Greinar (Vísindafélag Íslendinga) - 01.01.1976, Blaðsíða 29

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tral volcanoes, and iii) dyke swarms. Those due to central vol-
canoes are the most noticeable at low altitudes and are generally
sharply bounded. In the neovolcanic zone, subglacially formed
volcanoes are sometimes highly magnetic (1).
Although the central volcanoes in Iceland have only been partly
mapped yet and no precise definition or classification of these has
appeared in the literature, the term has been used for oval areas
of any age and several km extent (see (3)) that display some of
these characteristics:
A caldera; a swarm of dykes or fissures; acid or intermediate
rock outcrops; gabbro or diabase intrusions; seismic traveltime,
magnetic or gravity anomalies; evidence of high-temperature
geothermal activity; thick series of basalt lava “flow units”; vari-
able dip of strata; cone sheets or other intrusive sheets; plugs.
In fact, distinct gravity or magnetic anomalies are almost in-
variably present. These are generally positive (1,4; unpubhshed
aeromagnetic data from Mid-Western Iceland hy Th. Sigurgeirs-
son). It is estimated hy the author that these anomahes wi 11 gener-
ally allow identification of such a volcanic center, even after ero-
sion of several hundred meters thickness, or alternatively after its
being buried by several hundred meters of Tertiary flood basalts.
The presence of a coarse-grained mafic rock suoh as gabbro will
bias both the gravity and magnetic anomalies of these centers to-
wards positive values, whereas occurrences of acid rock or high
temperature hydrothermal alteration will tend to cause negative
anomalies. In magnetics, this is due to the high susceptibility of
gabbro (5) in contrast to acid rocks, 30 samples of which from
Iceland were found by the author to have a mean NRM intensity of
0.510-3 Gauss and a still lower induced magnetization (0.25T0"*G).
In some cases, dykes, or sheets, or caldera-filling formations, may
contribute to the geophysical anomalies (6).
2. REDUCTION AND PRESENTATION OF MAGNETIC DATA
A Varian V-75 marine proton magnetometer was used, with the
prohe towed 150-200 m behind the ship. Measurements were made
every three seconds, the output being recorded on analog chart with