Jökull - 01.12.2003, Blaðsíða 30

Thordarson et al.
of tephra produced by the eruption is greater than the
cumulative tephra volume from all four 20 th Cen-
tury Hekla eruptions. The Laki tephra deposit cov-
ered 7200km2 within the 0.5 cm isopach. For com-
parison the area within the 0.5 cm isopach for the
1947 and 1991 Hekla eruptions is ∼2000km 2 and
485km2, respectively (Thorarinsson, 1976, Larsen et
al., 1992). The Laki eruption plumes also dispersed
tephra as far as the Faeroe Islands, Scotland and main-
land Europe, implying that an area of ∼750,000km 2
may have been affected by tephra fall from Laki. In-
formation on the timing and dispersal of tephra falls
provides valuable information on the source and na-
ture of explosive activity during the Laki-Grímsvötn
eruptions. This facet of the eruption has been anal-
ysed in detail by Thordarson and Self (1993) and thus
only the key results are reiterated here.
In the first 3 months of the eruption, the contem-
porary accounts report fifteen occurrences of tephra
fall in association with the Laki-Grímsvötn eruptions.
Eight of the reported occurrences refer to tephra falls
that directly affected the Fire districts and five indi-
cate rain out of tephra in the highlands above the Fire
districts (Figure 5). The last two are reports of tephra
fall elsewhere in Iceland, one near Vestmannaeyjar off
the south coast and another in Skagafjörður, Northern
Iceland (Figure 1, inset).
Eight of the reported tephra falls originated at the
Laki fissures (Figure 5a–f). On five occasions the
tephra was spread over the Fire districts, whereas on
three occasions the tephra fall was confined to the
highlands to the west and north of the fissures. Each
of these tephra falls are derived from different fissure
segment on the vent system and the tephra stratigra-
phy indicates a systematic northeast propagation of
the explosive activity with time. Furthermore, textural
studies of the tephra fall units that make up the proxi-
mal deposit show that at least five of the fall units were
produced by explosive activity of subplinian character
whereas two were formed by phreatomagmatic activ-
ity (Thordarson and Self, 1993).
Sightings of fire fountains on 8 June indicate that
at the onset of the eruption the activity was confined
to the vents on fissure 1 (B8; Figure 3). The tephra
that fell in the Fire districts on the first day was pro-
duced by explosive activity on this fissure (Figure 5a).
The two additional fire fountains noticed on 11 June
indicate a northeast extension of the vent system and
were linked to formation of fissure 2 (B18; Figure
3). The following night explosive activity on these
fissures produced a tephra fall that was dispersed to
the west-southwest toward the Mýrdalsjökull glacier
(Figure 5a). The tephra that fell in the Fire districts on
14 June contained noticeably more of Pele’s hair than
previous falls (B27–B29) and the tephra stratigraphy
shows that it is derived from fissure 3 (Figure 3 and
5). Judging from the contemporary chronicles each of
these tephra falls lasted only for several hours (B4–
5, B29), indicating that the subplinian phases were
short-lived; peaking abruptly and followed by rapid
and steady decline in explosive intensity. Contempo-
rary observations show that in the period June to Au-
gust the eruption columns above the Laki fissures rose
to heights greater than 8.5 km (B71). These observa-
tions are compatible with model calculations, which
indicate that the subplinian eruption columns were at
least 11-13km high and that the principal tephra fall-
outs occurred from high altitude plumes. The vigour
of the explosive activity is verified by the abundance
of Pele’s hair in these tephra falls (e.g., B3–4, B27–
29), because its presence implies very high eruption
velocities in gas-charged fire fountains (Shimozuru,
1994). This evidence, along with extremely effective
vent degassing, is taken to indicate eruption by high
velocity gas-rich jets that drove the subplinian explo-
sive activity (Thordarson et al., 1996).
Again subplinian tephra fall occurred in the Síða
district on 9 July and continued through the 10 th
(B48–B50). This tephra fall appears to have had a
southeasterly dispersal because it did not reach the
Meðalland district (Figure 5d). The proximal tephra
stratigraphy shows that at this time all of the fissure
segments southwest of Mt. Laki erupted explosively
and that a significant amount of tephra was dispersed
to the north of the fissures. This dispersal direction is
consistent with the indicated weather pattern for 11–
12 July on reconstructed synoptic weather maps for
the North Atlantic region (Kington, 1988). Thus, the
explosive activity most likely continued through 11
and 12 July.
28 JÖKULL No. 53, 2003