'Fossil waves' left by huge underwater volcanoes offer clues on impact

Now, large caldera-forming eruptions are better-understood thanks to a multi-year study of ancient Santorini volcano deposits, which reveals new information on how future eruptions might affect the planet's climate.

Dr. Johan Gilchrist and Dr. Mark Jellinek examined the concentric terraces that surround the Santorini caldera—historically referred to as the Minoan eruption.

They found that, consistent with previous terraced caldera deposits, the terrace widths diminish with increasing distance from the vent and slope upward towards the caldera wall. Additionally, compared to calderas from subaerial or submarine eruptions, the terraces close to the caldera rim are far broader.

Dr. Gilchrist suspected that sedimentation waves collapsing periodically around the volcanic jet spread where they touched the ocean surface during shallow subsurface eruptions.

To verify this, the researchers imitated the undersea Minoan eruption by injecting particles into shallow water layers. They demonstrated that, depending on the eruption's severity and the water's depth, the descending sedimentation waves produced by shallow water eruptions can impact and propagate at the sea surface to produce tsunamis and scour the seafloor. 

A direct link between deposit shape and parental eruption

Significantly, the terraced deposits left a fingerprint of the eruption, shedding light on the magnitude of the sedimentation waves and how they interacted with the water and seafloor.

"This study... will guide a next generation of hydrovolcanic climate models aimed at understanding how the mass partitioning properties of eruptions like Hunga Tonga-Hunga Ha'apai—as well as the largest and most impressive volcanic phenomena in the geological record—minimize their effects on climate change," said Dr. Jellinek in the press release.

"For the case of three submarine caldera-forming eruptions, this study provides the first direct relationships between the deposit architecture and parental eruption conditions," added Dr. Gert Lube, a volcanologist with Massey University not involved in the study.

"The results of this study are intriguing and could possibly be extended to non-marine, caldera-forming, and smaller eruption events."

The full study was published in Nature Geoscience.

Study abstract:

Catastrophic, caldera-forming explosive eruptions generate hazardous ash fall, pyroclastic density currents and, in some cases, tsunamis, yet their dynamics are still poorly understood. Here we use scaled analogue experiments and spectral analysis of well-preserved concentric terracing of seafloor deposits built by submarine caldera-forming explosive eruptions to provide insights into the dynamics governing these eruptions and the resultant hazards. We show that powerful submarine eruption columns in collapsing regimes deliver material to the sea surface and seabed in periodic annular sedimentation waves. Depending on the period between successive waves, which becomes shorter with decreasing jet strength, their impact and spread at the sea surface and/or seabed can excite tsunamis, drive radial pyroclastic density currents and build concentric terraces with a wavelength that decreases with distance, or deposits that thin monotonically. Whereas the Sumisu (Izu–Bonin arc) caldera deposit architecture is explained by either a subaerial or deep-water model involving no interaction between sedimentation waves and the sea surface, those of the Macauley (Kermadec arc) and Santorini (Hellenic arc) calderas are consistent with a shallow-water model with extensive sedimentation wave–sea surface–seabed interactions. Our findings enable an explicit classification of submarine caldera-forming explosive eruption dynamics and quantitative estimates of eruption rates from their terraced deposits.