Jökull - 01.01.2020, Síða 38

Reviewed research article
The 1845–46 and 1766–68 eruptions at Hekla volcano:
new lava volume estimates, historical accounts and
emplacement dynamics
Rikke Vestergaard1∗, Gro Birkefeldt Møller Pedersen2,3 and Christian Tegner1
1Aarhus University, The department of Geoscience, Centre of Earth System Petrology
2Institute of Earth Sciences, University of Iceland, Reykjavík, Iceland
3Nordic Volcanological Center, University of Iceland, Reykjavík, Iceland
∗Corresponding author: rikke.vest94@gmail.com
https://doi.org/10.33799/jokull2020.70.035
Abstract — We use new remote sensing data, historical reports, petrology and estimates of viscosity based on
geochemical data to illuminate the lava emplacement flow-lines and vent structure changes of the summit ridge
of Hekla during the large eruptions of 1845–46 and 1766–68. Based on the planimetric method we estimate
the bulk volumes of these eruptions close to 0.4 km3 and 0.7 km3, respectively. However, comparison with
volume estimates from the well-recorded 1947–48 eruption, indicates that the planimetric method appears to
underestimate the lava bulk volumes by 40–60%. Hence, the true bulk volumes are more likely 0.5–0.6 km3
and 1.0–1.2 km3, respectively. Estimated melt viscosity averages for the 1766–68 eruption amount to 2.5 ×
102 Pa s (pre-eruptive) and 2.5×103 Pa s (degassed), and for the 1845–46 eruption 2.2×102 Pa s (pre-eruptive)
and 1.9×103 Pa s (degassed). Pre-eruptive magmas are about one order of magnitude more fluid than degassed
magmas. In the 1845–46 and 1947–48 eruptions, SiO2 decreased from 58–57 to 55–54 wt% agreeing with
a conventional model that Hekla erupts from a large, layered magma chamber with the most evolved (silica-
rich) magmas at the top. In contrast, the lava-flows from 1766–68 reveal a more complicated SiO2 trend. The
lava fields emplaced in 1766 to the south have SiO2 values 54.9–56.5%, while the Hringlandahraun lava-flow
that erupted from younger vents on the NE end of the Hekla ridge in March 1767 has higher SiO2 of 57.8%.
This shows that the layered magma chamber model is not suitable for all lava-flows emplaced during Hekla
eruptions.
INTRODUCTION
The Hekla volcano is one of the four most active vol-
canic systems in Iceland, having erupted about 23
times since the settlement of Iceland in the year 874
(Thórarinsson, 1967; Thordarson and Larsen, 2007;
Höskuldsson et al., 2007; Pedersen et al., 2018a,b).
Hekla therefore presents one of the major volcanic
hazards of Iceland (Einarsson, 2018; Barsotti et al.,
2019). So far, the older historical eruptions have
mainly been studied from literary sources in combi-
nation with tephra chronology, which provides a great
record of the explosive activity and its related hazards
(Thórarinsson, 1967; Sverrisdottir, 2007; Gudnason et
al., 2017, 2018; Janebo et al., 2016a,b, 2018). Hekla
eruptions generally start with subplinian to plinian ex-
plosive eruption plumes up to 12–36 km high (Thor-
arinsson, 1967; Höskuldsson et al., 2007), and typi-
cally gives no to little warning (Grönvold et al., 1983;
Soosalu et al., 2003; Einarsson, 2018). This makes
future Hekla eruptions hazardous for air traffic, es-
pecially since one of the busiest air traffic corridors
in the world runs straight over Hekla (Soosalu et al.,
JÖKULL No. 70, 2020 35