Eruptions Newsletter, Preview Edition: February 15, 2025

Welcome back to Eruptions, now in its new form. I was going to officially launch on March 2, but the world is complicated. So, as a preview, I wrote some information about the unrest around Santorini. Not to bury the lede here, but on February 15, tremor was detected near Santorini — and that tends to go with magma movement. That does not mean an eruption is imminent, but it does change some of the interpretations of the activity so far.

Remember, if you like what you read, subscribe (if you haven’t signed up already) or suggest it to your friends!

Santorini, Greece

Likely the most prominent unrest right now is in the Aegean Sea. There have been thousands of small-to-moderate earthquakes near the famed volcanic island of Santorini (also known as Thera in the volcanology community). A vast majority of the island’s residents have evacuated. The island is a major tourist destination, with its warm climate and dramatic scenery formed by a massive eruption at ~1600 BCE — the Minoan eruption that could have played a role in the collapse of civilizations in the eastern Mediterranean basin. The Minoan eruption is one of the largest eruption known in the last 10,000 years.

Santorini is far from a “silent caldera”. The massive Minoan eruption — thought to be a 7 on the Volcanic Explosively Index — was only one of multiple gigantic caldera-forming blasts since ~180,000 years ago (including the Skaros caldera at 70,000 years ago … a name Davros would approve).

The volcanic system at Santorini has experienced at least 10 eruptions, some as large as VEI 4, since the ~1600 BCE Minoan blast. The most recent activity formed lava domes within caldera, including Nea Kameni. This seems to be the area that the current unrest is focused, with deformation since middle 2024.

The earthquakes that have been rocking the area since January are located closer to Kolumbo, a submarine volcano north of the island that is last thought to have erupted around 1650. So, why is the concern about an eruption focused on Santorini? Part of this is because magmatic systems are complicated! When magma is intruding under a large system like Santorini, it will migrate upwards from its source tens of kilometers beneath the volcano, but also it can move laterally through weaknesses and established pathways in the crust.

I’m getting ahead of myself! Here is the real challenge right now: Earth scientists studying the earthquakes aren’t sure what is generating the temblors. They could be formed by magma intruding, hydrothermal fluids (hot water, more or less) or tectonic stress on the crust (so, not related to a potential eruption). In fact, although the earthquakes are coincident with Santorini and Kolumbo, many people monitoring the situation had been thinking the earthquakes are tectonic — not volcanic — in origin.

Without changes in other volcanic monitoring signals like increased inflation of the ground surface (complicated here by the fact that much of that surface is under the ocean), an increase in the release of volcanic gases like carbon dioxide or sulfur dioxide (again, complicated by the location), earthquakes like these by themselves aren’t enough to say “yes, an eruption is coming”.

Explaining all the nuances of volcanic seismicity (earthquakes) is something that takes weeks in my classes, but there are differences in the type of shaking caused by tectonic events (on faults) and magma movement. If magma is moving towards the surface prior to an eruption, volcanologists look for tremor — constant shaking that might not be noticeable at the surface. This constant rumble is caused by magma (remember, it is a very viscous liquid!) grinding through the rocks on its way up to the surface.

Although much of the evidence so far had made people think that the earthquakes are tectonic, on February 15 tremor began to be recorded near Santorini. This definitely pushes the situation more into a volcanic than tectonic one — in fact, Dr. Costas Synolaki of the Natural Disasters and Academy of Athens thinks, with the arrival of volcanic tremor, the earthquake swarms being dominantly volcanic in origin. One big question is the relationship between the multitude of earthquakes since January and this new development. Many volcanoes experience tremor without an eruption, so closely monitoring the tremor and the depth of earthquakes will help give experts the information they need to develop a model of what is going on.

Top image: Arian Zwegers, Flickr.