis, and the lack of a time-distance rule in that
respect, we come to a further point, which is a significant rule:
the valley volcanoes lie on the axcs of the valleys. These have
typically a NW-SE trend, that is, at nearly a right angle to the
dominating strike of the active lines in the Median Zone. It seems
plausible that the volcanoes under consideration are connected with,
or reveal the existence of fractures of the latter category, but why
did the volcanic manifestation in that case only show up in the
valleys? Two altemative explanations present themselves for con-
sideration: 1) In the valleys the extrusion of the magma would
be easiest from a purely hydrostatic point of view. But this ex-
planation is insufficient when we consider the behaviour of a fis-
sure eruption. In the initial phase the (hydrostatic) magma pres-
sure is at a maximum, enabling the magma to extrude all along
a fissure of even a very considerable length, more or less irrespec-
tive of differences in height of the surface. Only after this phase,
the extmsion is restricted to certain points. Moreover, these are not
necessarily the lowest points in the terrain, but rather the widest
points of the fissure. This was demonstrated in the Heimaey erup-
tion of 1973, where the extrusion was soon confined to the middle
of the originally active fissure, which happened to be about the
highest point in the terrain along the fissure. Thus, even if in our
cases the valley axes had happened to be the widest parts of the
SW-NE striking fissures, the initial phase of the eruptions should
have produced structures elongated in the SW-NE direction, which
is not the case. 2) The other altemative is that the axes of the
valleys indicate old fractures, and that the eruptions took place at
the crossing points of two near-orthogonal sets of fractures. We
accept this alternative, and consider the two sets to be conjugate
shear fractures in a stress field which at times was very extensive,
for both sets occur widely in Iceland, cf. Fig. 1. They are of basic
importance for the understanding of the tectonic and volcanic acti-
vity in the country as a whole (7, 6, 1).
From what has been described and quoted so far, we conclude
that in the en echelon arranged volcanic and tectonic fractures,
9