Rit (Vísindafélag Íslendinga) - 01.06.1967, Síða 139

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land in the period 1938—65 (Gerke 1967). On the other hand
the modern earthquake cracks show that whenever cracking
takes place, the cause is fundamentally the same.
Whether a progressive movement along the principal frac-
tures has taken place in late geologic times is still unsettled.
The valleys in the neo-volcanic zone have the same direction
as the main fractures and setting off of river courses is there-
fore not to he expected; at any rate I have seen no clear
evidence of large-scale movement.
Volcanism. The principal direction in middle southem
Iceland is about 45° east of north (Laki crater chain 44°,
Eldgjá 38—40°, Heljargjá graben 46°). The S-direction would
be nearly 60°. This is the direction of the 5 km long vol-
canic Hekla fracture and of the steep wall of Grímsfjall at
the foot of which the Grímsvötn volcano is situated. This
suggests that these great periodic volcanoes are situated on
exceptionally large secondary fractures at the boundary of
the main volcanic zone. In fissure eruptions the lava will
mostly cover the fissures, and exclude observation of their
directions, hut it seems worth while to inquire into the pos-
sible role of secondary fractures as lava feeders.
Magnetie anomalies. The magnetic anomalies found over
the Reykjanes Ridge (Heirtzler et al. 1966) and Iceland
(Serson et al. 1966) are seen to be connected with the frac-
ture system in Iceland. The anomaly picture found by Heirt-
zler et al. shows a straight central axis, the A magnetic zone.
It is about 40 km broad, and a magnetic body some 20 km
broad is inferred as a cause. About 80 km to each side are
the B magnetic zones, termed here Bw (western) and BE (east-
ern). A further 40 km away from A are the C magnetic
zones.
A little northeast of the area covered by the work of Heirt-
zler et al. one sees from the measurements of Serson et al.
(1966), part of which is shown in Fig. 10, that the A zone
makes a few small jumps eastwards, until it reaches
Reykjanes. Here comes a major jump, which is geologically