Jökull - 01.01.2020, Side 53

Vestergaard et al.
Figure 6. A) Lava-flow-lines of the 1766–68 and 1845–46 eruption (explanations given in Figures 3 and 4 are
applicable here) combined with sample location and SiO2 wt% content (Table 2). Background and boundary
outlines as in Figure 1. B), C) and D) are panels showing SiO2 of the lava samples versus their emplace-
ment ages respectively for the 1947–48, 1845–46 and 1766–68 eruptions. Sample points placed from older to
youngest lavas towards the right on the x-axis. – Þróun útbreiðslu hraunflákanna árin 1766–1768 og 1845–46
(sjá skýringatexta á myndum 3 og 4), ásamt staðsetningu sýnatökustaða og SiO2 hlutfalli í sýnum (tafla 2).
Bakgrunnur kortsins er sá sami og á 1. mynd. B), C) og D) sýna SiO2 hlutfall í sýnunum miðað við aldur
þeirra, sýnin eru frá eldgosunum 1947–48, 1845–46 og 1766–68. Elstu sýnin eru vinstra megin og þau yngstu
hægra megin.
change from ca. 57 to 54 wt% and ca. 58 to 55 wt%,
respectively. The 1766–68 eruption is more compli-
cated than such. Mainly one sample (18IS-16) from
the northern Hringlandahraun flow field differs from
the expected trend of a secular SiO2 decline (Figure
6a, d). This sample from the front of Hringlanda-
hraun flow field has higher SiO2 content (57.8 wt%)
compared to the samples from the southern lava-flows
(54.9-56.5 wt%) (Figure 6a). As described above,
Hringlandahraun erupted from the NE vent that was
active from March 18, 1767 (Figure 3a, b), thus orig-
inated from a later opening compared to the south-
ern lava-flow that emanated from craters active from
the beginning of the eruption (April 5, 1766). We do
not know if the sample from Hringlandahraun indi-
cates that the SiO2 content of the 1766–68 increased
during the eruption or if the SiO2 content fluctuated.
Nonetheless, we are certain of the emplacement pe-
riod of Hringlandahraun, because there are only de-
scriptions mentioning intense flow of lava to the north
50 JÖKULL No. 70, 2020