Jökull - 01.01.2020, Page 50

The 1845–46 and 1766–68 eruptions at Hekla volcano
4. mynd. – Þróun útbreiðslu hraunflákanna frá 1845–46 byggð á sögulegum heimildum. Heilar línur sýna
útbreiðslu hraunflákanna byggða á nákvæmum lýsingum. Brotalínur tákna áætlaða útbreiðslu hraunflóða.
Bakgrunnur kortsins er sá sami og á 1. mynd. B) Nærmynd af Hekluhryggnum, virkir gígar á árunum 1845–46
eru merktir á kortið. Loftmyndin er frá 1945–46. Heilar línur tákna virka gíga og brotalínur ógreinilegri gíga
á loftmyndinni.
Table 2. Whole rock analysis and viscosity estimates of the Hekla rock samples from the 1766–68 and 1845–46
lava-flows. Viscosity is estimated by means of the model by Giordano et al. (2008), emplacement time/period
of the samples are based on the historical sources (see text), and the morphology surface structures are based
on observations from the field and orthophotos. – Efnagreiningar og mat á seigju sýna frá hraununum sem
runnu 1766–68 og 1845–46. Seigjan er áætluð út frá líkani Giordano o.fl. (2008) og aldur sýnanna er byggður
á sögulegum gögnum (sjá texta). Formgerð sýnanna er byggð á rannsóknum í felti og loftmyndum.
Eruption 1766–68 1845–46
Approximate April 9– Sept. 9, Sept. 9, Sept. 9, March 18, Sept. 12, Nov. 14, Nov. 14, Nov. 19– Dec. 27, 1845–
emplacement date 21, 1766 1766 1766 1766 1767 1845 1845 1845 25, 1845 March 16, 1846
Sample no. 18IS-21 18IS-07 18IS-08 18IS-09 18IS-16 18IS-05 18IS-02 18IS-03 18IS-06 18IS-15
Morphology Complex Inflation Bulky Sheet-like Inflation Channelised Smooth Channelised Smooth
breakout flow flow breakout flow breakout
SiO2 (wt%) 54.90 56.50 56.40 56.40 57.80 57.10 55.20 56.50 54.70 54.00
Al2O3 (wt%) 14.80 15.00 15.00 15.00 15.20 15.10 14.90 15.00 14.80 14.70
Fe2O3 (wt%) 13.00 11.80 11.90 11.90 11.20 11.50 12.80 11.40 12.90 13.50
FeO (wt%) 5.85 5.31 5.35 5.35 5.04 5.17 5.76 5.13 5.80 6.07
FeOT (wt%) 17.54 15.92 16.05 16.05 15.11 15.51 17.27 15.38 17.40 18.21
CaO (wt%) 6.64 6.15 6.15 6.16 5.77 6.00 6.52 5.93 6.62 6.91
MgO (wt%) 2.82 2.43 2.43 2.42 2.15 2.34 2.73 2.31 2.80 3.04
Na2O (wt%) 4.16 4.33 4.29 4.32 4.41 4.35 4.26 4.29 4.18 4.07
K2O (wt%) 1.26 1.36 1.36 1.36 1.45 1.41 1.30 1.39 1.27 1.20
MnO (wt%) 0.29 0.27 0.27 0.27 0.26 0.27 0.29 0.26 0.29 0.30
TiO2 (wt%) 1.92 1.65 1.65 1.65 1.46 1.58 1.86 1.56 1.91 2.08
P2O5 (wt%) 1.03 0.83 0.83 0.83 0.70 0.78 0.99 0.77 1.02 1.16
H2O (wt%) 1.24 1.34 1.34 1.34 1.43 1.39 1.28 1.37 1.25 1.18
Viscosity
Pre-eruption 1060◦ log η (Pa s) 2.26 2.41 2.40 2.39 2.49 2.44 2.27 2.45 2.25 2.19
η (Pa s) 180.29 254.44 248.91 247.66 311.50 276.50 184.39 281.06 178.57 155.12
Degassed 1060◦ log η (Pa s) 3.15 3.40 3.39 3.38 3.56 3.47 3.19 3.47 3.15 3.04
η (Pa s) 1419.27 2500.54 2429.23 2415.85 3638.89 2963.15 1539.00 2955.40 1423.23 1091.63
Viscosity estimated by the VTF equation: log η (Pa s) = A + B/[T(K)-C] and model coefficients see Giordano et al. (2008).
Pre-eruptive water content estimated from the H2O/K2O relationship from the 1991 samples by Lucic et al. (2016). De-
gassed magma water content from Höskuldsson et al. (2007). – Seigjan er áætluð með VTF jöfnunni: log η (Pa s) = A + B
/ [T (K) -C] og líkanstuðlarnir eru frá Giordano o.fl. (2008). Vatnsinnihald fyrir gos er áætlað út frá H2O / K2O hlutfalli
úr sýnum frá 1991 eftir Lucic o.fl. (2016). Afgasað kvikuvatnsinnihald er frá Höskuldssyni o.fl. (2007).
ing H2O/K2O of 0.98 following the relationship from
Lucic et al. (2016) for the average of the 1991 sam-
ples. This results in an average of 1.3 wt% H2O in our
samples (Table 2). Based on these assumptions the
estimates of pre-eruptive melt viscosity ranges from
1.8×102 – 3.1×102 Pa s (1060◦C) for the 1766–68
lava-flows, and 1.6×102 – 2.8×102 Pa s (1060◦C) for
the 1845–46 lava-flows (Table 2). The whole rock
data of our samples are also comparable to the 2000
Hekla samples from Höskuldsson et al. (2007) from
which they measured 0.14 wt% water at a correspond-
ing liquidus temperature of 1136◦C as the component
representative of degassed magma. To estimate the
viscosity of degassed magma represented by our sam-
ples we apply this H2O content of 0.14 wt% (Table
2). This results in melt viscosity ranges of 1.4×103
– 3.6×103 Pa s (1060◦C) for the 1766–68 lava-flows,
and 1.1×103 – 2.9×103 Pa s (1060◦C) for the 1845–
46 lava-flows (Table 2). Our viscosity results show
one order of magnitude difference between unde-
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