Santorini and Kolumbo are two highly active and interconnected volcanic systems located in the Hellenic Volcanic Arc beneath the Aegean Sea. Recent seismic activity has revealed that these volcanoes, just 7 kilometers apart, share a single underground magma reservoir, complicating eruption prediction and risk assessment for the region. 

Santorini

Santorini is the remnant of a volcanic caldera that formed during the catastrophic Minoan eruption around 1600 BCE, one of the largest volcanic events in recorded history. 

Geology and history

• Volcanic field: Santorini is part of a larger Christiana-Santorini-Kolumbo volcanic field.
• Minoan eruption: The caldera was formed during a devastating Late Bronze Age eruption, which may have contributed to the collapse of the Minoan civilization and potentially triggered a volcanic winter.
• Modern activity: Subsequent eruptions built the islands of Palea and Nea Kameni inside the flooded caldera. The most recent eruption was in 1950.
• Seismic activity: Santorini experienced a period of magma replenishment and swelling between 2024 and 2025, which was largely unnoticed by inhabitants. 

Kolumbo

Kolumbo is a submarine (underwater) volcano located about 7 km northeast of Santorini. It is the largest of about 20 volcanic cones in a rift zone extending northeast from Santorini. 

Geology and history

• Largest eruption: Kolumbo's last major eruption occurred in 1650 CE, a highly explosive phreatomagmatic event that temporarily breached the sea surface.
• Destructive force: The 1650 eruption caused a tsunami and released toxic gases, killing dozens of people on Santorini and neighboring islands.
• Ongoing activity: The volcano remains active, with continuous hydrothermal venting and occasional seismic swarms. Its crater is 1.5 km across and 500 meters deep. 

Recent discoveries of a coupled system

Until recently, the magma systems for Santorini and Kolumbo were believed to be separate at shallow depths. However, new research published in September 2025 demonstrates a deeper connection. 

• Earthquake swarm: An intense earthquake swarm of over 1,200 quakes occurred around Santorini and Kolumbo in January 2025.
• Magma movement: Analyses of seismic data and surface elevation changes from 2024–2025 show that magma rose beneath Santorini, causing it to swell. When the seismic swarm started, quakes migrated towards Kolumbo, and both volcanoes began to simultaneously deflate.
• Shared reservoir: This simultaneous activity provides evidence that the volcanoes are hydraulically connected and share a mid-crustal magma system. The shared system was filled by nearly 300 million cubic meters of magma that rose from deeper within the crust.
• Increased risk: The revelation of a coupled magma system complicates future eruption prediction and risk assessment. Changes in one volcano's system can directly influence the other, increasing the need for high-resolution, real-time monitoring.

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