Rit (Vísindafélag Íslendinga) - 01.06.1967, Blaðsíða 103

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it becomes heated above the Curie-point when the dykes con-
solidate and cool down. The fact that the shortest known
events are detected at all shows that little mixing takes place.
The width of the postglacial volcanic zone in N. Iceland
is about 30 km. Formation of a new magnetized oceanic crust
must take place on a belt which is about an order of magni-
tude narrower.
With the rate of drift in the Iceland region indicated by
the magnetic profiles, the total drift during the Pleistocene
is about 70 km. In Iceland rocks of this age cover considerably
broader areas, which implies that there must be rocks of a
Tertiary age below younger formations.
In the uppermost layers of Icelandic basalts of Tertiary
age, the volume of the dykes is only a small fraction of that
of the lavas (Walker 1959). Eruptions on a comparable scale
at the bottom of the oceans would lead to a considerable
overlapping of rocks from different magnetic epochs, prevent-
ing the detection of short events, even taking into account
that the lava would pile up more closely around the craters.
Normally therefore no large eruptions take place; where this
happens seamounts or ridges, perpendicular to the mid-ocean
ridges are formed.
The new results from the magnetic surveys are somewhat
discouraging for the use of Iceland as a laboratory in the
study of mid-ocean ridges. It appears to differ from them
by excessive basalt extrusions and, perhaps because of this,
eruptions take place on an area larger than the active part
of the ridges. On the other hand, better knowledge about the
geology of the ridges and its difference from Icelandic geo-
logy should be a welcome aid in solving the question why
Iceland is a land and not an ocean floor. The Scotland-Green-
land Ridge should also be kept in mind.
The comparison of theoretical and observed profiles was
made by means of models of simple geometry and a two-
valued magnetization. More numerical work must be carried
out before all information in existing profiles is utilized. Be-
k