Jökull - 01.01.2020, Side 52

The 1845–46 and 1766–68 eruptions at Hekla volcano
Pedersen et al. (2018a). This suggests that the plani-
metric method may underestimate the lava bulk vol-
ume in the order of 40-60%. Wadge (1978) similarly
estimated that the planimetric method may underesti-
mate volumes by approximately 50%. Thus, consid-
ering this, it is likely that the volume of the 1845–46 is
0.5–0.6 km3 and 1766–68 may be 1.0–1.2 km3. Tak-
ing this into account, the estimates thus confirms the
estimates provided by Thórarinsson (1967).
The production rate, which is defined as erupted
material in an eruption divided by pre-eruption repose
period, varied from 7.4×106 to 40×106 m3yr−1 for
the Hekla eruptions during the 20th century (Peder-
sen et al., 2018a). Our data yields production rates of
15×106 m3yr−1 and 7.2×106 m3yr−1 for the 1766–
68 (repose period of 73 years) and 1845–46 (repose
period of 77 years) eruptions, respectively. Peder-
sen et al. (2018a) stated that production rates at Hekla
are variable on 10–100 year time scale in contrast to
a steady production rate which was proposed earlier
(see references therein). Our production rates respec-
tively for the 1766–68 and 1845–46 agree with the
statement from Pedersen et al. (2018a).
Viscosity estimates
Viscosity estimates showed that pre-eruptive magmas
(2.5×102 Pa s and 2.2×102 Pa s in average for 1766–
68 and 1845–46 eruptions, respectively) were about
one order of magnitude more fluid than the degassed
magmas (2.5×103 Pa s and 1.9×103 Pa s in average
for 1766–68 and 1845–46 eruptions, respectively). In
addition to water, decreasing temperature accelerates
the increase in viscosity exponentially (Figure 5c).
Lowering the temperature has the effect of acceler-
ating the increase of viscosity by one to two orders
of magnitude (Figure 5c). We expect a large range
in the viscosity of lava during emplacement, result-
ing in variable shear strain rates and resulting sur-
face crust structures (Pedersen et al., 2017). Simi-
larly, Kolzenburg et al. (2017) showed that viscosity
can increase 3 orders of magnitude during the eruption
of Holuhraun. From field observations and studying
the orthophotos the sampled lava-flows are from infla-
tion structures (18IS-05, -07 and -08), complex mor-
phology (18IS-21), channelised flow (18IS-02 and -
06), sheet-like-flow (18IS-16), bulky type of break-
out structures (18IS-09) and smooth type of breakout
structures (18IS-03 and -15). There is not a simple
relationship between the estimated viscosities for the
samples and the observed flow morphologies (Table
2). This is most likely due to the large variation in
viscosity due to variable degassing, cooling and crys-
tallisation during emplacement. We can therefore not
assume that our estimates fully capture the entire span
of viscosity for the Hekla lavas during the 1766–68
and 1845–46 eruptions, since this would require ac-
tual emplacement temperature and actual crystal con-
tent measurements.
Combining Hekla’s emplacement time-lines and
SiO2 contents
Several models of the magma reservoir beneath
Hekla’s central volcano have been suggested to ex-
plain the chemically-zoned tephra deposits. Hekla
tephras typically change from silicic (white) of rhy-
olite composition at the base grading upwards to an-
desite composition (black) at the top, followed by
lavas of the composition andesite (ca. 58 wt% SiO2)
to basaltic andesite (54 wt% SiO2) as typical end com-
position (Sigmarsson et al., 1992; Sverrisdottir, 2007;
Chekol et al., 2011; Janebo et al., 2018). This has
been explained by a stratified magma chamber model
with the most SiO2 rich magmas at the top. This pop-
ular model was first proposed by Sigmarsson et al.
(1992) and has been slightly modified by Sverrisdottir
(2007) and Chekol et al. (2011). Thus, the tapping
of such a stratified plumbing system starts with the
most silica-rich magmas (rhyolite) during the initial
explosive phase (Thórarinsson, 1967; Sigmarsson et
al., 1992; Sverrisdottir, 2007; Janebo et al., 2016b).
The SiO2 content of the later tephras and lavas de-
cline to andesite and basaltic andesite (Sigmarsson
et al., 1992; Sverrisdottir, 2007). Therefore, it is to
be expected that the silica content of the large lava-
flow fields will also follow this pattern, showing a de-
cline of SiO2 over time. Our data enables us to test
this hypothesis by comparing the emplacement time-
lines with the SiO2 of the samples (Table 2). For
the 1845–46 and 1947–48 eruptions, the SiO2 evo-
lution is indeed decreasing with time, supporting the
stratified magma chamber model (Figure 6b, c). In
these cases, the SiO2 contents of the studied samples
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