Jökull - 01.12.1994, Page 44

INTRODU CTION
The main objectives of the present study are to in-
vestigate the grain shape of basaltic hydromagmatic
and magmatic tephra from the Reykjanes volcanic
system in Iceland, and to test hypotheses conceming
the relationship between shape parameters of the
tephra and magma properties and environmental fac-
tors controlling eruptive mechanisms and eruption
types during eruption. In addition, the suitability of
two-dimensional parameters derived from computer-
ized image analysis of tephra grains is tested as a
means of differentiating between various types of
tephra populations. The project was instigated by the
need for quantification and automation of the study of
tephra grain shape.
Volcanism in Iceland is related to spreading at the
plate boundary between the North-American and
Eurasian plates. In South-Iceland it is mainly confined
to two volcanic zones, the Eastern Volcanic Zone
(EVZ) and the Westem Volcanic Zone (WVZ) but in
North-Iceland it runs along the Northern Volcanic
Zone (NVZ) from Vatnajökull to Öxarfjörður, where it
becomes shifted westward along the Tjömes Fracture
Zone (TFZ) on the North-Icelandic shelf (Fig. 1).
Tephra is commonly divided into two groups:
magmatic, and hydromagmatic tephra. The magmatic
one is formed through the expansion of escaping
gases in the vent or in lava fountains, while the hydro-
magmatic tephra is generated by the disintegration of
steam explosions upon contact with water, either sea-
water, glacial melt-water, or ground-water. Basaltic
tephra that is formed in magmatic eruptions is gener-
ally subdivided into hawaiian and strombolian tephra,
the former consists mainly of lava spatter of limited
distribution, the latter consists mainly of scoria pro-
duced in slightly more explosive eruptions.
The shape of tephra grains is related to magma
properties and environmental factors during emption.
The original shape properties of fresh tephra may be
modified during subsequent weathering, erosion, and
transport leading to redeposition of volcaniclastics in
sedimentary basins. The study of the grain shape of
tephra units formed in recent eruptions where the phys-
ical environment of the eruption is well documented is
of fundamental importance for volcanologists wishing
to interpret the origin of ancient deposits. It is also im-
portant for sedimentological studies of basin rocks
formed in plate margin tectonic settings, to obtain ref-
erence values from presently active volcanic areas for
Fig. 1 Geological map of Iceland
showing bedrock stratigraphy and
major volcanic and tectonic
zones.
1. mynd. Jarðfrœðikort afíslandi.
Sýndur er aldur berggrunns og
afstaða helstu gosbelta og
skjálftabelta.
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JÖKULL, No. 44