Jökull - 01.06.2000, Blaðsíða 2

Holocene eruptions within the Katla volcanic system, south
Iceland: Characteristics and environmental impact
Guðrún Larsen
Science Institute, University of Iceland, Dunhaga 3, 107 Reykjavík
Abstract – Holocene volcanism within the Katla volcanic system is characterized by: 1) explosive (hydromag-
matic) basaltic eruptions along volcanic fissures within the Mýrdalsjökull caldera; 2) explosive silicic eruptions
from vents associated with the caldera and 3) predominantly effusive basaltic eruptions involving both the cen-
tral volcano and the fissure swarm. Typical Katla eruptions are accompanied by basaltic tephra fall, lightning
and glacial floods (jökulhlaups) of meltwater, ice and volcanic debris. Twenty eruptions have occurred in the
last 11 centuries. The volume of airborne tephra varies by three orders of magnitude, with an estimated vol-
ume of 1.5 km

of freshly fallen tephra from the largest historical Katla eruption. The length of documented
eruptions varies from 2 weeks to over 5 months. The average repose period since 1500 AD is 47 years with
maximum deviations of 33 and 34 years. All Katla eruptions during the last 400 years have begun in the spring-
fall season. At least 12 silicic Katla eruptions are known from the period ca. 1700 BP and 6600 BP. The silicic
magma was most likely erupted by hydromagmatic explosive eruptions. The tephra dispersal axes indicate vent
locations within the caldera or along the caldera rim. The volume of airborne silicic tephra varies by orders
of magnitude, the largest and most widespread is tephra layer UN with uncompacted tephra volume of 0.3 km

.
Intervals between the silicic eruptions have varied from ca. 100 to ca. 1000 14C yrs. Two major “fires” and
5-10 relatively minor, partly effusive eruptions have occurred during the Holocene. The 10th century Eldgjá
and the 6800 BP Hólmsá fires are the largest known Holocene eruptions within the Katla system. A


75 km
long, discontinuous and partly subglacial eruptive fissure was active during the Eldgjá eruption. The opening
phase on most fissure segments was explosive, followed by an effusive phase on the subaerial segments. The
eruption produced a voluminous basaltic tephra layer with a minute silicic component, two major lava fields
and possibly a hyaloclastic flow deposit. Large jökulhlaups accompanied the eruption. The combined volume
of erupted material may exceed 19 km

DRE. Eruptions on the Katla system have caused extensive environ-
mental changes during the past 1100 years. The Eldgjá fires radically changed the landscape, hydrology and
utilization potential of large areas in South Iceland. Since then, jökulhlaups accompanying eruptions within
the caldera have escaped eastwards, raising the Mýrdalssandur plain and extending its coastline southwards.
INTRODUCTION
The Katla volcanic system, South Iceland, as defin-
ed by Jakobsson (1979), is a Holocene feature super-
imposed on upper Pleistocene hyaloclastites and lava
flows (Jóhannesson et al., 1990). A central volcano
partly covered by the ice cap of Mýrdalsjökull and an
associated fissure swarm form the SW-NE trending 80
km long system (Figure 1). The hyaloclastite massif
reaches an altitude of 1380 m a.s.l. under the ice cover.
The massif encompasses an ice-filled caldera with an
area of 110 km  and a depth of 700 m (Björnsson et
al., 1993; this volume). Three glaciers descend to the
south, east and northwest from the ice cap onto the
lowlands through deep gaps that they eroded in the
caldera walls. Seismic activity has been mostly con-
fined to two areas within and immediately to the west
of the caldera (Einarsson, 1991).
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