New Explosive Shallow-Marine Volcanoes on the Milos Shelf and near Kos Revealed by Seismic Reflection Data

One of the objectives of MULTI-MAREX-2 expedition (MSM135) aboard RV MARIA S. MERIAN was to assess potential geohazards associated with shallow-marine explosive volcanism along the South Aegean Volcanic Arc. In this context, we identified two previously undocumented polygenetic shallow-water volcanic systems on the Milos shelf and in the area of the Kos–Nisyros–Yali island group, i.e. in proximity to two major volcanic centers.

Bathymetric data reveal a circular depression approximately 2 km in diameter at a water depth of ~200 m and south of Milos. Seismic reflection data show outward-prograding, upward-concave reflection geometries within an approximately 50 m thick outer ring, which we interpret as volcaniclastic deposits formed by an explosive eruption. The interpretation of this structure as a shallow-water volcano is supported by a ring-shaped positive magnetic anomaly. The volcano-forming eruption may be linked to the formation of the Green Lahar deposits on Milos (T. Cavailhes, pers. comm.). Beneath the main edifice, seismic data image additional outward-prograding sedimentary units, likewise interpreted as volcaniclastic deposits, indicating a polygenetic evolutionary history. Hydroacoustic data reveal gas flares in the water column, documenting ongoing hydrothermal activity that may be responsible for the formation of the mapped sinkholes in the area.

In Kefalos Bay and along the southern coast of Kos, multibeam bathymetry reveals another shallow-water volcano with a crater diameter of approximately 2 km at the seafloor. The southern flanks of this edifice are collapsed, most likely as a result of lateral spreading above mechanically weak volcaniclastic deposits related to the Kos Plateau Tuff eruption at 161 ka. The crater itself is infilled by younger volcaniclastic deposits with a flat top in a water depth of ca. 180 m.

These findings demonstrate that explosive shallow-marine volcanism has occurred at multiple locations along the South Aegean Volcanic Arc more frequently than previously thought, and represents an underestimated geohazard, particularly in coastal regions close to populated areas. This study also demonstrates that seismic data are required to distinguish between monogenetic and polygenetic submarine volcanoes.