Jökull - 01.11.1998, Page 15

(Þórarinsson, 1951, 1953; Walker, 1992: Jurado-
Chichay, 1993). Furthermore the common occurrence
of lava fragments, both as free clasts and as the core
of armored bombs, demonstrates that the explosive
activity involved disintegration of partially solidified
lava which is typical for rootless vent eruptions
(Þórðarson et al., 1992).
A fairly accurate timing of the rootless vent erup-
tions at Leiðólfsfell can be inferred from the detailed
accounts on the progress of the eruption (Þórðarson
1990, 1991). The Laki eruption began on 8 June and,
as indicated above, the lava had passed the Leiðólfs-
fell area by 24 June. It is also known that ash from the
Laki fissures fell in the Leiðólfsfell area on 12 June
and probably produced the primary strombolian
tephra layer at the base of the Leiðólfsfell scoria. Thus
the rootless vent eruptions at Leiðólfsfell must have
occurred between the 12 and 24 June 1783. A reddish
blaze and "sparks of fire” (glowing blebs of magma)
were seen rising above the moors of Geirlandsheiði on
17 June (Steingrímsson and Olafsson, 1783; Stein-
grímsson, 1788). This was observed at the Prestbakki
farm in the Síða district. The line of sight from Prest-
bakki over Geirlandsheiði points to Leiðólfsfell, which
is located 20 km west of Prestbakki and 15 km south
of the Laki fissures. From this it is evident that the
eruptive activity observed by Steingrímsson on this
day was not activity at the Laki fissures. If we consid-
er the line of sight and the timing of this event it is al-
most certain that Steingrímsson was watching the
rootless vent eruptions that produced the Leiðólfsfell
cones and the associated scoria fall deposit.
Tephra fall deposits produced by rootless vent ex-
plosions are not widely cited in literature, but are not
an unknown phenomena. Þórarinsson (1951, 1953)
identified two locally dispersed scoria fall deposits
that were formed by rootless vent explosions associat-
ed with the Younger (-2300 BP) and the Older (-3500
BP) Laxárhraun lava flows in Northern Iceland. Dur-
ing the ongoing Pu'u'O'o-Kupaianaha eruption at Ki-
lauea, sustained rootless vent explosions occurred
when lava entered the sea. On several occasions this
activity formed small scoria cones and fall which was
dispersed >500 m away from source (Þórðarson and
Self, 1991; Mattox and Mangan, 1997). Other recent
examples of rootless vent explosions are those that oc-
curred within the pyroclastic flow deposits from the
1980 eruption at Mt. St. Helens (Moyer and Swanson,
1987) and the 1991 eruption at Pinatubo (Pinatubo
Volcano Observatory Team, 1991). Thus the Leiðólfs-
fell scoria deposit is by no means a unique occurrence
and it is very likely that other rootless cone groups in
Iceland produced similar fall deposits, although these
have yet to be identified.
REFERENCES
Grönvold K., Oskarsson N„ Johnsen S. J., Clausen H.
B„ Hammer C. U„ Bond G„ and Bard E. 1995. Ex-
press Letter: Ash layer from Iceland in the Green-
land GRIP ice core correlated with oceanic and land
sediments. Earth and Planetary Science Letter 135,
149-155.
Gudmundsson J. (1844). Skaftárgljúfur 1783: Lýsing
Jóns Guðmundssonar (1807-1875) klausturhaldara á
Kirkjubækarklaustri á ferð sinni til eldstöðvanna við
Blæng árið 1842. Handrit í vörslu Landsbókasafn Is-
lands, Lbs. 2045 4to, safn Þorvaldar Thoroddsens
um eldfjöll á íslandi.
Jónsson J. 1978. Eldstöðvar og hraun í Skaftafellsþingi.
Náttúrufrœðingurinn 48,196-232.
Jónsson J. 1985a. Eldstöðin við Leiðólfsfell og sögnin um
Tólfahring. Náttúrufrœðingurinn 55, 73-81.
Jónsson J. 1985b. Rústirnai' við Réttarfell og Leiðólfsfell.
Arbók hins íslenska fornleifafélags 1985, 129-136.
Jónsson J. 1990. Tólfahringur og Leiðólfsfell. Arbók
hins íslenska fornleifafélags 1990, 121-131.
Juardo-Chichay Z. 1993. The Pohue Bay flow, Mauna
Loa, Hawaii: High volumetric flow-rate tube-fedpa-
hoehoe and associated features. MS Thesis, Univer-
sity of Hawaii at Manoa, Honolulu. 117 pp.
Larsen G. 1978. Gjóskulög ínágrenni Kötlu. BS Honors
Thesis, University of Iceland, Reykjavík. 59 pp.
Larsen G. 1979. Um aldur Eldgjárhrauna. Náttúrufrœð-
ingurinn 49, 1-26.
Larsen G. 1984. Recent volcanic history of the Veiðivötn
fissure swarm, southem Iceland - An approach to
volcanic risk. Journal of Volcanology and Geother-
mal Research 22, 33-58.
Larsen G. 1990. The lOth Century Eldgjá-Katla erup-
JÖKULL, No. 46, 1998
13