Seismic imagery from volcanoes on the Azores Plateau implies that explosive deep-water eruptions are more common than previously thought.

It is generally assumed that volatile-rich magmas erupt predominantly effusively at water depths of several hundred meters, as the high hydrostatic pressure suppresses explosive activity. This view is supported by numerous multibeam surveys showing that most submarine volcanoes lack explosive craters. Here, we present high-resolution reflection seismic and bathymetric data from submarine volcanoes located at more than 2 km water depth along the southern transtensional diffuse plate boundary of the Azores Plateau. The seismic reflection characteristics of the investigated edifice flanks resemble those of volcanoes elsewhere that have been verified by drilling to consist of highly fragmented volcaniclastic deposits, including ash, lapilli or pumice. Additional evidence for explosive activity comes from a crater that was subsequently filled with mounded coarser volcanic deposits, or even effusive lava, during the waning stages of eruption. Consequently, earlier multibeam studies were unable to detect indicators for explosive eruptions. Our results demonstrate that submarine volcanic morphology alone is insufficient to rule out explosive eruptions and highlight the value of seismic imagery to systematically identify explosive submarine volcanism. Deep volcanic explosions seem more common than generally admitted, even on oceanic plateaus. This has implications for magmatic volatile outputs and their regional to global environmental impacts.