Jökull - 01.01.2020, Page 47

Vestergaard et al.
dashed outline). Mentions of lava were once again
noted on September 19, where lava flowed both to the
west and to the east in the direction of Krakatindur
(Figure 3a green dashed outline). The eruptive activ-
ity in general is described as being variable between
April 9 and September 19 with explosive activity and
earth tremors. It is therefore likely that an extension
of the vent opening of the Hekla ridge and possibly
new vent openings related to the larger vent extension
to the NE may explain how the lava could flow to-
wards Krakatindur (Figure 3b purple dashed outline
and solid light-green outline). The lavas that flowed
generally towards S-SW from the Hekla ridge display
complex morphology, e.g. lava-flows resembling ar-
teries which are signs of vertical stacking of numer-
ous surges of lava. In addition, inflation structures are
prominent, implying stalling of lava, and (often) in
connection to breakout structures. It appears from the
aerial orthophotos that there are at least two types of
breakout structures representing two flow rates. One
type looks thinner and smoother, slightly similar to
the sheet-like-flows observed. The second type lavas
look bulky and have not flowed as far unlike the afore-
mentioned type. The literary sources from Thórar-
insson (1967) and Einarsson (1949) mention a "lava
flood"-type and a "toothpaste-tongue"-type, respec-
tively, thus there is a consistency between the observa-
tions of the aerial orthophotos and the literary sources
of two different types of breakouts. The lava-flow to-
wards E in the direction of Krakatindur is a sheet-like-
flow (Figure 3a green dashed outline E from Hekla
ridge), thus, in contrast to the main lava-outpouring
to the S-SW of the Hekla ridge, this lobe likely repre-
sents a single surge of lava. In the beginning of the
year 1767 Hekla seemed to "loose its breath" with
the whole of February being quiet. On March 18,
1767, a large eruption began again with detonations,
an eruption column, some ash fall, and rapid lava-flow
flowing to the North (the lava-flow called Hringlanda-
hraun) until March 26 (Figure 3a turquoise solid out-
line). This suggests that a new vent (or vents) became
active on the north-eastern part of the summit ridge
(Figure 3b green outline). Similar to the lava flowing
towards Krakatindur, the Hringlandahraun is a sheet-
like-flow, thus representing a faster, single surge of
lava. Hereafter, there are no descriptions of lava ac-
tivity. From April 1 to June 1, 1767, Hekla’s activity is
described as fluctuating in regard to its explosiveness,
emission of ash, fire and smoke. Hekla was quiet the
whole month of June. In July and August little activity
in the form of ash and smoke emerged. After August
and the remainder of 1767, Hekla was quiet. In March
and April 1768 "Hekla seemed to wish to waken from
its slumber again". Smoke and fire were observed by
people on May 11 and 30. Following May 30, 1768,
the eruption ended.
The detailing of the effusive activity is rather
sparse, therefore, few dates of lava emplacement can
be deduced and outlined on the map (Figure 3a). De-
tailed description of the southern lava flow field exists
for April 9, while further deduction of extent is uncer-
tain. We are certain that the descriptions of new in-
creased explosive activity resulting in new vent open-
ings on the north-eastern side of the Hekla ridge on
ca. March 18, 1767 resulted in the emplacement of
the Hringlandahraun lava field (Figure 3a turquoise
solid outline), because the previously described vents
are unable to emplace lava at that location. Further-
more, the observed morphology of Hringlandahraun
is a sheet-like-flow, which fits the description of rapid
lava emplacement. Thus, the Hringlandahraun was
emplaced at or after March 18, 1767, implying it is
younger than the lava field to the S-SW and E of the
Hekla ridge.
The entire area of the exposed flow field is
49.8 km2. For the planimetric method, 44 thickness
profiles were measured along the lava fronts and un-
evenly distributed indifferent to the nine zone divi-
sions. The estimated flow thicknesses vary from 5.3–
22.8 m and the average thickness for the entire flow is
13.7 m. The estimated bulk volume is 0.7 km3.
The 1845–46 eruption
The following account is based on the description
of Schythe (1847) and Thórarinsson (1967). After
a repose period of 77 years, Hekla showed activity
again on September 2, 1845. The opening phase was
plinian, lasted approximately one hour and produced
7.5×1010 kg and 0.13 km3 tephra deposited from a 19
km high eruption plume to the ESE of Hekla (Gudna-
son et al., 2018). Following the plinian phase, both
44 JÖKULL No. 70, 2020