A volcanic debris field
The rocky outcrops of Bozcaada contain extensive deposits of volcanic tuff, a type of rock formed from the consolidation of material ejected during an explosive eruption. These layers are the physical record of violent Pleistocene-era events. Petrographic analysis of the tuff reveals a rhyolitic composition. The rock contains phenocrysts—larger, well-formed crystals—of quartz, plagioclase, biotite, and amphibole (specifically hornblende) within a fine-grained matrix of volcanic glass and lithic fragments, which are pieces of other rocks caught up in the eruption.
Rhyolitic magma is characterized by high silica content, making it extremely viscous. This thick consistency traps volcanic gases, allowing pressure to build to extreme levels before being released in a powerful, explosive eruption. The resulting ash, pumice, and rock fragments were carried by the wind and settled over a wide area, including present-day Bozcaada. Over time, these layers compacted and cemented together to form the tuff visible today. The presence of these specific minerals shows the origin of the magma's origin and the explosive nature of the eruption that created it.
Geochemical detectives
The volcanoes responsible for Bozcaada's tuff are no longer visible. Geological activity and rising sea levels since the Pleistocene have left them submerged beneath the Aegean Sea. Identifying the exact source of these ancient ash layers requires the science of tephrochronology. Researchers use geochemical fingerprinting to match the tuff on Bozcaada to a specific eruption event. The chemical composition of the volcanic glass shards within an ash layer—specifically the ratios of trace elements—is unique to the parent magma and the eruption. This signature is a marker, allowing a layer of ash found in one location to be correlated with the same layer found hundreds of kilometers away.
The work is part of a larger effort to map the volcanic history of the Eastern Mediterranean. A 2017 oceanographic expedition using the research vessel "RV Poseidon" collected dozens of sediment cores from the Aegean seafloor. These cores contain a nearly complete record of eruptions over the last 200,000 years, with over 220 distinct ash layers identified. This entire volcanic system is driven by the northward subduction of the African tectonic plate beneath the Eurasian plate, a process that has generated magma and fueled eruptions across the Aegean for millions of years. By analyzing the tuff on Bozcaada and matching its fingerprint to the sub-sea record, geologists can reconstruct the history of now-vanished volcanoes.