Jökull - 01.01.2005, Síða 71

Holocene tephra stratigraphy and eruption frequency of Katla, S-Iceland
age repose time for explosive basalt eruptions at Katla
volcano in Holocene times is on the order of 25 years,
emphasising that in recent times the volcano has been
relatively quiet. The historical eruption frequency is a
factor of two lower than that observed for prehistoric
times.
Implications for future activity of Katla volcano
The calculated SAR ages for the prehistoric Katla lay-
ers in our composite soil profile indicate that the time
interval between the deposition of successive tephra
layers has been as short as 2 years and as long as 164
years. This suggests that the true repose time between
prehistoric explosive Katla eruptions is highly vari-
able and spans two orders of magnitude (< 1 year
to > 80 years). This is interesting in the context of
the time that has passed since the last visible erup-
tion from the Katla fissure (i.e. AD 1918), which cur-
rently stands at 87 years and is second in the record to
the 95 years between the Katla eruptions in AD 1262
and AD 1357. This observation suggests that activity
at Katla volcano is declining. This notion, however,
does not eliminate the possibility of imminent erup-
tion, but rather suggests that in view of longer time-
scales the volcano is either calming down or changing
behaviour.
The periodic intensification in prehistoric eruption
frequency over specific time intervals most likely re-
flects an increase in the flux of basaltic magma into
the upper regimes of the Katla plumbing system. The
net result is an increase in heat flux from depth that
may enhance the production of felsic magma at shal-
lower levels via partial melting of the surrounding
crust. This can explain the observed correlation in
the timing of the eruption frequency peaks for basalt
and dacite eruptions. Consequently, the lower erup-
tion frequency in historical time does not support the
occurrence of an imminent felsic eruption, unless the
behaviour of the volcano is changing.
This study has focused on the frequency of explo-
sive eruptions at the Katla system and changes therein
during the Holocene. Although eruption frequency
and its variations are likely to be directly correlated
with magma productivity, the exact relationship be-
tween these two variables is still poorly constrained.
Magma productivity at the Katla system, defined as
the volume of erupted material per unit time, has not
yet been assessed quantitatively but is relevant for
future research. However, the proposed hypothesis
for production of felsic magma can be assessed from
major- and trace-element composition of the individ-
ual tephra layers and their isotope ratios, thus permit-
ting further discussion of the magma plumbing system
beneath the Katla system and its behaviour during the
last millennia.
CONCLUSIONS
The Holocene eruption history of the Katla volcanic
system is recorded by tephra layers in soil profiles
in the surrounding regions. A composite soil sec-
tion east of Mýrdalsjökull containing 208 tephra lay-
ers and spanning 8400 years was measured and sam-
pled for this study. Eighteen of the tephra layers were
formed during historical time (i.e. the last 11 cen-
turies), whereas 190 layers are prehistoric and repre-
sent 7300 years of volcanic history. About 88% of
these tephra layers, or 182 in total, are based on field
identification assigned to explosive eruptions within
the Katla volcanic system. This tally includes 172
basaltic and 10 felsic layers. A Katla origin of 111
basaltic layers was confirmed by major element anal-
yses.
All of the tephra layers have been dated using a
SAR (soil accumulation rate) age model, established
by measuring the average SAR between seven 14C-
dated marker layers. In turn, this data was used to
evaluate the long-term frequency of explosive erup-
tions within the Katla system over the last 8400 years.
The mean eruption frequency during prehistoric times
was on the order of 4 eruptions per century, compared
to ∼2 eruptions per century in historical time. Erup-
tion frequency appears to have peaked at 6–7 erup-
tions per century in the periods 7.0–8.5 ka and 2.5–4.5
ka. The total number of explosive basalt eruptions at
the Katla volcanic system over the last 8400 years is
estimated to be a minimum of 300.
Chemical analyses show that the magma com-
position produced by the Katla volcano throughout
the Holocene is mostly evolved Fe-Ti basalts, show-
ing small-scale but systematic compositional variation
JÖKULL No. 55 71