Jökull - 01.01.2012, Blaðsíða 31

Deciphering eruption history and magmatic processes from tephra in Iceland
2012). Factors that control eruption frequency may
be pulses in the mantle plume affecting magma pro-
duction, tectonic environment, structure of the magma
plumbing systems and environmental factors such as
ice cover on volcanoes. All or some of these fac-
tors affect the tephra formation, hence, the tephra
layer frequency and thereby the estimated eruption
frequency of volcanic systems.
TEPHRA, ERUPTION HISTORIES AND
MAGMATIC PROCESSES – DIFFERENT
SCALES AND APPROACHES
Years of individual eruptions in Iceland are relatively
well established from written resources of Iceland for
the last millennium (e.g. Thorarinsson, 1958, 1967,
1974, 1975, 1976, 1980; Thorarinsson and Sigvalda-
son, 1962, 1972; Jóhannesson, 1977; Steinthorsson,
1977; Larsen 1979, 1982, 1984, 2000; Larsen et al.,
1998, 1999, 2002a; Grönvold et al., 1983, 1995; Sæ-
mundsson, 1991; Thordarson and Self, 1993; Thor-
darson and Larsen 2007; Sigurgeirsson, 1995; Gud-
mundsson et al., 1997, 2012). The prehistoric vol-
canic activity has not been as thoroughly studied
and remains fragmentary (e.g. Thorarinsson, 1952a,
1952b, 1965, 1971; Larsen and Thorarinsson, 1977;
Jakobsson, 1979; Jóhannesson et al., 1981; Vil-
mundardóttir and Kaldal, 1982; Sæmundsson, 1991;
Róbertsdóttir et al., 2002a, 2002b; Sigurgeirsson,
1992; Sigvaldason et al., 1992; Sigvaldason 2002;
Gudmundsson, 1998; Boygle, 1999; Larsen et al.,
2001; Larsen and Eiríksson 2008a, 2008b; Thordar-
son and Höskuldsson 2008; Óladóttir et al., 2008,
2011a; Guðmundsdóttir et al., 2012).
As previously mentioned the eruption history can
be looked at from different scales, focusing on every-
thing from the lifetime of a given volcano down to a
single phase during a given eruption. Following are
four case studies showing how it is possible to use
tephra to improve understanding of volcanic activity
at different scales.
Katla volcano, its Holocene tephra record and
likely enhanced future activity
A composite tephrostratigraphy from the Katla vol-
cano recording ∼8400 years gives insight into the de-
velopment of the volcano and information on mag-
matic processes taking place (Óladóttir et al., 2008).
Major and minor element chemistry revealed changes
in the volcano plumbing system, as illustrated by
changes in K2O concentration with time (Figure 4).
Two cycles of plumbing system development are
likely, starting with a simple vertical magma trans-
fer (constant K2O value) replaced by a sill and dike
complex (irregular K2O value) that developed into a
magma chamber (increasing K2O value, Figure 4a).
Changes in the magma system configuration seem to
correlate with variations in the eruption frequency.
The eruption frequency increases when a simple ver-
tical magma transfer is replaced by a sill and dyke
complex and falls again when a magma chamber has
developed (Figure 4b). According to this model the
historical time period is characterised by a simple
magma system of principally vertical magma transfer,
consistent with the low eruption frequency observed
(Óladóttir et al., 2005), and if the volcano evolves into
a sill and dyke complex like its past behaviour indi-
cates an increase in eruption frequency can be pre-
dicted (Óladóttir et al., 2008).
Hekla volcano, magma composition and magni-
tude of explosive opening phases vs. previous re-
pose periods in the last millennium
Hekla is one of few Icelandic volcanic systems that
produce abundant silicic and intermediate magma.
Compared to most volcanic systems in Iceland, pro-
ducing mainly basaltic material of similar composi-
tion over long periods of time, the composition of
Hekla products are highly variable (e.g. Sigmarsson
et al., 1992). The chemical composition does not only
vary between eruptions but also changes during erup-
tions with the initial erupted phase having more silicic
composition than the final phase (e.g. Thorarinsson,
1967; Sigmarsson et al., 1992; Sverrisdóttir, 2007).
The larger an eruption is at Hekla, the larger is the
compositional variability of its products. During his-
torical time all the 18 historical eruptions of the Hekla
volcano have begun with an explosive, plinian or sub-
plinian opening phase and all but one were followed
by lava effusion (Thorarinsson, 1967, 1970; Grönvold
et al., 1983; Gudmundsson et al., 1992; Höskulds-
JÖKULL No. 62, 2012 29