Not far off from the Cycladic paradise of Santorini, the Kolumbo volcano remains dormant, but forever threatening. Alarmingly, researchers have detected rising magma, which are often associated with earthquake swarms and makes a future eruption highly likely.
The main problem geologists have is that it is impossible to predict just when it will be explode - it could be one day from now or in a century. Due to this is unpredictable behaviour, the undersea volcano needs to be closely monitored, scientists warn.
As Haaretz reported, although the Levantine side of the Mediterranean would be less affected, Kolumbo's eruption would pose an acute risk to the Greek islands around it, including the world famous Santorini.
"We know the system is active. We know that magma is moving from a deeper system to a shallower reservoir – episodically" but not constantly, said Professor Michele Paulatto of the Imperial College London, coauthor of the new paper in the journal Geochemistry, Geophysics, Geosystems.
He adds: "Our results show that Kolumbo poses a serious threat and deserves a real-time monitoring facility."
Kolumbo lies about eight kilometers (five miles) northeast of Santorini. Its crater is presently about half a kilometer beneath the surface of the sea, which it last broke through when erupting in 1649-1650.
That eruption almost 400 years ago apparently lasted for months. The lava built up a cone that rose above the Aegean Sea waves, but in time it eroded and the volcanic slopes collapsed into its caldera, sending tsunamis slamming into nearby islands.
Kolumbo is part of the Hellenic Arc of volcanoes which includes Thera, the volcano that erupted in 1,600 B.C.E and is wrongfully accused of destroying the Minoan culture.
This means that Kolumbo and Thera and over a dozen more vents are part of the same volcanic system, perching on the same deep tectonic feature.
In 2020 K.L. Konstantinou of the National Central University, Taiwan estimated that Kolumbo produced five major eruptions in the last 70,000 years, and posited that "an eruption in the near future is unlikely."
But that was before the new team revealed the previously undetected body of magma moving beneath the subsea mount.
Now, with the knowledge that it magma chamber is growing anew, Kajetan Chrapkiewicz, Paulatto and colleagues warn in the new paper of potential regional hazard, having found using an imaging technology called full-waveform inversion, which, like medical ultrasound, uses sound waves to image hidden structures, that the molten rock in the chamber may be approaching a volume similar to that last eruption.
It bears mention that submarine volcanoes are not rare: Estimates of how many there are range from tens of thousands to hundreds of thousands.
According to the Smithsonian Institute, in the last 11,000 years, the period of human civilization, there were 119 submarine eruptions, but we cannot always know when a seabed fissure is leaking lava, especially at great depths and pressure.
We cannot spot deformation of the deep seabed like we can when magma starts rising in terrestrial volcanoes.
In any case, undersea eruptions have the potential to be highly explosive, as a function of water depth and magmatic composition, among other things.
"The level of explosiveness does depend on the size and nature of the water-gravity contact and the depth of the eruption," confirms volcanologist Prof. Yishai Weinstein of Bar-Ilan University.
The type of magma Kolumbo and Thera erupt is quite explosive irrespective of the water column; when it interacts with water, it becomes even more explosive, Paulatto explains.
That is what happened in the case of the enormous Hunga Tonga-Hunga Ha'apai eruption in the South Pacific almost exact a year ago: Extremely hot magma superheated the water and the upshot was an exacerbated explosive effect, he says.
“The level of explosiveness does depend on the size and nature of the water-gravity contact and the depth of the eruption,” confirms Weinstein.
The bottom line is that undersea volcanoes are difficult to spot and monitor, but having detected a potentially lively specimen, it bears investment in the expensive technology to keep an eye on it.
Indeed, since the discovery that Kolumbo's magma chamber is growing, its monitoring has improved, Paulatto says.
The team is not saying that an eruption is imminent, or even inevitable. But with the new information, they can say how big its eruption might be and what hazards might ensue, Paulatto explains.
"The magma reservoir is estimated to be about 1.4 cubic kilometers in size, which sounds large, but it's quite small in terms of volcanoes. For comparison, one of the biggest we know of is in the Andes and is 500,000 cubic kilometers in size," he says.
The maximum eruption size is perhaps half the volume of its chamber, he says: "Typically, not the whole reservoir erupts."
A Kolumbo submarine eruption would have similarities to the Hunga-Tonga Hunga blast in 2021, but Hunga-Tonga Hunga's ejecta and energy was probably a good ten times greater than what Kolumbo might produce, Paulatto reassures.
On the logarithmic volcanic explosivity index, which ranges from zero (non-explosive) to eight (mega-colossal), Hunga-Tonga Hunga produced a category-5 eruption but Kolumbo might produce a category-4.
But based on the historic and archaeological record of its 1650 blast, its eruption will likely generate tsunamis perhaps up to dozens of meters in height, he projects. That will be highly destructive in the local Aegean.
But because the trigger of the tsunamis would be a geographical pinpoint, not tectonic unrest causing a rent in the sea bed, the amplitude of the tsunami ought to rapidly decay with distance. Over in Israel and Lebanon their energy may have largely dissipated, though danger cannot be dismissed.
Weinstein notes, the nearby isles such as Santorini and the Cycladic Islands would be the worst affected because the interaction of magma and seawater usually produces ‘base surges’, gas-rich blasts with velocities of hundreds of kilometers per hour, that can travel laterally over the surface of the sea and slam into nearby communities within minutes.
The 1650 eruption sent ash-fall all the way to Anatolia, today's Turkey, but historical records don't indicate a problem beyond white powder covering the fields, which would have washed away with the first rain.
"And Israel and Middle East are even further," Paulatto points out. "We can put better constraints on this with modeling."
With cutting-edge monitoring systems in place, an eruption can be predicted as much as a few days ahead of the event. But given that the growing magma chamber beneath submarine Kolumbo took scientists by surprise, the researchers warn that there are probably more like it.