Rit (Vísindafélag Íslendinga) - 01.06.1984, Side 15

1. GENERAL INTRODUCTION
Oceanic volcanic islands fa.ll into three major groups regarding productiv-
ity, tectonic setting and rock composition:
1) Volcanic arc islands, relatively productive, connected to subduction
zones, and having calc-alkaline to tholeiitic rock suites (Lesser Antilles,
South Sandwich Islands, Aleutian Islands).
2) Major islands or island groups of high productivity, connected to oceanic
ridges or other extremely hot areas, and having tholeiitic rock suites with
subordinate transitional and alkaline rocks (Iceland, Hawaii, Gala-
pagos).
3) Small isolated volcanoes of low magmatic productivity, variably posi-
tioned in relation to oceanic ridges, fracture zones or other global
tectonic elements, having dominantly alkaline and occasionally transi-
tional or tholeiitic rock suites (Gough, Reunion, Easter Island,
Bouvetoya).
Jan Mayen falls into this third group. These small islands have gained
much attention by geologists and petrologists in spite of their insignificance
when compared to world production of volcanic rocks. This interest has
resulted in numerous descriptions of the volcanic geology of these islands
and a number of papers on their petrology and geochemistry have been
published. The petrological or chemical trends of the islands have been
related to their distance as measured normally from oceanic ridges, and the
activity has been explained as resulting from convective plumes of varying
nature. Generally crystal fractionation is claimed to be a major process in
the rock suite evolution.
There is no single theory which explains the variety in the tectonic
settings, productivity, rock suite range and composition, and other features
observed among these third group islands, or their general role in the global
evolution. The reason for this may be that no such general common features
of this type are found among these islands. A more precise knowledge of
various relationships in the case of the individual islands could lead to
patterns allowing the compilation of such a theory. However, such know-
ledge is not yet available and it must be collected on a world wide basis
before the evolution of such a theory is possible. The study of the oceanic
islands is thus a continuing research topic.
The volcanic geology ofjan Mayen has been described and mapped in
various detail by a number of authors, most recently by the present author
(Imsland, 1978a). A redrawn simplified version of the geological maps is