In Star Wars, Episode 1: The Phantom Menace, there’s a scene where Qui-Gon and young Obi-Wan are “flying” in a submarine ship through this massive lake after visiting Jar Jar Binks’ underwater city, when all of a sudden this massive fish thing swims out of the black abyss, seemingly hungry and very menacing (see what I did there :)). But then, out of nowhere, an even larger sea monster swims in and bites the smaller fish in half. Liam Neeson’s character, Qui-Gon Jinn, serenely responds, “There’s always a bigger fish,” to a frightened young Obi Wan Kenobi. It’s a pretty cool scene, if you’re into that sort of thing.
I know what you’re thinking. How the heck do sea monsters and Star Wars relate to geology? But believe me, they do. And it’s crazy. Let me tell you a story about how one very large volcano was swallowed up by an even bigger volcano.
Take a step back in time—50 million years ago. It’s hard to imagine because most of us won’t live past ninety, but just know that it was a very long time ago. The first island of Santorini—Profitis Ilias—was thrust up out of the ocean over 30 million years from a giant fault. It now stands 565 meters above the sea (Friedrich 2009) in the southeastern quadrant of modern day Santorini. Thus began the formation of one of the most visited islands in the world.
Then, 19.4 million years later, to the west, tens of volcanic domes started forming under the ocean, and as lava slowly piled up they eventually creeped up above the water. Imagine huge Imperial Star Destroyers appearing out of light speed (the volcanoes, however, did not move at the speed of light, of course).
This formed what is now the Akrotiri peninsula, the westward facing finger in the southern part of the island. We actually visited two of these domes last week, one near the Akrotiri lighthouse on the very tip of the Peninsula, and the other on Cape Mavrohidi, colloquially called Red Beach due to the dark reddish-purple color of the volcanic rock.
530 thousand years ago, less than a mile to the north, andesitic dikes started bringing magma up from the chamber through an opening in the underwater crust and began building something called a stratovolcano, or composite cone, now named Peristeria. Picture a Mount Fuji-like, towering, triangle-shaped volcano with the ability to wipe out an entire civilization.
Over the next 100,000 years, Peristeria steadily stacked up lava until it eventually protruded above the surface of the Aegean Sea, becoming the tallest peak Santorini has ever seen. By 430 Ka (thousand years ago), it had reached a height of around 4,000 to 4,500 feet high, or 1,300 meters.
This Wednesday we went on a hike with a panoramic view of the current caldera and the remaining piece of Peristeria that still stands today. Though my class did not, you can actually hike up to the “top” of what is now called Mikros Profitis Ilias, which reaches 314 meters above sea level. But that’s actually only about an eighth of how tall the composite cone Peristeria actually was!
All that to say, Santorini looked much different 500 thousand years ago than what it does today. Instead of Thera being one connected island, it was a cluster of different volcanoes and basement rock.
But that’s only the first half of the story…little did Peristeria know that a larger, more deadly volcano was brewing under its surface, waiting for the right moment to take its place as the alpha volcano.
After Peristeria reached its maximum height, the northern part of the island was eerily dormant for over 200,000 years. Then, two large eruptions occurred back to back called the Lower Pumice Series. All the ash, lava and volcanic rock ejecting from the mouth of the volcano covered the entirety of Santorini. In the last stage of the eruption, immediately after depositing those explosive layers, it collapsed in on itself and over half of the whole island fell into this massive hole in the ground, irrevocably changing the appearance of Santorini forever. This massive hole is called a caldera. Think sarlacc, only bigger and it’s a volcano.
To further explain, volcanoes erupt because they have chambers of magma under the crust that feed the lava flows and explosive ejections through dikes and conduits. What happens in a caldera is that the force of the explosion is so powerful that it guts out the entire magma chamber, leaving an empty cavity of space underneath the heavy volcanic material above the surface.
Inevitably, the weight is too much for the roof of the magma chamber alone to carry, so it all collapses in on itself, catastrophically eating up everything that was once peacefully resting right above it.
This is precisely what happened to Peristeria. The “bigger fish” volcano grew large enough to sneak up on Peristeria and bite it in half, just like ‘ole Sando Aqua Monster. There have actually been four of these caldera-forming eruptions in Santorini’s history. The most recent one excavated the island so much that ocean water literally filled in the depression. This is why Santorini has its large “bay”.
So, next time you see a volcano, remind yourself that there are many fish in the sea, and like all things in nature, only the strongest volcano will survive.
Friedrich, W.L., 2009, Santorini—Volcano, Natural History, Mythology: Denmark, Aarhus University Press, p. 37.