Jökull - 01.06.2000, Page 23

Guðrún Larsen
has been that of the accompanying jökulhlaups, be-
cause of the short warning time for people and live-
stock exposed to the floods and because of the damage
inflicted on the environment. This hazard is reduced
with the current monitoring of the volcano. The most
serious effects of future Katla eruptions could, how-
ever, be on power transmission in Southern Iceland
as a result of tephra fall and lightning in the eruption
cloud. The latter is potentially the greatest hazard to
people and livestock in areas within 30-40 km of the
eruption site. A modern community with power trans-
mission lines, television aerials and electrical fences
(rural areas) is more vulnerable to this menace than
earlier communities in the vicinity of the volcano.
SUMMARY AND CONCLUSIONS
Intracaldera basaltic eruptions, the typical hydromag-
matic Katla eruptions, occur at intervals of decades.
The average frequency during the last 9 centuries is
close to two eruptions per century, with the shortest
and longest interval of 13 and 80 years, respectively.
Intracaldera silicic eruptions occurred at intervals
of hundreds of years in the period 6600 - 1700 BP.
The average frequency was close to two eruptions per
millennium, and the shortest and longest interval were
ca. 100 and ca. 1000 years, respectively.
Large, predominantly effusive fissure eruptions,
involving both the central volcano and fissure swarm
occur at intervals of thousands of years. Two such
episodes are known in the Holocene. They represent
major rifting episodes that may modify the volcanic
system, including the subcaldera conditions.
Katla eruptions of known calendar dates all began
in the spring-fall season. Future eruptions may follow
that trend. The vent area in most historical Katla erup-
tions was apparently in the eastern part of the caldera.
The best documented locations lie within the south-
eastern corner of the caldera.
Twelve silicic eruptions took place in the ca. 5000
year period between the Hólmsá and Eldgjá fires, dur-
ing which time the composition of silicic magma re-
mained fairly homogenous. The interval since the last
silicic eruption ca. 1700 14C years ago is the longest
on record. The reason as to why silicic activity has not
recurred may be subcaldera changes brought about by
the Eldgjá event. A substantial silicic eruption cannot,
however, be excluded in the future.
Vent areas in the silicic eruptions have been asso-
ciated with the caldera. Accompanying jökulhlaups
may have escaped from the caldera through any of the
three gaps, depending on pre-Eldgjá topography and
the ice thickness in the caldera.
The 75 km long Eldgjá fissure is one of the longest
known fissures active in a single Holocene eruption.
The Eldgjá tephra is the third or fourth largest his-
torical tephra layer in Iceland and the lava flows are
the among largest in recorded history. A hyaloclastite
flow that escaped from the caldera through the Kötlu-
jökull gap appears to be part of its products.
Since activity on the Katla system was renewed
in the 12th century, after an apparent repose of some
200 years, all associated jökulhlaups have escaped
through the Kötlujökull pass onto Mýrdalssandur.
Possible explanations are changes in ice thickness,
subglacial topography of the caldera and/or the loca-
tion of the eruption site.
The immediate environmental effects of the Eld-
gjá event included permanent or temporary destruc-
tion of large vegetated areas by tephra fall, lava flows
and jökulhlaups. Between 3000 and 4000 km

were
severely affected, whereof several hundred km

were
permanently laid waste. The drainage pattern of at
least a 2000 km

subaerial and subglacial area was
permanently altered.
Long-term environmental changes brought about
by the Eldgjá event are exemplified by the post-Eld-
gjá jökulhlaups and include 1) lateral extension and
raising of the surface of the Mýrdalssandur plain; 2)
extension of the sandur coast line to the south; 3)
changes in the course of rivers and 4) continuing de-
struction of vegetated areas by jökulhlaups.
The immediate and long term changes following
the Eldgjá event may be the most extensive environ-
mental changes caused by volcanic activity in Iceland
since settlement 11 centuries ago.
ACKNOWLEDGEMENTS
Many people have helped me in various ways in
the field and otherwise during my many trips in the
Katla-Eldgjá area. Sigurður Þórarinsson introduced
22 JÖKULL No. 49