Jökull - 01.11.1998, Síða 13

Fig. 5. The historic eruption chronology for the Leiðólfsfell
area, as inferred from the measured soil profiles. The left
hand panel shows the eruptions from the Katla volcano and
the right hand panel shows eruptions from other volcanoes.
A volcano and an eruption column indicate eruptions from
central volcanoes and the light shaded saw-tooth symbol in-
dicates fissure eruptions outside a central volcano. The root-
less vent eruptions at Leiðólfsfell are also shown as spray
fountain rising up from the Laki lava flow (black). The text
labels indicate the source volcano or the vent system and the
year of the eruption: K, Katla; H, Hekla; G, Grímsvötn; Ö,
Öræfajökull; Lei, Leiðólfsfell; L. Laki; Ve, Veiðivötn; E, El-
dgjá; Va, Vatnaöldur. The source volcano for the olive gray
layer is uncertain.
- Myndrœn framsetning á gjóskulagatímatali Leiðólfsfells-
svœðisins frá því um 800 e.Krfram á okkar tíma. Vinstri
dálkurinn sýnir gjóskulög mynduð í Kötlugosum og hœgri
dálkurinn sýnir gjóskulög frá öðrum eldstöðvum. Eldkeila og
gosmökkur gefa til kynna eldgos í megineldstöð, en skorðótt
tákn vísa til sprungugosa utan megineldstöðva. Eldgosið sem
myndaði gervigígana við Leiðólfsfell er einnig sýnt sem gos-
strókur upp af Skaftáreldahrauni. Merkingarnar vísa til gos-
stöðva og gosársfyrir einstök gjóskulög: K, Katla; H, Hekla;
G, Grímsvötn; Ö, Örœfajökull; Lei, Leiðólfsfell; L. Laki; Ve,
Veiðivötn; E, Eldgjá; Va, Vatnaöldur.
ORIGIN OF THE CONE GROUP
The logical conclusion to be drawn from the evi-
dence above is that the scoria cones and the associat-
ed fall deposit at Leiðólfsfell were formed by rootless
vent eruptions when the first lava from the Laki fis-
sures flowed over the area. Such eruptions occur
when molten interior of a lava comes in contact with
an external water source, and they are driven by ex-
plosive vaporization of the water (Þórarinsson, 1953;
Þórðarson et al., 1992). Rootless cone groups are com-
mon within the Laki lava flow and formed where the
lava came in contact with running water or advanced
over water-saturated ground (Þórðarson and Self,
1993). The water source at Leiðólfsfell was undoubt-
edly the former channel of the Hellisá River, which
now follows a southerly course to the east of the
mountain (Fig. 1). Historical accounts indicate that
the Hellisá River had its passage to the north of
Leiðólfsfell mountain prior to the Laki eruption in
1783. Reverend Jón Steingrímsson, who witnessed
Katla Others
the eruption, states that on 24 June 1783 an explo-
ration party from the Skaftártunga district "saw the
lava flow down the channel of the Hellisá River" (Ste-
ingrímsson, 1788). Pálsson (1794) asserts that the
Laki lava advanced into the waterways of Hellisá,
forcing the river into a new course further to the east.
Guðmundsson (1844) who resided at Kirkjubæjar-
klaustur and visited the Laki fissures in 1842, appears
to be re-phrasing common knowledge when he writes
that before the 1783 eruption the Hellisá River flowed
to the north of Leiðólfsfell, but the lava forced it into
its present course. Immediately to the west of the
Hellisá River, where it turns sharply to the south and
follows the edge of the Laki lava field, is a dry
riverbed (labeled A on Fig. 1). This old riverbed is in
alignment with the course of the river above the turn
and its westem end is blocked by the Laki lava flow.
JÖKULL, No. 46, 1998
11