“Core on deck!”
For two months, whenever I heard that cry, I would run to the deck of the JOIDES Resolution to watch the crew pull up a 10-meter-long cylindrical tube filled with layered, multi-colored rocks and sediments they had drilled from the seabed beneath our ship.
In the winter of 2022, I spent two months sailing the southern Aegean Sea aboard the International Ocean Discovery Program’s JOIDES Resolution as part of IODP Expedition 398. My geology colleagues and I used this former oil exploration vessel to drill deep into the seafloor, revealing the volcanic history of the area off the coast of Santorini, Greece.
As a scientist who studies the chemistry of volcanic rocks, I use my expertise to correlate volcanic sediments with the eruption that produced them. I also try to understand the conditions that magma experiences, both deep beneath a volcano and during an eruption.
Our expedition’s seafloor drilling revealed a massive but previously unknown volcanic eruption that occurred more than 500,000 years ago. This discovery increases our understanding of volcanic activity in the chain of volcanoes that form the South Aegean Volcanic Arc, which will allow for a more accurate hazard analysis of this region.
Building a more complete volcanic history
Archaeologists have long been fascinated by Santorini’s late Bronze Age eruption around 1600 BC, which is associated with the decline of the Minoan civilization on the nearby island of Crete. Geologists are also interested in the region, due to the volatility of volcanic and seismic activity in the area, which has a population of about 15,000 and attracts about 2 million tourists a year.
Although there is considerable documentation of the Santorini volcano on land, scientists know that this record is incomplete. On land, erosion, vegetation, and additional eruptions often cover or obscure older volcanic deposits, resulting in a patchy history. The deep-sea drilling made possible by IODP’s JOIDES resolution gives researchers access to a geologic record that has rarely been preserved on land.
After a volcanic eruption, pyroclastic materials—pieces of rock and ash formed during the eruption—sink through the water column and collect on the seafloor. There, clays and biological material, such as the shells of small marine organisms, continually rain down, covering the volcanic rock deposits. This process preserves a record of an individual eruption as a single layer. Layers build up over time, with each successive volcanic event creating a nearly continuous chronological record of the region’s volcanic history.
The mission of Expedition 398 was to access these deep-sea data to document the extensive eruptive history in each area of concentrated volcanic activity.
IODP Expedition 398
IODP Expedition 398 collected cores to better understand the volcanic history and return interval of the Santorini, Christiana and Kolumbo volcanoes in this region. The JOIDES Resolution crew drilled 12 sites to a maximum depth of 2,950 feet (900 meters) below the seafloor. We retrieved over 11,000 feet (3,356 meters) of total cores, divided into 780 cores.
As technicians cut the core into 1.5-meter pieces, scientists gathered to see what material had been recovered. After the cores were pressurized, the team split them lengthwise, photographed them, analyzed them for physical properties such as magnetic susceptibility, and described the material. Core describers measure and record the geologic composition of each rocky unit within.
As the geochemistry lab leader, I collected small samples of multiple layers of volcanic rock and ash to dissolve and analyze for their trace element composition. During an eruption, magma crystallizes and mixes with elements in the water and rock it comes into contact with. The resulting chemical changes in the magma are unique to the circumstances of that particular eruption. So once I can determine the chemical composition of the deposit samples, I can determine their volcanic origin.
Our discovery: The Archaeos Tuff
During the expedition, our group of researchers discovered a thick, white pumice layer in several places, in different basins. Biostratigraphy on board ships dated each occurrence of the layer to the same age: between 510,000 and 530,000 years ago. Geochemical correlations suggested that the composition was also the same in all boreholes.
By finding the same layer in these basins, our research team can model how large the eruption that caused it might have been. We used seismic data collected during the expedition to determine that the bulk volume of the volcanic sediment is about 21 cubic miles (90 cubic kilometers), with thicknesses of up to 490 feet (150 meters) in places. In addition, we determined that this layer of volcanic rock spread across 1,100 square miles (3,000 square kilometers) of this region in the southern Aegean Sea.
Our team named this deposit Archaeos Tuff, derived from the Greek word archaeology for old. The name reflects the Greek origins of the rock, and also the fact that it was considerably older than much of the volcanic activity known on land.
The characteristics of the Archaeos Tuff help us understand the nature of the volcanic eruption that formed it. Its thickness and distribution over a large area suggest that the Archaeos Tuff is the result of a single, very intense eruption. The numerous vesicles, or small holes, in the rock indicate that a large amount of gas was released at the same time as the liquid magma. These small gas bubbles paint a picture of a powerful eruption that released a large amount of volatile gas quite quickly.
But despite its apparent size and intensity, this eruption did not correlate with previously known land deposits or major eruptions. The relative lack of material on land suggested a primarily submarine eruption. Once we knew what we were looking for, our team was able to match our newly discovered deep-sea layer of volcanic sediment to a few small, previously uncorrelated land deposits on the islands of Santorini, Christiana, and Anafi. The presence of these deposits suggests a break in the sea surface during the eruption, again supporting our picture of an energetic eruption.
Further research into the composition and age of the Archaeos Tuff confirmed the unique nature of the rock deposit left by this eruption. Based on the data collected, our team believes that the Archaeos Tuff is the result of an eruption six times larger than the Bronze Age Minoan eruption, leaving rock deposits 30 times thicker. The presence of such a large volcanic deposit tells us that the South Aegean Volcanic Arc is more capable of producing large submarine volcanic eruptions than scientists previously thought.
The identification of the Archaeos Tuff expands our knowledge of volcanic processes in the southern Aegean. It suggests a greater propensity for dangerous submarine volcanism than previously thought – and that officials should reassess the volcanic hazards to surrounding populations.
This article is republished from The Conversation, a nonprofit, independent news organization that brings you facts and reliable analysis to help you understand our complex world. It was written by: Molly Colleen McCanta, University of Tennessee
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Molly Colleen McCanta receives funding from IODP.