Unlocking the Mysteries of Volcanic Tsunamis: Santorini’s Seafloor Reveals Clues

A team of researchers from the GEOMAR Helmholtz Centre for Ocean Research Kiel, in collaboration with other institutions, has delved into the aftermath of Santorini’s Minoan eruption, unearthing the origins of the devastating tsunami that occurred 3,600 years ago. By analyzing seismic data and bathymetry of the seafloor, the study sheds light on the significance of understanding flank instabilities in assessing tsunami risks associated with active volcanoes.

The Aegean seafloor bears crucial imprints that serve as valuable indicators for a more accurate evaluation of volcanic tsunami hazards.

The colossal eruption of Santorini during the Late Bronze Age stands as one of the most significant explosive volcanic events in the Holocene period. This catastrophic event shattered Santorini, hurling massive quantities of scorching volcanic material and gas across the eastern Mediterranean. The ensuing colossal tsunami wreaked havoc on the coast of Crete, devastating numerous Minoan settlements. Today, the island remains as the only evidence of Santorini’s immense past, with its large caldera rims protruding from the sea.

Driven by a desire to unravel the cause of the massive tsunami that occurred 3,600 years ago, a team led by marine geophysicist Dr. Jens Karstens embarked on a comprehensive study of the ocean floor’s enduring impact. In October 2019, they collected new geophysical data using the research vessel R/V Poseidon around the Santorini Caldera, employing sound waves to create a detailed structural image of the subsurface. Combining these new seismic profiles with previously collected swath bathymetry data from international expeditions, the researchers gained insights into the water depth and intricate morphology of the seafloor.

The high-resolution seismic profiles revealed distinct undulating seafloor bedforms, resembling wavy sediment structures, which can be observed radially around the caldera and extend up to 25 kilometers from the volcano. In their recently published study in the scientific journal Earth and Planetary Science Letters, the researchers demonstrate the importance of these structures in understanding the genesis of volcanic tsunamis.

Volcanic tsunamis have long remained enigmatic due to the complex interplay of events that trigger them. Dr. Jens Karstens explains, “Reconstructing the seafloor morphology is a step forward in comprehending the generation of tsunamis during large eruptions.”

Seafloor bedforms, akin to ripples or dunes found in riverbeds or on beaches, develop at the interface of water and the ocean floor as sediment is transported by flowing water. In the case of Santorini, these sediments were formed when currents of dense hot gas and volcanic rocks, known as pyroclastic flows, cascaded into the ocean at high speeds from the volcanic flanks. Another process that contributes to these undulating bedforms is the destabilization of sediments on the volcano’s slopes.

Dr. Karstens describes their analysis of the new seismic reflection data, stating, “We have discovered that the deposits around Santorini are not uniform. They are thicker to the north and thinner on the other flanks.” The undulating seafloor bedforms to the north were formed by pyroclastic flows, while the researchers observe evidence on the other flanks indicating that these sediment structures are linked to instabilities of the volcanic flanks, believed to have been formed or reactivated during the Minoan Eruption. “While we cannot precisely reconstruct the dynamics of the mass movement, including the speed at which the rock masses slid down the slopes of Santorini, our findings suggest their contribution to the destructive Minoan tsunami,” adds the geophysicist.

During the massive caldera-forming eruption, a tremor triggered segments of the volcanic slope to slide hundreds of meters downward, displacing vast amounts of water and generating a colossal tsunami wave. The researchers estimate that approximately two cubic kilometers of material moved down the volcanic flanks during the Minoan eruption. In comparison, a recent collapse-generated tsunami in 2018 at Anak Krakatau, which devastated the coasts of the Sunda Strait in Indonesia, displaced only one-tenth of the equivalent total volume.

Deformations of the subsurface, reaching 200 meters below the seafloor, indicate that slope instabilities can also be reactivated by regional tectonic earthquakes with magnitudes exceeding 7 (M7+) in the Santorini area. These earthquakes have caused destructive tsunamis in the past.

Dr. Morelia Urlaub, professor for marine geomechanics at GEOMAR and leader of the PRE COLLAPSE project, emphasizes, “This study underscores the importance of understanding flank instabilities in assessing the tsunami hazard at active volcanoes.” The Minoan eruption remains one of the most extensively studied volcanic eruptions globally, and Santorini presents a unique opportunity to correlate the formation of undulating seafloor deposits with volcanic processes.

In August 2023, members of the PRE COLLAPSE research group will visit Anak Krakatau with the research vessel RV Sonne to conduct similar seismic and bathymetric surveys, studying the eruptions that occurred in 1883 and 2018. Leveraging the newly acquired knowledge about undulating seafloor bedforms, they aim to interpret and compare the subsurface structures at Krakatau and Santorini, and evaluate how these findings can contribute to hazard assessment at other active marine volcanoes.

Reference: “Formation of undulating seafloor bedforms during the Minoan eruption and their implications for eruption dynamics and slope stability at Santorini” by Jens Karstens, Jonas Preine, Steven Carey, Katherine L.C. Bell, Paraskevi Nomikou, Christian Hübscher, Danai Lampridou, and Morelia Urlaub, 7 June 2023, Earth and Planetary Science Letters.
DOI: 10.1016/j.epsl.2023.118215

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More about volcanic tsunamis

• GEOMAR Helmholtz Centre for Ocean Research Kiel: Website
• Earth and Planetary Science Letters: Journal
• University of Rhode Island: Website
• University of Hamburg: Website
• Ocean Discovery League: Website
• University of Athens: Website
• R/V Poseidon: Vessel Information
• PRE COLLAPSE Project: Project Information
• Anak Krakatau: Volcano Information
• RV Sonne: Vessel Information