Greinar (Vísindafélag Íslendinga) - 01.01.1977, Blaðsíða 72

70
direction within individual lavas in a lava succession, while
there are clear jumps from one lava to the other. A further
test is the identity of the direction of magnetization of a bak-
ed sediment and the lava which caused the baking. It must
thus be tested from case to case whether the original direction
has been retained or not through the processes of regional
hydrothermal alteration. It seems possible that the direction
might change greatly if the region was under stresses other
than load during the hydrothermal alteration. This would be
of interest in the case of the Deccan traps, as during burial
they might have been affected by stresses connected with the
Alpine orogeny. Further study of the magnetization of the
Deccan traps seems called for.
By the great many irregularities in Cretaceous and older
paleomagnetic poles, not least in such cases which demand the
reference to continental drift, we prefer to think that by deep
burial the direction of the rock magnetization is usually chang-
ed, until evidences to the contrary have been given.
Linear magnetic anomálies on the ocean floor. In connection
with changes of original magnetization, we shall mention briefly
the ocean floor anomalies. In an earlier paper (39) we have
rejected the usual explanation of these anomalies, on grounds
which are still valid. Furthermore, in Part I of these studies
(15), we have demonstrated the absence of spreading in the
Median Active Zone of Iceland for about the last one million
years. By the figure for spreading in this zone, 2 cm/year, as
it is endlessly repeated in the geoscientific literature, the ob-
servable spreading ought to have been around 20 km. This
absence of spreading shows that the original, and largely still
retained explanation of the linear magnetic anomalies along
the crestal zone of the Reykjanes ridge is wrong.
We propose to approach the question of the origin of the sea-
floor magnetic anomalies in a different way.
The seismicity on the approximately 1500 km broad sub-
marine ridge systems is due to strike-slip and dip-slip fractur-