Jökull - 01.01.2020, Page 48

The 1845–46 and 1766–68 eruptions at Hekla volcano
tephra and lava erupted. Lava emission on the first day
was described as being very large and flowing swiftly.
In the beginning (September 2, 1845), lava mainly
erupted from the central crater and the crater on the
SW shoulder of the Hekla ridge (Figure 4b green and
turquoise outlines). Additionally, three craters are re-
ported locating closely together (forming a fissure)
next to the crater on the SW shoulder of the ridge (Fig-
ure 4b dark red dashed outline). The smallest, low-
est and south-westernmost on the ridge of the three
craters is considered the main source of the 1845–
46 eruption, and from which the youngest 1845–46
lava erupted. It is, however, not clear when the small-
est crater took over the main activity, but it is certain
that all five craters were active from the beginning of
September 2, 1845. The lava flowed mainly W and
NW in the direction of the Melfell ridge and towards
the old Næfurholt farm (Figure 4a). The morphol-
ogy is mainly characterised by channelised flows in
the central area of the flow field and vertical stack-
ing of lavas closest the Hekla ridge. On September
9, the lava fronts were nearly 2 km east of Melfell
(Figure 4a orange outlines). On September 12, the
lava-flow began to advance westwards on both sides
of Melfell (Figure 4a red outlines). On September 21,
the lava was described having advanced 183 m fur-
ther westwards between Melfell and Markhlíð (Fig-
ure 4a light-pink outline) and in the following two
days the people residing at the Næfurholt farm fled
because the lava cut off their water supply, and they
feared that the lava would bury their farm. However,
at about that time the lava is described having changed
its course to flow towards NNW on the east side of
Melfell (Figure 4a light-pink dashed outline). Thus
the lavas stalled and resulted in inflation structures
visible on the aerial orthophotos. In general, inflation
structures are observed at flow margins and in connec-
tion with breakout structures, e.g. the lava flowing be-
tween Melfell and Markhlíð is the result of a breakout
(Figure 4a light-pink and dark-purple solid outlines).
In some areas (e.g. NW of Litla Hekla) the break-
outs somewhat resembles cauliflowers (maybe faster-
flowing breakout?). On October 18, a new, small lava-
flow began at the summit ridge crater, or as a break-
out from the main existing lava stream, and flowed
SW in the direction of a place later called Þrætu-
stígur (Figure 4a dark-pink outline). On November
14, the main lava-flow field (and breakout) had com-
pletely encircled Melfell (Figure 4a dark-purple out-
line), and on November 19 it had flowed down into
the gully nearest the Næfurholt farmstead. The lava
stopped at its maximum length of ca. 10 km from the
summit ridge on November 25 having passed close
by the Næfurholt farmstead (Figure 4a blue outline).
After November 25 and to the end of November,
the eruption apparently paused. After this approx-
imately month long pause, lava activity is reported
(unclearly) occurring on December 27, 1845, January
26-February 5, 1846 and March 3–16, 1846 mainly
with lava emission flowing towards N-NW, passing
Litla Hekla on its west side (Figure 4a green dashed
outlines). Thórarinsson (1967) reports pauses in the
lava emission and the explosive activity in the peri-
ods November 17-30, 1845 and January 25 to March
2 and March 23-24, 1846 which contradicts the above
mentioned dates of lava activity, thus it is uncertain
when the pauses occurred, and what the pauses ex-
actly mean. The 1845–46 eruption ended April 5–10,
1846. A possible small recurrence in the form of a
dense cloud rising from the volcano emerged August
13–16, 1846.
The entire area of the flow field is 24.8 km2. The
area of the nine zones and their average thickness
(measure by a total of 37 thickness profiles) reveal
thicknesses from 4.4–28.1 m (Figure 2f). The esti-
mated bulk volume is 0.4 km3, and the average thick-
ness for the entire flow field is 14.7 m.
Bulk rock compositions and viscosity
This study’s samples have an average MgO content
of 2.55 wt% which corresponds to a melting tem-
perature of 1060◦C according to Thy et al. (2006).
Hence, we apply the temperature 1060◦C in the Gior-
dano et al. (2008) model. Our samples are mostly
basaltic andesite with 54–58 wt% SiO2 (Table 2),
and the two eruptions 1766–68 and 1845–46 demon-
strate similar trends of MgO decreasing and K2O in-
creasing respectively with increasing SiO2 (Figure 5a,
b). These compositions are comparable to the 1991
Hekla samples from Lucic et al. (2016). Hence, pre-
eruption H2O of our samples can be estimated assum-
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