Sinkholes observed at Vesturbunki, Surtsey

We report observations and monitoring of sinkholes and tensional fissures occurring along the outer margin of the lava shield within Vesturbunki, one of Surtsey’s two tuff cones. The features were first noted in summer 2015 and traced nearly continuously along roughly 500 m of the lava–tuff contact. In 2015, 49 sinkholes and fissures were observed; subsequent field work in 2017, 2019 and 2025 recorded approximately 20 features per visit, including some previously unrecorded openings. Field data includes GPS locations and photographs. To contextualize these observations, we compare them with geodetic and seismic data from the area and report maximum surface temperatures for the two tuff cones, Vesturbunki and Austurbunki, from surveys we conducted between 2008 and 2025. Earlier geodetic data on Surtsey indicates ongoing subsidence of the island, with a decaying rate since the eruption and Austurbunki is more stable than Vesturbunki. Seismicity near Surtsey during the years 2009–2024 was examined and was found to be low to moderate with five events ≥M 2.5. The seismic hypocentre locations are uncertain because of sparse station coverage. Surface temperature monitoring shows Vesturbunki is consistently hotter, up to 100°C, and fluctuates by up to 4.5°C between measurements, while Austurbunki is stabler and slightly cooler, up to 96°C, with only 2°C fluctuations. The sinkholes were likely produced and maintained by a combination of island-wide subsidence, local compaction of volcanogenic material, and possibly by seismicity that may trigger opening of fissures in the palagonite tuff cone that enhance upward heat flow locally and temporarily. Continued monitoring and mapping, as well as installation of a local seismic station on Surtsey, is recommended to better resolve the processes that produce these features.