Jökull

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Jökull - 01.12.1994, Qupperneq 48

Jökull - 01.12.1994, Qupperneq 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. 46 JÖKULL, No. 44
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