Jökull - 01.11.1998, Side 14

Further to the south (at point B on Fig. 1) the channel
morphology suggests that the Hellisá River captured
the waterway of the rivulet in Arnagil and regained
access into the Skaftá River gorge via Landárgljúfur
to the south of Hólahaft. This geologic evidence are
consistent with the northem passage of the Hellisá
River prior to 1783 and that the Leiðólfsfell cones
were formed by rootless vent explosions as the Laki
lava entered the channels of the river.
The dispersal, characteristics and chemistry of the
Leiðólfsfell ejecta further support this interpretation.
The Leiðólfsfell scoria layer can be traced with cer-
tainty up to 1.5 km away from source but beyond that
distance it cannot be separated from the tephra fall
that originated at the Laki fissures. Over this distance
the thickness of the deposit decreases from >300 cm
to <1 cm and the maximum clast size decreases from
~300 mm to ~1 mm. This evidence indicates that the
tephra deposit was produced by explosive activity of
low intensity. The low vesicularity (< 30 vol.%) of the
scoria clasts is consistent with data from other known
basaltic rootless vent emptions which contain clasts
that are typically much less vesicular than those gen-
erated by explosive activity at the primary vents
Fig. 6. Close up photograph of the -935 AD Eldgjá tephra deposit at location 2. The deposit consists of five distinct fall units:
1. Black fine grained ash, 6.0-9.0 cm thick; 2. Black fine lapilli scoria, 0.5-1.0 cm thick; 3. Brown medium grained ash, 4.0 cm
thick; 4. Black medium grained ash, 3.0 cm thick; and 5. Black fine to medium lapilli scoria, 8.0-10.0 cm thick. Tephra unit 1 is
phreatomagmatic and units 2-5 are strombolian. The Settlement layer (Va-870) is visible at the bottom of the photograph. The
scale to the right is 40 cm long.
- Nærmynd af Eldgjárgjóskunni í jarðvegssniði 2. Hér samanstendur lagið affimm gjóskueiningum sem eru talið fi'á botni og
upp: 1. Svörtfínkorna aska, þykkt 6,0-9,0 sm; 2. Svart meðalgróft gjall,þykkt 0,5-1,0 sm; 3. Brún meðalkorna aska,þykkt 4,0 sm;
4. Svört meðalkorna aska, þykkt 3,0 sm; 5. Svartfínt-til meðalgróft gjall, þykkt 8,0-10,0 sm. Eining 1 erfreatómagmatísk gjóska,
einingar 2-5 eru strombólsk gjóska. Landnámslagið (Va-870) sést neðst á myndinni og mœlikvarðinn til hœgri er 40 sm.
12
JOKULL, No. 46, 1998