Seafloor deposits reveal previously unknown volcanic eruption 520,000 years ago within the southern Aegean

“Core on deck!”

For two months, each time I heard that scream, I ran to the deck of the JOIDES Resolution to observe the crew hoist up a ten-meter-long cylindrical tube stuffed with layered, colourful rock and sediment they’d drilled from the seabed beneath our ship.

In the winter of 2022, I spent two months cruising the southern Aegean Sea aboard the International Ocean Discovery Program's JOIDES Resolution as a part of IODP Expedition 398. My fellow geologists and I used this former oil exploration vessel to drill deep into the seabed and uncover the volcanic history of the world off the coast of Santorini, Greece.

As a scientist who investigates the chemistry of volcanic rocksI exploit my expertise to correlate volcanic deposits with the eruption that caused them and to know the conditions magma was exposed to each deep beneath a volcano and through an eruption.

Our expedition’s drilling on the ocean floor revealed a massive but previously unknown volcanic eruption which occurred greater than 500,000 years ago. This discovery expands our understanding of the volcanic activity within the chain of volcanoes that South Aegean Volcanic Arcwhich is able to enable a more accurate hazard evaluation of this region.

Building a more comprehensive volcanic history

Archaeologists have long been fascinated by the eruption of Santorini within the late Bronze Age around 1600 BC. This eruption is related to the decline of the Minoan Civilization on the nearby island of Crete. Geologists even have great interest within the region attributable to the volatility of volcanic and seismic activity in the world, which is about 15,000 inhabitants and pulls around 2 million tourists per yr.

Although there’s extensive documentation of Santorini volcano on land, scientists realize it is incomplete. On land, older volcanic deposits are sometimes covered or obscured by erosion, vegetation and extra eruptions, leading to a fragmentary history. The deep-sea drilling enabled by the IODP's JOIDES resolution gives researchers access to a geological documentation rarely preserved on land.

After a volcanic eruption, pyroclastic materials – rock fragments and ash formed through the eruption – settle through the water column and collect on the ocean floor. There, clay and biological material, akin to the shells of tiny marine organisms, continually rain down and canopy the volcanic rock deposits. This process preserves the record of a single eruption as a single layer. The layers construct over time, with each successive volcanic event creating an almost continuous chronological record of the region's volcanic history.

The mission of Expedition 398 was to access these deep-sea records to document the extensive history of eruptions in each area of ​​concentrated volcanic activity.

IODP Expedition 398

IODP Expedition 398 collected cores to raised understand the volcanic history and return interval of the Santorini, Christiana and Kolumbo volcanoes on this region. The JOIDES Resolution crew drilled at 12 locations to a maximum depth of 900 meters below the seafloor. We recovered a complete of over 780 cores with a complete length of greater than 3,356 meters.

While technicians cut the core into 1.5-meter-long sections, scientists gathered to see what material had been recovered. After subjecting the cores to surface pressure, the team split them lengthwise, photographed them, analyzed them for physical properties akin to magnetic susceptibility, and described the fabric. Core describers measure and document the geological composition of every rock unit contained inside.

As head of the geochemistry lab, I took small samples from multiple layers of volcanic rock and ash to dissolve and analyze them for his or her trace element composition. During an eruption, magma crystallizes and mixes with elements within the water and rock it comes into contact with. The resulting chemical changes within the magma are unique to the conditions of that specific eruption, so once I determine the chemical composition of the deposit samples, I can determine their volcanic origin.

Our discovery: The Archaeos Tuff

During the expedition, our research group discovered a thick, white layer of pumice in several places in numerous basins. Biostratigraphy each occurrence of the layer was dated to the identical age: between 510,000 and 530,000 years ago. Geochemical correlations suggested that the composition was also the identical in all boreholes.

Finding the identical layer in these basins allows our research team to model how large the eruption that caused it might need been. Using the seismic data collected through the expedition, we were in a position to determine that the full volume of volcanic sediment is about 90 cubic kilometers, and in some places it’s as much as 150 meters thick. In addition, we found that this layer of volcanic rock was distributed over 3,000 square kilometers of this region within the southern Aegean.

Our team named this accretion Archaeus Tuff, after the Greek word Archaea for ancient. The name reflects the Greek origin of the rock and the proven fact that it was much older than much of the volcanic activity we all know of on land.

The properties of the Archaeos tuff help us understand the character of the volcanic eruption that formed it. Its thickness and distribution over a large area suggest that the Archaeos tuff is the results of a single, high-intensity eruption. The quite a few bubbles, or tiny holes, within the rock indicate that a considerable amount of gas was released similtaneously the liquid magma. These small gas bubbles paint an image of a robust eruption that released a considerable amount of volatile gas quite quickly.

Yet, despite its apparent size and ferocity, this eruption didn’t correlate with previously known land deposits or large eruptions. The relative lack of fabric on land suggests a primarily submarine eruption. Once we knew what we were searching for, our team was in a position to match our newly discovered deep-sea layer of volcanic sediment with some small, previously uncorrelated land deposits on the islands of Santorini, Christiana, and Anafi. The presence of those deposits suggests a breach of the ocean surface through the eruption, which again suits our picture of a high-energy eruption.

Further studies of the composition and age of the Archaeos Tuff confirmed the distinctiveness of the rock deposit left by this eruption. Based on the info we collected, our team believes that the Archaeos Tuff is the results of an eruption six times larger than the Bronze Age Minoan eruption and left rock deposits 30 times thicker. The presence of such a big volcanic deposit tells us that the South Aegean Volcanic Arc is more capable of manufacturing large submarine volcanic eruptions than scientists previously thought.

The identification of the Archaeos tuff expands our knowledge of volcanic processes within the southern Aegean. It suggests that the danger of submarine volcanism is larger than previously thought – and that authorities must reassess the volcanic hazards to the encircling population.