Jökull - 01.01.2012, Síða 24

B. A. Óladóttir et al.
come less abundant, reaching a point where no writ-
ten documentation exists, as is the case for most of the
geological history. Most of the time only volcanic de-
posits provide a record of eruption activity and under-
standing gained through observation of actual erup-
tions in real time is needed to reconstruct course of
events preserved in the physical remains of past vol-
canic eruptions. Individual tephra layers are therefore
important records of not only the eruption history but
also related magmatic processes.
In Iceland, the tephra record is formed by dis-
tinctive layers found in different environments, ter-
restrial (e.g. Thorarinsson, 1958, 1967; Larsen and
Thorarinsson, 1977; Larsen, 1982, 1984; Larsen and
Vilmundardóttir, 1992; Larsen, 2000; Róbertsdóttir
et al., 2002a, 2002b; Kirkbride and Dugmore, 2005,
2006, 2008; Óladóttir et al., 2008, 2011a, 2011b),
glacial (e.g. Steinthorsson, 1977; Larsen et al., 1998),
lacustrine (e.g. Boygle, 1999; Hardardóttir et al.,
2001; Jóhannsdóttir, 2007; Jagan, 2010; Larsen et al.,
2011) and marine (e.g. Eiríksson et al., 2000; Haflida-
son et al., 2000; Larsen et al., 2002a; Kristjánsdóttir et
al., 2007; Guðmundsdóttir et al., 2011, 2012). In ad-
dition, studies of cryptotephra (e.g. Dugmore, 1989;
Dugmore et al., 1995a; Wastegård et al., 2001; van
den Bogaard and Schminke, 2002; Davies et al., 2008,
2010; Abbott et al., 2012) have greatly expanded the
area of correlated tephra layers outside Iceland.
Icelandic tephra studies carried out in the terres-
trial part have focused on diverse topics such as single
eruptions (e.g. Thorarinsson, 1958), periods in a vol-
cano’s history (e.g. Thorarinsson, 1967; Larsen, 1984,
2000; Róbertsdóttir et al., 2002b; Óladóttir et al.,
2008), specific tephra layers (Larsen and Thorarins-
son, 1977; Róbertsdóttir et al., 2002a), history of par-
ticular regions (e.g. Steinthorsson, 1977; Larsen et al.,
1998, Óladóttir et al., 2011a) and how tephra helps
in dating glacial movements (e.g. Kirkbride and Dug-
more, 2005, 2006, 2008). Review papers have also
been published on the history of explosive eruptions
in Iceland over specific periods (Larsen and Eiríksson,
2008a, 2008b). This paper describes how field-based
information on tephra is obtained and evaluated and
discusses how effectively eruption history can be read
from the tephra record. It also presents four cases of
improved understanding of volcanic processes taking
place during tephra formation.
ERUPTION HISTORY
The term eruption history is used here to cover vol-
canic activity and related physical processes as a func-
tion of time at all scales. Examples are eruption his-
tory of a single volcano or volcanic system assessed
from combined tephrostratigraphy spanning a long
time period; detailed insight into the course of events
during a single eruption from in-depth investigation
of a single tephra layer; deciphering different erup-
tive phases of a particular eruption by studying indi-
vidual tephra grains (Figure 1). Obviously the gen-
eral eruption history of a volcanic system does not in-
clude as many details as that of individual eruptions
but it gives useful information on the long-term be-
haviour of the volcano and/or volcanic system under
consideration. Tephra layer sequences extending over
thousands of years can provide information on erup-
tion frequency and on magma evolution with time in a
volcanic system (e.g. Óladóttir et al., 2008) that can-
not be accessed otherwise and are of importance when
predicting future volcanic hazards. Here, the focus is
on how eruption frequency is assessed, and eruption
histories and magmatic processes observed at differ-
ent time scales from the tephra record.
TEPHRA FORMING PROCESSES
Eruptions of basaltic magma, which has relatively
low volatile contents and viscosity, are normally ef-
fusive, whereas silicic eruptions are frequently explo-
sive. Approximately 9 out of 10 eruptions in Iceland
are basaltic according to Thordarson and Höskuldsson
(2008), yet four out of every five tephra layers are of
basaltic composition (Larsen and Eiríksson, 2008a).
The reason is that large parts of the active volcanic
systems in Iceland are covered by glaciers resulting in
phreatomagmatic activity and, hence, tephra forming
basaltic eruptions.
Although silicic eruptions account for only 5–
10% of all Icelandic eruptions they are very important
for tephrochronology in Iceland and Northern Europe.
Due to higher intensity and more effective fragmenta-
22 JÖKULL No. 62, 2012