Eruption at Sundhnúksgígar, August 2024 – preliminary petrographic and geochemical data
Eruption at Sundhnúksgígar, August 2024 – preliminary petrographic and geochemical data
Samples of quenched lava were collected on the second and third day of the eruption that started near Sundhnúksgígar on August 22nd, and on the fourth day, a sample of freshly fallen tephra was also collected. In all cases, the samples come from the area north of Stóra-Skógfell. The volcanic glass in the samples was analysed with the electron microprobe of the Institute of Earth Sciences, University of Iceland. Petrographically, the samples strongly resemble those collected in the earlier eruptions since December 2023 near Sundhnúksgígar (see earlier reports), and contain vesicular glass with plagioclase, olivine and clinopyroxene crystals (Fig. 1). The quenched lava samples have abundant microlites, in contrast with the tephra glass which is relatively microlite-poor.
Fig. 1. Backscattered electron image of a quenched lava sample (left side) from day 2 and a highly vesicular tephra sample (right side) from day 4 of the August 2024 eruption at Sundhnúksgígar. Plagioclase and olivine crystals can be seen in the quenched lava and plagioclase and clinopyroxene in the tephra.
Geochemical characteristics
The basaltic glass in the samples contains MgO in the range 5-7 wt%, similar to the previous 2023-24 eruptions at Sundhnúksgígar. The K2O/TiO2 of the glass is 0.11-0.16, nearly identical to the ratio measured in samples from the May-June 2024 eruption, but significantly lower than in earlier eruptions at Sundhnúksgígar when it was 0.21-0.23. This suggests that magmas with similar compositions have accumulated in the reservoir beneath Svartsengi since late April. The persistence of low K2O/TiO2 in basalts from May to August 2024 suggests that magma production and accumulation at depth has evolved towards a more steady-state, less dynamic system, which was a characteristic feature of basalts erupted during the medieval Reykjanes Peninsula Fires.