Jökull - 01.06.2000, Page 14

Holocene eruptions within the Katla volcanic system,
pumice volumes or the floods that transported them
were particularly large.
Predominantly or partly effusive basaltic eruptions
Major events: The "Fires"
Effusive basaltic eruptions are the least common
events on the Katla volcanic system. Between 5 and
10 relatively minor eruptions, that were at least partly
effusive and produced small lava flows, and two major
"fires" producing large lava flows, are known to have
taken place during the Holocene. The former occurred
on short fissures on the western and northern periph-
ery of Mýrdalsjökull central volcano and are thought
to be more than 4000 14C yrs old (Jóhannesson et al.,
1990). In the two "fires", fissures also opened up on
the NE trending fissure swarm. They are the largest
and most hazardous Holocene events of the Katla vol-
canic system. This chapter focusses mostly on the
younger fires.
The older "fires" (Hólmsá fires) are about 6800
14C yrs old. The lava flows, now partly covered by
the products of the younger fires, can be shown to fit
into the regional tephra stratigraphy of S-Iceland at a
specific stratigraphical level (Figure 5). The lavas fol-
lowed depressions and river channels down the Álfta-
versafréttur area at least as far as Atlaey (Figure 8)
and possibly all the way to the coast of that time.
Their southward extension is hidden below an exten-
sive cover of younger lava. The volume of lava is ten-
tatively estimated to be ca. 5 km

. The length of the
eruptive fissure is not known, but a minimum length of
8-10 km outside the present ice margin near Öldufell
is inferred by the paths taken by the lava.
The younger "fires" (Eldgjá eruption) took place
in the early 10th century (Larsen, 1979), most likely
around 934-938 AD (Hammer et al., 1980; Zielin-
ski et al., 1995). The ca. 75 km long fissure extends
from the Katla caldera beneath the Mýrdalsjökull ice
cap in the southwest, through the mountainous terrain
northeast of the ice cap to Eldgjá proper and contin-
ues intermittently to Stakafell mountain (Figure 8). It
is the longest known eruptive fissure to be active in
historical time (last 11 centuries). The eruption pro-
duced a widespread basaltic tephra layer, composed of
several distinct units, and two major lava fields (Rob-
son, 1957; Miller, 1989; Larsen, 1996; Thordarson et
al., in press). Possibly, a hyaloclastite flow accom-
panied the eruption. Jökulhlaups occurred along the
subglacial part of the fissure.
The Eldgjá event is of particular importance be-
cause of its magnitude and because it is the cause
of the most extensive environmental changes brought
about by volcanic activity in Iceland during the last 11
centuries.
The Eldgjá fires of the 10th century: The fissure and
the products
About one-fifth of the total length of the Eldgjá fissure
lies below the present Mýrdalsjökull ice cap. The ex-
act locations of the subglacial fissure segments are not
known but isopachs of individual units of the Eldgjá
tephra layer define at least two major segments. The
southwesternmost of the two is located to the west of
Kötlujökull, lying either within the present caldera or
along its eastern margin/fracture. Another major seg-
ment lies to the west of Öldufellsjökull, possibly sepa-
rated from the adjoining subaerial segment by a minor
discontinuity or a dextral shift.
The 60 km long subaerial part of the fissure runs
through hillocky landscape and becomes increasingly
discontinuous to the northeast (Figures 1 and 8). In-
dividual fissure segments occupy the low areas and
are connected by shallow graben structures extend-
ing across topographical highs. Locally the fissure
opened up in a pre-existing valley or a depression.
This section of the eruption fissure is an 8 km long,
400 m wide and 150 m deep chasm after which the
eruption is named, Eldgjá proper. The depression was
occupied by a river at the time of eruption as evi-
denced by the scarps of a pre-eruption waterfall which
was sealed off by the eruption products in the NW-
wall of the chasm, close to the present waterfall.
By far the greatest part of the airborne Eldgjá
tephra was erupted on the subglacial part of the Eldgjá
fissure (Figure 9). Hydromagmatic explosive activity
appears to have been dominant throughout the erup-
tion there. Isopachs of the tephra layer as a whole in-
dicate that the main source area of the airborne tephra
was west of Öldufellsjökull. Isopachs of individual
units show that the first tephra to become airborne
emerged there and at S-Eldgjá.
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