Greinar (Vísindafélag Íslendinga) - 01.01.1976, Qupperneq 133

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active in the late phase of our A- and B-sections of the Median
Zone, as well as in the volcanism of our present area, we see the
manifestations of variations in this stress field. The field has been
active over a long time, cf. the arrangement of tertiary dykes
(NE to N), the main coastal faults, and the valleys, but its mani-
festations have been of variable intensity and, alternately, of
tectonic and volcanic character. When we take into account that
the maximum shear is half the difference of the two (here hori-
zontal compressive) principal stresses, Tmax — /4(oi-a2), we see
that the shear magnitude is not only very sensitive to even rela-
tively small variations in the principal stresses, Fig. 6 shows, more-
over, how such variations might even ohange right-lateral shear
on a certain fracture into left-lateral shear. It is then hardly sur-
prising when manifestations which are due to shear, change both
in intensity and character. We cannot here go deeper into the
theory of stresses and their effects but a few remarks and indica-
tions accompany Fig. 6.
Finally, the reason why the valley volcanoes consist nearly ex-
clusively of tuff-breccias, seems easy to explain: the river water,
and very soon the lake water, would penetrate from the beginn-
ing into the orifice and make the eruption explosive. Towards the
end, the eruption might become effusive, when the funnel had
become sealed off from the water, and we see this in the lava cap
of Hvalfell (cf. also 2, Fig. 28).
c) The large “Dolerite” Shield-volcanoes.
To the west of Thingvellir Lake lies a wide expanse of inter-
glacial lavas, the Mosfellsheidi shield, reaching the coast between
Esja and Hafnarfjördur. The main center of the shield, Borgar-
hólar (410 m), lies 10 km west of the recent Thingvellir axis,
and so does another center, Lyklafell farther SW. A small shield
center, Hædir, lies closer to the lake. Neither at Borgarhólar nor
at Hædir is there a trace of a crater or a plug, but Lyklafell is
clearly the ruin of a volcano in an advanced state of destruction.
Sections here reveal the shieldvolcano-type of lavas. The latter are