Jökull - 01.06.2000, Side 20

Holocene eruptions within the Katla volcanic system,
The field observations are supplemented by de-
scriptions of the conditions that met the first settlers
in the late 9th-early10th century in the Book of Set-
tlement, and these two independent sources support
each other on certain points. One description places
a large lake in the Álftaver district. Present geologi-
cal conditions support the existence of a lake behind
the moraine in the northern part of Álftaver (Figure 8),
where rootless cones indicate that the lava flowed over
a wet area or into a lake before banking against the
moraine. Descriptions and definitions of the early set-
tlements imply that large parts of the area now known
as Mýrdalssandur were suitable for farming, i.e. ex-
tensively vegetated. This implies that rather stable or
low energy conditions had prevailed in the area for an
extended period.
The 9th century coastline can be crudely recon-
structed from the descriptions in the Book of Set-
tlement and from younger descriptions (S.t.s. Ísl. IV,
1907-15). The former refers to a fjord in the area to
the west of cape Hjörleifshöfði, with the head towards
the cape. It has recently been argued that the fjord was
a lagoon behind a sandbarrier (Imsland and Larsen,
1993), analogous to the present inlets on the south and
southeast coast (Figure 8). The younger sources de-
scribe conditions along the coastal cliffs between Vík
and Höfðabrekka where no beach existed until after
1660 A.D.
Consequences of the Eldgjá fires
Immediate effects of a catastrophic event
The Eldgjá eruption changed the landscape, hydrol-
ogy and utilization potential of large areas in S-
Iceland. About 800 km

of land were covered by
new lava that raised the topography, blocked water-
ways and permanently changed the run-off pattern of
an area extending from the Mýrdalssandur in the west
to Landbrot in the east. A considerable part of the 800
km

overrun by the lavas was vegetated. Over 20.000
km

were affected by the tephra fall on land. Some
2600 km

were covered by over 20 cm thick tephra
and severely damaged. Of these, roughly 600 km

were buried below more than100 cm thick deposits
and permanently laid waste.
The changes were most radical within 30 km
east of the Mýrdalsjökull massif. Álftaversafréttur,
Skaftártunga and Álftaver were affected by heavy
tephra fall, lava flows, a hyaloclastite flow and jökul-
hlaups. The lava from S-Eldgjá followed river valleys
and gorges to the low areas, forcing rivers in Álfta-
versafréttur out of their beds. At Álftaver the lava
banked against and was deflected westwards by the
moraine, filling in a lake/wet area in the process, then
turning southwards to the coast of that time, possibly
extending it seawards. The lava fronts on the sandur
east of Kötlujökull blocked previous routes of melt-
water and jökulhlaups to the east. Hyaloclastite flows
at Kriki may have changed the topography below
Kötlujökull and consequently the pre-eruption paths
of meltwater. A tephra blanket more than 1 m thick
suffocated the existing vegetation and filled in gul-
lies and depressions. Extensive soil erosion followed,
which may have resulted in complete denudation lo-
cally, e.g. in Álftaversafréttur. Where the thickness
exceeded 0.5 m the tephra prevented recovery of the
vegetation for centuries. After more than a millen-
nium some of these areas still have only a thin, easily
punctured soil and vegetation cover.
The extent and effects of jökulhlaups accompany-
ing the Eldgjá event cannot be realistically estimated
because the Eldgjá lavas and post-Eldgjá jökulhlaups
in the Mýrdalssandur area have covered most of their
tracks. The nickname Aurgoði (Lord of the mud)
of a second generation Norse settler in that area im-
plies that water-transported sediments (aur = mud)
occurred within his estate. Broadly speaking, ar-
eas along the eastern and northern periphery of the
Mýrdalsjökull ice cap may have been affected to a
greater or lesser extent by the Eldgjá jökulhlaups.
Topographical changes below the Mýrdalsjökull
ice cap can only be guessed at. Accumulations of vol-
canic debris within the caldera and elsewhere along
the subglacial part of the erupting fissure are likely to
have caused changes locally. Erosion by meltwater
may have been substantial in some areas. Subsidence
of the caldera floor as a consequence of such large
eruption is also a definite possibility. Whatever the
causes, drainage from the caldera was permanently al-
tered as a consequence of the Eldgjá event; pre-Eldgjá
jökulhlaups escaped through the Sólheimajökull and
JÖKULL No. 49 19