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The Weekend Neos Kosmos : 29 June 2019
14 THE WEEKEND NEOS KOSMOS | SATURDAY 29 JUNE 2019 DIGITAL.NEOSKOSMOS.COM ‘We probed Santorini’s volcano with sound to lea EMILIE HOOFT T he island of Santorini in the Mediterranean has attracted people for millennia. Today, it feels magical to watch the sun set from cliffs over the deep bay, surrounded by cobalt blue churches and whitewashed houses. This mystical place attracts about 2 million tourists per year, making it one of the top destinations in Greece. Not all those visitors recognise that Santorini is an active volcano. In 1630 B.C., the volcano exploded and collapsed leaving behind an almost circular hole. This is the caldera – visible today as a bay filled with seawater and lined by cliffs. The large explosion covered a Bronze Age town, burying buildings in volcanic ash two stories deep. The latest lava flows erupted in 1950 and expanded the islands that have grown at the centre of the caldera. Recently, in 2011-2012, the volcano went through a period of unrest. The ground bulged up and out, and many small earthquakes occurred. Scientists concluded that a small amount of magma was injected about 2.5 miles (4 kilometers) under the northern portion of the caldera. What attracted me to this iconic place is that most of the volcano is submerged under water. I am a geophysicist interested in how magma moves deep in the Earth. Over the past decade, I've been using advanced technology to improve how we "see" magma's otherwise hidden pathways below volcanoes around the world. USING SOUND TO SEE WHAT'S BENEATH THE SURFACE In the 1780s, French scientist Ferdinand Fouquet travelled to Santorini to view an ongoing eruption. He was the first to realise how the volcanic surface depression known as a caldera was formed. As magma emptied out of its underground reservoir during the eruption, the roof of rock that had been covering it collapsed. The flanks of the volcano that remained form the ring of islands visible above water today. My research project aimed to delve deeper, literally, than what we can see from the surface to figure out what's going on within this still active volcano. A blanket of water over everything except the very top of the Santorini volcano meant I could use deep-penetrating marine sound sources to 'illuminate' the subsurface structures. My international collaborators and I wanted to find the location and depth where magma was collecting and how much magma there is right now. We conducted our work from the R/V Marcus Langseth, an American marine seismic ship. It is the only academic ship with a sound source capable of imaging the deep insides of a volcano. This technology is controversial because of the potential impact of loud sounds on marine wildlife and its intensive use by oil exploration companies. We spent months doing environmental permitting and finding the optimal design for the experiment. The ship carried a team of experienced biological observers who surveyed the sea both above and below water for sound-sensitive or endangered species. If any were observed at a distance, we were to follow a prescribed set of actions to ensure they wouldn't be disturbed. After all this preparation, though, we saw almost no wildlife during the expedition. Our 'active source seismic imaging' method is like making a CAT-scan picture of the inside of the Earth. 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. University of Oregon graduate student Brandon VanderBeek capturing an ocean-bottom seismometer after it resurfaces. The caldera cliffs of Santorini are in the distance. The black fresh lavas of the island inside the caldera are in front, on the left. Emilie Hooft, CC BY-ND In this instance, 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. We have to use very accurate timing to measure how long it takes the sound energy to go through the different parts of the volcano. The energy from the sound source will travel more slowly through rocks that are broken or that are hot and contain magma. 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. To get the data back from the seafloor, we send a special sound signal to the sensor – like a bird call – that commands the instrument to drop its anchor. Then everyone scans the sea looking for the instrument. During the day we search for a cheerful orange flag, at night a strobe light makes this task easier. Our ship manoeuvres alongside the instrument and a crew member leans over the side, hooks the instrument on a long pole and pulls it back on board. The data is in hand. FILLING OUT THE SUBSURFACE PICTURE Analysis of the seismic data is an enormous task. It required experienced inspection by PhD student Ben Heath and master's student Brennah McVey. We then The Greek islands of Santorini form the perimeter of a volcano whose last major explosion happened about 3,400 years ago. Now the center of the crater-like caldera is filled with seawater.
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