Jökull - 01.12.1994, Síða 48

Icelandic annals. The R-9 unit was sampled for shape
analysis in a soil section c. 8 km from the eruption site.
Heimaey
The 1973 Heimaey eruption took place on a 1500
- 2000 m long volcanic fissure across the island of
Heimaey on the South Icelandic shelf (Fig. 1). The
eruption was characterized by lava fountaining along
the fissure, but activity was soon confined to a single
crater that was built up on the centre of the fissure
(Thorarinsson et al., 1973). A detailed account of the
eruption was published by Einarsson (Einarsson,
1974), and Self et. al. (1974) have described the
course of events, and the type and distribution of
tephra during the first month of the eruption. Accord-
ing to them the eruption was a typical strombolian
eruption, and the tephra was classified as strombolian
on basis of grain size characteristics. Grain size pa-
rameters and isopach maps of the Heimaey tephra
were measured and compiled by Egilson (1974).
Laki
The Laki volcanic fissure (Fig. 1) is an about 40 km
long crater row belonging to the Grímsvötn volcanic
system on the EVZ. The 1783 Skaftáreldar eruption
produced one of the most voluminous post-glacial lava
flows anywhere (14.7 km3). The tephra production was
estimated 0.75 km3 (0.4 km3 dry rock equivalent) by
Thórdarson (1991). According to his studies the Laki
tephra may be split into six units, four of which are
strombolian (S1 - S4) and two phreatomagmatic (P1 -
P2) ones, the latter being related to tuff cones on the
crater row. At the base of each strombolian unit there is
a thin band of fine grained phreatomagmatic tephra.
All the phreatomagmatic Laki tephra was produced in
explosions due to contact with groundwater according
to Thórdarson (1991), and he concluded that the activ-
ity had evolved from a predominantly phreatomagmat-
ic phase (but with some strombolian affinity), into a
predominantly strombolian phase characterized by
lava fountains (but with some phreatomagmatic affini-
ty persisting). Units S3 and P1 were sampled for the
present study of grain shape.
Surtsey
The island of Surtsey (Fig. 1) was formed in a sub-
marine eruption 30 km off the south coast of Iceland
on the South Iceland shelf during the years 1963-
1967. The course of the eruption has been described
by Thorarinsson et al. (1964), Thórarinsson (1966)
and Einarsson (1965). Hydrovolcanic explosions took
place when the magma came into contact with sea-
water. Two adjacent tuff cones were built up above
sea level. Lorenz (1974) concluded that the bulk of the
tuff cones were built up in water-saturated base surge
explosions. This was based on the presence of porous
layers of vesicular tuffs, pisolites, and rippled layers
in the crater rims. Grain size analyses of the Surtsey
tephra show that it is generally poorly sorted and fine
skewed (Sheridan, 1972). After the craters were built
up above sea level and the sea ceased to have access to
the vents the explosive activity was replaced by lava
fountaining and the island was partly covered by lava.
Both air fall tephra (collected mainly on ships during
the eruption) and base surge deposits from the crater
rims were analyzed in the present study.
Fig. 6 SEM images of tephra particles from Laki, Surtsey,
and Heimaey. Sample numbers refer to Table 1. Length of
scale bar 0.1 mm.
6. mynd. Rafeindasmásjármyndir af gjóskukornum frá Laka,
Surtsey og Heimaey. Númerin vísa til töflu 1. Lengdarkvarði
0,1 mm.
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JÖKULL, No. 44