Jökull - 01.06.2000, Síða 4

Holocene eruptions within the Katla volcanic system,
Holocene volcanism within the Katla volcanic sys-
tem seems to fall into three categories (Larsen, 1994):
1) Explosive hydromagmatic basaltic eruptions on
short volcanic fissures below the Mýrdalsjökull ice
cap are the most common events of the Katla system.
These usually occur within the caldera and during
recent centuries have concentrated close to its east-
ern boundary. These eruptions are accompanied by
tephra fall and jökulhlaups (glacial floods), that since
the 12th century have apparently followed the path of
Kötlujökull onto Mýrdalssandur. They appear to be
the typical Katla eruptions as far back as the record
from soil-sections goes. The number of Holocene
basaltic eruptions is unknown but may exceed 170.
2) Explosive silicic eruptions from vents below the
ice cap, apparently within the caldera. They are ac-
companied by tephra fall and probably by jökulhlaups
as well. At least 12 eruptions are known from soil-
sections in the surrounding area and several others are
anticipated. These are the second most common erup-
tions in the Katla system.
3) Predominantly effusive basaltic eruptions within
the fissure swarm and along the margin of the central
volcano. Most of these eruptions probably had an ex-
plosive component as well in cases where the fissures
reached below the ice cap. The longest fissures are
up to 75 km long. Huge lava flows have accompanied
some of these eruptions, which are the least common
type of activity in the Katla system.
In some eruptions of the first and third category
a very minor component of silicic glass has been de-
tected in the basaltic tephra, and fragments of light
coloured rock are among the scant lithics in some
tephra layers. Comprising much less than 1% of the
erupted material in all known cases, these occurrences
are considered too small to justify an additional cate-
gory of “mixed” silicic and basaltic eruptions.
Katla eruptions and/or jökulhlaups have been
mentioned in documents since the 12th century and
have been described in contemporary writings since
the 16th century (Biskupa Sögur, 1858; Storm, 1888;
Annálar 1400-1800, 1922-87; S.t.s. Ísl. IV, 1907-15;
P. Sveinsson, 1919; G. Sveinsson, 1919; Jóhannsson,
1919). Several authors have described various as-
pects of the Holocene explosive and effusive basaltic
eruptions and their products (Thoroddsen, 1894; Rob-
son, 1957; Thorarinsson, 1955, 1959, 1975, 1980;
Jónsson, 1978; Jakobsson, 1979; Larsen, 1979, 1993,
1996; Einarsson et al., 1980; Miller, 1989; Zielin-
ski et al., 1995; Guðmundsdóttir, 1998; Thordar-
son, et al., in press). The jökulhlaups have been
described in contemporary writings and treated by
more recent authors (Thorarinsson, 1957; Rist, 1967;
Haraldsson, 1981; Jónsson, 1982; Sigurðsson, 1988;
Maizels, 1993; Björnsson, 1993; Larsen, 1993; Karls-
son, 1994; Larsen and Ásbjörnsson, 1995; Tómasson,
1996). The Holocene silicic eruptions have only re-
cently been studied (Ólafsson et al., 1984; Larsen,
1994; Larsen et al., in press; Newton, 1999). Rhyo-
lites exposed as nunataks around the caldera rim and
as pyroclastic flows outside the ice cap are thought to
be of Late-glacial age (Jóhannesson et al., 1990; La-
casse et al., 1995). Pre-Holocene eruptions have been
treated by several authors (e.g. Lacasse et al., 1995)
but are beyond the scope of this paper.
In this paper, the characteristics of the three types
of eruption will be summarized and their environmen-
tal impact briefly evaluated. The main emphasis will
be on the historical period (i.e. the last 1100 years).
CHARACTERISTICS OF THE
HOLOCENE VOLCANISM
Explosive basaltic Katla (K) eruptions
Typical Katla eruptions are explosive, hydromagmatic
eruptions accompanied by often widespread tephra
fall, lightning in the eruption cloud and enormous
jökulhlaups consisting of meltwater, ice and volcanic
debris. The only historically recorded volcanic prod-
ucts consist of airfall tephra and water-transported de-
bris. During the opening stages the Katla eruptions
are subglacial, but apparently melt their way through
the overlying  400 m thick ice cover in a matter of
hours, if precursory earthquakes felt in nearby areas
can be taken as a mark of the beginning of the erup-
tions. The last Katla eruption to break through the ice
and become subaerial occurred in 1918. It began on
October 12 and lasted for about 3 weeks.
JÖKULL No. 49 3