Jökull - 01.06.2000, Page 17

Guðrún Larsen
emerged there and at S-Eldgjá. The third tephra unit
was erupted on the Caldera segment. Activity then
continued on the Öldufell segment, while airborne
tephra emerged only intermittently on the Caldera
segment. This tephra is highly fragmented and closely
resembles typical Katla tephra.
On the subaerial segments of the Eldgjá fissure,
explosive activity caused by high extrusion rates and
vigorous degassing took place simultaneously with
the effusion of lava (Miller, 1989), but was proba-
bly also enhanced by high ground water table and
surface water during the initial phases. This activ-
ity was pronounced on the Central-Eldgjá and Eld-
gjá proper segments, and resulted proximally in ex-
tensively welded spatter deposits and distally in sco-
ria deposits interfingering with hydromagmatic tephra
from the subglacial part.
The Eldgjá tephra was mainly carried southeast
but a smaller lobe extends northwest (Figure 9). At
least 15 individual units from various parts of the fis-
sure can be discerned. The tephra from the subglacial
part of the fissure is distinctly bedded, with a maxi-
mum observed thickness of 4.5 m at ca. 5 km from the
source and up to 2 m and 1 m at a distance of ca. 10
km and 20 km, respectively. Along the subaerial part,
scoria and spatter form proximal deposits up to 15 m
thick, thinning rapidly away from the fissure, occa-
sionally forming sheets of spatter-fed lava that flowed
up to 5 km from the source (Jakobsson, 1979; Miller,
1989). The land area covered by tephra is at least
20.000 km

but the area at sea has not been estimated.
The volume (compacted) of the Eldgjá tephra on land
is close to 2.7 km


within the 0.5 cm isopach, corre-
sponding to 0.9 km


calculated as dense rock equiv-
alent (Larsen, 1996). The total volume has not been
calculated, due to large dispersal to the sea, but is es-
timated to exceed 4 km


or 1.3 km


DRE.
A hyaloclastite flow deposit is found along the
northern margin of Kötlujökull glacier at Kriki, east
of the Kötlujökull pass (Figure 8). It is extensively
gullied by water, providing numerous exposures of
its internal structures, and is partly overlain by a thin
moraine. The main body consists of three subunits
or facies but its base is nowhere exposed. The low-
ermost exposed subunit consists of irregularly jointed
lava and pillow lava with a highly irregular surface.
Vertical or subvertical protrusions of pillow basalt ex-
tend into the middle subunit, which is mainly made up
of hyaloclastite breccia consisting of poorly consoli-
dated scoriaceous ash to bomb size clasts, fragments
of pillows or irregularly jointed lava and small iso-
lated pillows. The protrusions of pillow basalt occur
mostly in the lower half, occasionally extending into
the upper part where thin horizontal lava sheets are
also intercalated with the breccia. In one instance a
couple of thin (20-50 cm) dykes emerge from a protru-
sion, extending up through the middle and the upper-
most subunit, forming a small sheet. The uppermost
unit consists of poorly consolidated layered hyalo-
clastite tuff with occasional cross-bedded or slumped
layers. The Kriki hyaloclastite flow is very similar to
the standard hyaloclastite unit defined and described
by Bergh (1985) and Bergh and Sigvaldason (1991),
with the exception that the lowermost facies, regularly
jointed lava, has not been observed. Bergh (1985) in-
terpreted the observed features to have formed when
the flows were discharged from a subglacial into a
subaqueous environment, a condition that cannot be
met at Kriki. Walker and Blake (1966) envisaged such
flow in a tunnel created by a preceeding jökulhlaup.
The Kriki hyaloclastite flow deposits emerge out from
under the present margin of Kötlujökull at 600 m a.s.l.
and can be followed for 6-7 km until they disappear at
ca 300 m a.s.l. below younger alluvials from glacial
rivers and jökulhlaups. Contacts between the Álfta-
ver lava and the Kriki deposits are nowhere exposed.
Water transported debris, which directly overlies rem-
nants of primary, bedded Eldgjá tephra at 160 m a.s.l.
on the southwest slope of Rjúpnafell, is tentatively
correlated to the Kriki deposits. The implication is
that the debris flow was emplaced during the Eldgjá
eruption. The volume of the Kriki deposits outside
the glacier margin is about 0.5 km


.
Voluminous lava flows, comparable in volume to
those of the 1783 Skaftá fires, were erupted on the
subaerial part of the Eldgjá fissure. The lavas were
channelled along river gorges and valleys down to the
lowland where they formed extensive lava fields in the
districts of Álftaver, Meðalland and Landbrot (Fig-
ure 8). Productivity was highest on the

8 km long
16 JÖKULL No. 49