Making a ‘CAT-scan image’ of the inside of the Earth

It’s November 2015, on the Cycladic island of Santorini. In defiance of the bitter cold, a large group of scientists of various specializations from the United States, Britain and Greece stand on the bridge of the Marcus Langseth, one of the largest seismic vessels in the world, scanning the horizon for traces of an orange flag.

A short time earlier the boat had sent a signal for seismographs on the seabed to rise to the surface of the sea. The crew would spot them from the orange flag on top of them.

If one didn’t know it, it would be hard to guess that all this was related to the Santorini volcano, nor that a pioneering research project was under way: the worldwide mapping of the magma chamber of an active volcano, the results of which were published recently (Hooft et al., 2019, EPSL).

American geophysicist Emilie Hooft, professor of geology and volcanology at the University of Oregon, who has always been fascinated by the Santorini volcano, was the leader of this research mission.

“It is an emblematic place where the largest part of the volcano is submerged under the water,” she says. But with her research she wanted to go deeper – literally – to see what is hidden beneath the surface and to look into the heart of an active volcano.

In recent years, Hooft has developed an innovative method of “scanning” volcanoes using sound.

“It’s like making a CAT-scan picture of the inside of the Earth,” she explains. “Instead of building an image using X-rays, though, we use sound waves generated by 36 heavy, metal canisters – called airguns – that are towed deep in the water behind the ship. When the airguns open, compressed air pushes on the seawater, creating a sound wave that travels through the Earth.”

In this case, the sound travels through the rocks beneath the volcano. “Then seismic sensors resting on the seafloor on the other side of the volcano record when the sound reaches them. The team installed 65 of these stations on land, across Santorini and the nearby islands, and dropped another 90 stations to the seafloor.”

“When we probe the structure from many different directions and at many different depths, we can recover a detailed picture of the interior of the Earth,” Hooft says.

Surprising results

The results obtained from the data analysis impressed the scientists.

“Surprisingly, we found a narrow zone of collapsed rock hiding within the broad caldera at Santorini. We had thought the entire caldera would be filled with this type of broken rock at shallow depths. Our finding meant that the collapsed portion of the caldera was much narrower and deeper than it appears from the surface,” she says.

It was also a surprise for the Greek part of the research team, including Paraskevi Nomikou, who is from Santorini and is an assistant professor in the Faculty of Geology and Geoenvironment at the University of Athens and a member of the Institute for the Study and Monitoring of the Santorini Volcano (ISMOSAV).

“Indeed, although we would like to have some ideal dimensions for the magma chamber – which is like a magma tank – our first results showed a cylindrical, 3-kilometer-wide anomaly that extends from the surface to a depth of about 3 kilometers in the northern basin of the caldera, and corresponds to very low density, porous material filled with warm seawater,” the Greek academic tells Kathimerini.

“At any rate, the location of this anomaly is identified with the point of adjustment observed during the volcano-seismic disturbance in 2011-12. Moreover, it is precisely this area in the northern caldera that revealed the different phases of the Minoan eruption 3,600 years ago.”

Perhaps the most striking find is that, under this rock, magma continues to accumulate slowly, thousands of years after the eruption that created the caldera.

For Nomikou, as for all Santorini islanders, the volcano has played a crucial role in her life.

“When you grow up in a volcano you feel and hear the power of nature and you are inspired to explain how a ‘living organism’ such as a volcano was created and how it evolves,” she says.

“It was because of the volcano that the people of Santorini carved the pumice stone to build their homes, that they cultivated their products (tomatoes, white eggplant, the katsouni cucumber and grapes) without adding water, knowing that the fertile volcanic soil contributes to the mist that covers the island in the mornings providing the plants with moisture and keeping their roots cool throughout the day. And it was because of the ‘breathing’ of the volcano, which always reminded them that it is active, that they built many churches to watch over them,” she adds.

Participation in this mission was very important for the Greek scientists.

“November 2015 was the first time that an ‘active seismic experiment’ was carried out in a water-filled caldera where the secrets of its activity lie beneath the sea floor,” Nomikou says.

“By sketching the magma chamber of the Santorini volcano and the Kolumbo submarine volcano just 7 kilometers northeast of Santorini, we are trying to understand the movements of the magma, the geological structure of the subsoil and the way the caldera was created. Understanding the magma chamber’s formation, how it evolves and is recharged, we will gain insights into the next volcanic eruption.”

As Hooft explains, “if the volcano shows signs of awakening – these would be earthquakes, ground inflation, and/or degassing – our results will help interpret these signals.”

“This is because our work provides a picture of the internal structure of the volcano that makes a framework for deciphering the meaning of the unrest.”

According to the experts, the underwater space will be part of intensive research for years to come, “because the birth of volcanoes starts there.”