“Core on deck!”
For two months, whenever I heard that scream, I ran to the deck of the JOIDES Resolution to watch the crew pull a 30-foot (10-meter) cylindrical tube filled with layers of multicolored rocks and sediment drilled into the seabed. below our ship.
In the winter of 2022, I spent two months sailing through the southern Aegean Sea aboard the International Oceanic Discovery Program’s JOIDES Resolution as part of IODP Expedition 398. My fellow geologists and I used this old oil exploration vessel to drill deep into the bottom from the sea and reveal the volcanic history of the area off the coast of Santorini, Greece.
As a scientist who studies the chemistry of volcanic rocksI use my experience to correlate volcanic sediments with the eruption that caused them and to understand the conditions that magma experienced both deep within a volcano and during an eruption.
Drilling the seabed by our expedition revealed a massive but previously unknown volcanic eruption which occurred more than 500 thousand years ago. This discovery expands our understanding of volcanic activity in the chain of volcanoes that comprise the South Aegean Volcanic Arcwhich will allow a more accurate hazard analysis of this region.
Constructing a more complete volcanic history
Archaeologists have long been fascinated by the eruption of Santorini in the late Bronze Age, around 1600 BC. This eruption is associated with the decline of the Minoan civilization on the neighboring island of Crete. Geologists also have a significant interest in the region due to the volatility of volcanic and seismic activity in this area which is home to around 15,000 residents and attracts around 2 million tourists per annum.
Although there is significant documentation on land from the Santorini volcano, scientists know that this record is incomplete. On land, erosion, vegetation, and other eruptive events often cover or obscure older volcanic deposits, resulting in a fragmented history. Deep-sea drilling made possible by IODP’s JOIDES Resolution gives researchers access to a geological record rarely preserved on land.
After a volcanic eruption, pyroclastic materials – pieces of rock and ash formed during the eruption – settle in the water column and accumulate on the sea floor. There, clays and biological material, such as shells from tiny marine organisms, rain down continuously, plugging the volcanic rock deposits. This process preserves the record of an individual eruption as a single layer. The layers are built up over time, with each successive volcanic event creating a nearly continuous chronological record of the region’s volcanic history.
Expedition 398’s mission was to access this deep-sea record to document the extensive history of eruptions in each area of concentrated volcanic activity.
IODP Expedition 398
The IODP 398 Expedition collected drill cores to better understand the volcanic history and recurrence interval of the Santorini, Christiana and Kolumbo volcanoes in this region. The JOIDES Resolution team drilled 12 locations to a maximum depth of 2,950 feet (900 meters) below the seabed. We recovered more than 11,000 feet (3,356 meters) of total core across 780 cores.
As technicians cut the core into 1.5-meter (4½-foot) sections, scientists gathered to see what material had been recovered. After bringing the cores to surface pressure, the team would split them lengthwise, photograph them, analyze their physical properties such as magnetic susceptibility, and describe the material. The main descriptors measure and record the geological composition of each rock unit contained within it.
As the geochemistry lab leader, I took small samples from multiple layers of volcanic rock and ash to dissolve in solution and analyze their trace element composition. During an eruption, magma crystallizes and mixes with elements in the water and rocks with which it comes into contact. The resulting chemical changes in the magma are unique to the conditions of that specific eruption. Once I discover the chemical composition of the deposit samples, I will be able to identify their volcanic origin.
Our Discovery: The Archaeos Tuff
During the expedition, our group of researchers discovered a thick, white layer of pumice in several locations in several basins. On board biostratigraphy 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 holes.
Finding the same layer in these basins allows our research team to model the size of the eruption that caused it. We used seismic data collected during the expedition to determine that the volume of the volcanic sediment is about 21 cubic miles (90 cubic kilometers), with thicknesses up to 490 feet (150 meters) in some places. Furthermore, we determined that this layer of volcanic rock was spread across 3,000 square kilometers of this region in the southern Aegean Sea.
Our team named this deposit Archaeos Tuff, from the Greek word archaea for old. The name reflects the rock’s Greek origin, as well as the fact that it is significantly older than much of the volcanic activity we know on land.
Based on the characteristics of the Archaeos Tuff, we can understand the nature of the volcanic eruption that formed it. Its thickness and distribution over a wide area suggest that the Archaeos Tuff is the result of a single high-intensity 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 tiny gas bubbles paint the picture of a powerful eruption in which a large amount of volatile gas was rapidly released.
However, despite its evident size and ferocity, this eruption did not correlate with any previously known terrestrial deposits or large eruptions. The relative lack of material on land suggests a primarily submarine eruption. Once we knew what we were looking for, our team was able to match the newly discovered layer of volcanic sediments on the seafloor with some small, previously uncorrelated terrestrial deposits on the islands of Santorini, Christiana and Anafi. The presence of these deposits indicates some disruption of the sea surface during the eruption, which again fits our image of an energetic eruption.
Further study of the composition and age of the Archaeos Tuff confirmed the unique nature of the rock deposit left by this eruption. Based on the data we collected, our team believes that the Archaeos Tuff is the result of an eruption six times larger than the Bronze Age Minoan eruption, leaving behind 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 underwater volcanic eruptions than scientists previously recognized.
The identification of Tuff Archaeos expands what we know about volcanic processes in the southern Aegean Sea. It suggests a greater propensity for dangerous underwater volcanism than previously thought – and that authorities need to reassess the volcanic dangers to the surrounding population.
This article was republished from The conversation, an independent, nonprofit news organization that brings you trusted facts and 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.
