Jökull - 01.01.2020, Page 51

Vestergaard et al.
Figure 5. Whole rock and viscosity estimates of the Hekla 1766–68 and 1845–46 samples analysed in this study.
Panels show A) MgO and B) K2O versus SiO2 content, and C) our viscosity estimates versus temperature for
respectively pre-eruptive (undegassed) and degassed melts. – Efnagreiningar og mat á seigju Heklusýnanna frá
hraunbreiðunum 1766–68 og 1845–46. Myndirnar sýna A) MgO, B) K2O samanborið við SiO2 innihald og C)
Mat á seigju miðað við hitastig kvikunnar áður en hún gaus og eftir (afgösuð kvika).
gassed and degassed magma. Emplacement dates and
morphology surface structures related to the sampled
lava-flows are listed together with the samples’ bulk
rock composition and viscosity estimates (Table 2).
DISCUSSION
Lava bulk volumes and accuracy of the planimet-
ric method
New volume estimates for the 1766–68 and 1845–46
lava-flow fields are provided. Thórarinsson (1967)
previously estimated the volume to 1.3 km3 and
0.6 km3, respectively, but the basis of these estimates
remains unclear (e.g. method of thickness estimates
and the number and location of measurements). In
contrast the bulk volume estimates of this study gives
volumes of 0.7 km3 (47% smaller) and 0.4 km3 (42%
smaller), respectively. Because of the inherent er-
rors connected with the assumptions of the plani-
metric method (e.g. the limitation of thickness esti-
mates to the lava margins), it is impossible to esti-
mate an error. However, it is possible to get an idea
of the accuracy by comparing the volume estimates
for the 1947–48 lavas calculated with the planimetric
method with the volume estimates from Pedersen et
al. (2018a). They made accurate lava-flow thickness
maps for the 20th century lava-flows by differenc-
ing pre- and post-eruption DEMs generated from his-
torical stereo photographs. The planimetric method
yields for the 1947–48 lava-flow field a volume esti-
mate of 0.4 km3, which is 44% lower compared to the
volume result of 0.742±0.138 km3 by Pedersen et al.
(2018a). Likewise, Montalvo (2013) provided a vol-
ume estimate of 0.07 km3 for the 1980-81 lava-flow
field using the planimetric method, which is nearly
60% smaller than the estimated 0.169±0.016 km3 by
48 JÖKULL No. 70, 2020