Jökull - 01.12.1994, Page 45

maturity and provenance analyses.
Volcanism in Iceland ranges from submarine to
subglacial and subaerial eruptions and a considerable
range is also present in the chemical composition from
basaltic up to rhyolitic. Basaltic tephra falls naturally
into two broad groups: magmatic and hydromagmatic
tephra. While magmatic tephra is formed by the expan-
sion of magmatic gases in the volcanic vent and in lava
fountains, hydromagmatic tephra results from frag-
mentation of magma through steam explosions caused
by magma-water interactions (Sheridan & Wohletz,
1983; Wohletz, 1983). The hawaiian and strombolian
categories of subaerial basaltic volcanism were defined
by Walker (1973). According to Walker’s classifica-
tion, Hawaiian eruptions are characterized by a low
level of magma disintegration and small tephra distri-
bution dominated by lava spattering. Strombolian ac-
tivity produces higher volumes of scoria which typical-
ly shows greater dispersal. In the present study, the sub-
aerial activity in the Reykjanes Volcanic System has
Fig. 2 Location map of Reykjanes. Inset figure shows the
configuration of volcanic systems on Reykjanes Peninsula.
2. myncl. Staðháttakort af Reykjanesi. Á innfelldu myndinni
sést dreifing eldstöðvakeifa á Reykjanesskaga.
been of the strombolian-hawaiian type (Sigurgeirsson,
1992). As to the hydromagmatic tephra, a distinction is
made between surtseyan and phreatomagmatic tephra
in accordance with the original definition of these
terms (Walker, 1973; Walker & Croasdale, 1972). Surt-
seyan tephra is produced by explosive basaltic erup-
tions in the sea or in lakes. The definition is based on
the initial phase of the Surtsey eruption in 1963-64. A
phreatomagmatic eruption results when magma disin-
tegrates upon contact with groundwater (Fisher &
Schminke, 1984). It has been shown that the
water/magma ratio has a decisive effect on the explo-
sive power and extent of magma disintegration of hy-
drovolcanic eruptions and the resulting tephra texture
(Sheridan and Wohletz, 1983; Wohletz, 1983). The
explosivity is highest when the water/ magma ratio
amounts to 0.1 - 0.3, resulting in the production of
very fine grained tephra, while a ratio lower than 0.1
results in the production of scoria and lava flows.
In a reconnaissance study of the shape of sand-
sized tephra grains from several Icelandic volcanoes,
Eiríksson and Wigum (1989) found statistically sig-
nificant variation between magmatic and hydromag-
matic eruptive environments and also demonstrated a
correlation between shape and SiCH content. They
employed three-dimensional shape parameters based
on direct measurements of the long, intermediate, and
short axes of the particles. Their results indicated that
a two-dimensional analysis would effectively reveal
the variation in grain shape present in their samples.
GEOLOGY OF THE SAMPLE
LOCALITIES
The Reykjanes volcanic system was selected for
the present study of tephra grain shape because of re-
cent studies of the holocene volcanic history of the
area revealing some new aspects as to the relationship
between craters, tephra layers and craters (Sigurgeirs-
son, 1992). Samples from a few other eruptions in
Iceland were selected for comparison (Table 1). The
Laki, Surtsey, and Heimaey samples are all from units
from the EVZ. All tephra samples are from easily dis-
persed units showing minimal alteration. Most of the
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