Eruption at Sundhnúksgígar, April 2025 – Preliminary petrographic and geochemical results

Eruption at Sundhnúksgígar, April 2025 – preliminary petrographic and geochemical results

Quenched lava samples were collected on the 1^st April from the southern end of the fissure. The samples were prepared for microscopy and chemical analysis at the Institute of Earth Sciences (IES), University of Iceland. The images and data shown here were obtained using the electron microprobe at IES.

The initial results show that the erupted lava has a similar composition to the previous eruptions at Sundhnúksgígar (Figure 1). The MgO content reflects the extent of cooling and crystallisation of the magma; the preliminary data from this eruption shows an MgO content within the range of the previous eruptions (Figure 1A). The K₂O/TiO₂ ratio records changes in the magma supply from depth. Although the preliminary data suggests a small change relative to the most recent three eruptions, it is well within the range seen since December 2023. Together, this shows there has been no significant change in the conditions of magma storage beneath Svartsengi since the onset of these events.

Figure 1: Time series of the volcanic glass composition of lavas erupted at Sundhnúksgígar. The data for the first four eruptions is from Matthews et al. (2024). The shading indicates the range in volcanic glass composition from the Medieval Reykjanes Fires from Caracciolo et al. (2023).
 

Crystals are abundant in the lava (Figure 2). The small crystals formed during rapid ascent to the surface, consistent with the short period of unrest prior to eruption. The larger crystals formed during extended storage in the magma domain beneath Svartsengi, consistent with the magmas having accumulated slowly over months.

Figure 2: Back Scatter Electron Image of the lava showing crystals of plagioclase (plag; dark grey), crystals of olivine (olv; intermediate grey), and volcanic glass (light grey).

References

Caracciolo, A., Bali, E., Halldórsson, S. A., Guðfinnsson, G. H., Kahl, M., Þórðardóttir, I., Pálmadóttir, G. L., & Silvestri, V. (2023). Magma plumbing architectures and timescales of magmatic processes during historical magmatism on the Reykjanes Peninsula, Iceland. Earth and Planetary Science Letters, 621. https://doi.org/10.1016/j.epsl.2023.118378

Matthews, S. W., Caracciolo, A., Bali, E., Halldórsson, S. A., Sigmarsson, O., Guðfinnsson, G. H., Pedersen, G. B. M., Robin, J. G., Marshall, E. W., Aden, A. A., Gísladóttir, B. Y., Bosq, C., Auclair, D., Merrill, H., Levillayer, N., Low, N., Rúnarsdóttir, R. H., Johnson, S. M., Steinþórsson, S., & Drouin, V. (2024). A dynamic mid-crustal magma domain revealed by the 2023 to 2024 Sundhnuksgigar eruptions in Iceland. Science, 386(6719), 309-314. https://doi.org/10.1126/science.adp8778