Jökull - 01.01.2020, Side 42

The 1845–46 and 1766–68 eruptions at Hekla volcano
Table 1. Overview of remote sensing data sources used for this study of the Hekla lavas 1766–68, 1845–46 and
1947–48. – Yfirlit yfir fjarkönnunargögn sem notuð voru til að rannsaka Hekluhraunin frá 1766–68, 1845–46
og 1947–48.
Acquisition Source Data (Resolution and m/pixel) Reference
23 Sep. 1945 & 20 Sep. 1946 Aerial photographs Ortho (1) + DEM (10) Pedersen et al., 2018a
20 July 1960 & 4 Aug. 1960 Aerial photographs Ortho (0.5) + DEM (5) Pedersen et al., 2018a
Mosaic 2011–2013 TanDEM DEM (12) Rizzoli et al., 2017
29 Aug. – 4 Sep. 2015 Aerial photographs & lidar Ortho (0.2) + DEM (1) Pedersen et al. (in prep.)
and data are processed in the geographic informa-
tion system program, ArcGIS. The orthophotos and
DEMs were provided by Pedersen et al. (2018a),
and were created using digital photogrammetric
techniques generated from repeated aerial stereo-
photogrammetric surveys conducted over Hekla since
1945 (see Pedersen et al., 2018a). The orthophotos
from 1945–46 are particularly important because they
reveal the full extent of the 1845–46 lava-flow field
and large parts of the 1766–68 lava-flow field before
being partly covered by the lavas from the 1947–48
eruption and later eruptions. The DEM from 2015
is mainly applied for the planimetric method (see
below), except for areas covered by younger flows,
e.g. the southern lava-flows of the 1766–68 eruption.
Where younger lava-flows cover the 1766–68 and
1845–46 lava-flow fields, the 1945-46 DEM is used,
and where the 1947–48 lava-flow field is covered by
younger lava-flows, the 1960 DEM is applied. More-
over, maps delineating Hekla’s lava units (inclusive
the 1766–68 and 1845–46) are provided by Pedersen
et al. (2018b) (Figure 1).
The planimetric method (Stevens et al., 1999) is
the only available approach to estimate the bulk vol-
umes of the 1766–68 and 1845–46 eruptions. We
follow the example of Montalvo (2013) who also
used the planimetric approach to estimate volumes
for the Hekla lava-flow fields from the eruptions in
1878, 1913 and 1980–81. Similarly, the recent in-
vestigation of El Reventador Volcano, Ecuador by
Naranjo et al. (2016) also used the planimetric method
together with topographic satellite radar-based mea-
surements to better distinguish between different lava-
flow eruptions from the total lava volume change
from 2002–2009. The bulk volume is calculated by
multiplying the area covered by lava with an esti-
mated mean lava thickness. The planimetric method
works best if the flow field is divided into morpholog-
ical zones where the thickness can be assumed rel-
atively constant (Montalvo, 2013). These morpho-
logical zones are mapped based on orthophotos and
slope maps from post-eruption DEMs. The area is
extracted for each zone (Figure 2f). Thickness esti-
mates are obtained by calculating the average thick-
ness based on multiple thickness measurements of
profiles along the lava-flow margins (Figure 2a). To
obtain reliable thickness estimates, it is important that
the area outside the flow field is fairly flat, and we
decide that thickness is extracted from the profiles as
soon as the lava-flow flattens, thus remaining consis-
tency throughout our thickness measurement extrac-
tions (Figure 2b,c,d,e). For zones where no thick-
ness estimates could be obtained, e.g. zones on steep
slopes, the average of all the other zone thicknesses
are used instead.
Historical sources and emplacement time-lines
The historical sources include primarily the study
by the Icelandic volcanologist Sigurdur Thórarins-
son (1912–1983) who carried out extensive work on
Hekla (Thórarinsson, 1967, 1976; Björnsson, 1983),
and secondarily descriptions by Danish geologist Jør-
gen Christian Schythe (1814–1877) and Hans Finnsen
(1739–1796) (Finnsen, 1767; Schythe, 1847; Thór-
arinsson, 1967; Bricka, 2019). Sigurdur Thórarins-
son collected and summarised literary sources and
compiled them into a chronological account of Hekla
eruptions from 1158 to 1947. The narratives from
J.C. Schythe and H. Finnsen recount the 1845–46 and
1766–68 eruptions, respectively. It should be noted
that H. Finnsen did not witness the beginning of the
1766–68 eruption (April 5, 1766) as he was in Copen-
hagen, but he arrived on July 16, 1766 at Skálholt
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