Jökull - 01.01.2012, Side 27

Deciphering eruption history and magmatic processes from tephra in Iceland
RECONSTRUCTING ERUPTION
FREQUENCY OF VOLCANIC SYSTEMS
Correlations of tephra layers from different environ-
ments allow the optimal eruption record to be estab-
lished. In order to use the tephra in reconstructing the
eruption history of volcanoes and volcanic systems,
the volcanic source and age of the tephra has to be
identified.
Identifying volcanic source
In the pioneering days of tephrochronology, the prin-
cipal method for tracing the origin of a tephra layer
was by mapping of its dispersal (e.g. Thorarins-
son, 1944, 1958, 1967). The emphasis was on sili-
cic tephra layers that had fairly distinct macroscopic
characteristics such as colour. Mapping of basaltic
tephra layers was more difficult due to less distinct
field characteristics and the presence of many lay-
ers. Most tephra layers were traced to the correct
volcanic source by such mapping, but occasionally
the method led to incorrect source identification of
basaltic tephra. For instance, the 1477 tephra layer
was wrongly assigned to the Kverkfjöll volcano be-
fore major-element analyses proved its origin at the
Bárðarbunga-Veiðivötn volcanic system (Thorarins-
son, 1976; Larsen, 1982). The use of electron mi-
croprobe became an inseparable part of tephra studies
and the major element composition of tephra is now
widely adopted for fingerprinting and source tracing
(e.g. Larsen, 1981; Hunt and Hill, 1993, 2001; Dug-
more et al., 1995a; Larsen et al., 1999; Wastegård et
al., 2001; Óladóttir et al., 2008, 2011b; Lowe, 2011;
Hayward, 2012). During the last decade, the mea-
surement of trace element concentrations have been
added to tephra studies, facilitating recognition of in-
dividual tephra layers and identification of the vol-
canic source as well as shedding light on the petro-
genesis of magma forming the tephra (e.g. Pearce et
al., 2007; Shane, 2000; Lowe, 2008; Óladóttir et al.,
2011b; Abbot et al., 2012; Lane et al., 2012).
Tephra age determination
When tephra layers are rapidly dispersed and de-
posited they can form effective isochrons. The preci-
sion of these marker horizons can vary from hours to
days and weeks, but in practical terms they form un-
usually well-defined environmental marker horizons.
A tephra layer is, therefore, useful for relative dating
and correlation even though its exact age is not known
and all outcrops that contain the same tephra layer can
be synchronised. The ages of only a fraction of the
Icelandic tephra layers have been confirmed by writ-
ten descriptions, the 14C-method or ice-core chronol-
ogy. These are in particular silicic tephra layers and
widespread basaltic deposits (e.g. Thorarinsson, 1964,
1971; Hammer et al., 1980; Hammer, 1984; Dug-
more et al., 1995b; Grönvold et al., 1995; Zielin-
ski et al., 1995; Larsen et al., 2001; Gudmundsdót-
tir et al., 2011). By using the accumulation rate of
ice, soil or sediments between dated tephra markers,
an estimated age can be assigned to other tephra lay-
ers (e.g. Steinthorsson, 1977; Larsen, 1982; Larsen et
al., 1998; Jóhannsdóttir, 2007; Óladóttir et al., 2005,
2008, 2011a; Gudmundsdóttir et al., 2012). Examples
of tephra layers dated by various methods are shown
in Table 1.
Tephra correlation
Tephra correlation is fundamental to unravel eruption
history. Tephra layers can be identified and correlated
based on their macroscopic and microscopic char-
acteristics, their chemical characteristics and their
stratigraphic position within the local or regional
tephrostratigraphy. To obtain the complete history
of explosive activity, all tephra layers around a partic-
ular volcano have to be securely correlated because
the combination of explosive activity and contempo-
rary wind patterns can provide very narrow sectors
of fallout around the eruption site. This would pre-
vent tephra layers from a single eruption, deposited
in more than one discrete sector, to be counted as
multiple layers. Tephra layers, either as single lay-
ers or series of layers, with specific characteristics
are termed marker tephra layers and serve for first-
order correlations. The majority of these are silicic
in composition (Table 1) and most originate from the
Hekla volcano. Although several volcanic systems
in Iceland produce basaltic tephra that can be traced
to its source by chemical composition, the composi-
tional variability within each volcanic system is often
too small to distinguish between individual eruptions
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