I am currently sitting on a lounge chair by our hotel pool in beautiful Fira, Santorini, blogging and learning about the Aegean Sea and the active volcano that lies about 3.4 kilometers from my location.
I signed up to study abroad in Greece to gain a deeper understanding of volcanism, a specialized study under the large umbrella of geology. But you may be asking, why specifically did I come to Santorini? Out of all of the scattered islands in the Aegean Sea, why was Santorini the island that was chosen? Being (roughly) a sophomore level student, I am still fairly new to geology. I’d say I have a pretty good grasp on the fundamental concepts and history of the Earth, but I do still have quite a ways to go. With that said, I have learned if there is one thing that answers most geological questions, it almost always stems back to the concept of plate tectonics.
When you think of the world we live on, it is now known that our planet is spherical in shape. Scientists have figured out that the Earth’s crust [also known as the lithosphere] is composed of large masses or plates that “float” atop of the asthenosphere, the layer beneath the crust. Since the world is round, when one plate moves, other plates must move relatively to accommodate for the motion. Plates usually move either by convergence (moving together), divergence (moving apart), or strike-slip (sliding past each other). One of the best examples of a divergent plate boundary is the Mid-Atlantic Ridge, which we flew over while traveling to Greece.
When plates are in motion, hitting each other whilst sliding past, diving or jumping over each other, friction is created. When this friction slips, the release of pressure initiates earthquakes. We have the technology to track seismic activity, which mostly takes place at or near plate boundaries. The majority of recorded earthquakes take place near convergent plate boundaries specifically, where there is a large amount of force and pressure from plates moving towards each other and subducting beneath other plates. At subduction zones, the sinking plate can only go so deep before it starts to release excess or trapped water causing the overlying asthenosphere to melt. This melting furthermore creates magma that needs to escape, usually through volcanos.
The plate tectonics of the Aegean Sea are slightly more complex, which is exactly why we are here studying the geology of the island. In basic terms, the African Plate is moving north and subducting beneath the Eurasian Plate. While it’s easy to group these large plates in terms of relative motion, please note that among all of these massive plates are many microplates, or smaller plates, that makeup the grouping. The region consisting of the northern Aegean Sea is known as the Aegean microplate. As seen in the figure below, this microplate is shown with the surrounding plates almost twisting it counterclockwise. To the north, there is a fault [North Anatolian Fault] initiating that movement, with the southern portion moving west and the northern portion moving east (also known as a right lateral strike slip fault in geology). To the east near Turkey, the Dead Sea Fault moves in a left lateral strike slip motion, where the western portion is moving southwest and the eastern portion is moving northeast.
This pushing motion coming from the northeast is fighting against what is known at the Hellenic Trench. This trench, which was created by it being above a major subduction zone, is located just south of the Greek islands. In an ideal situation, plates would collide head-on. But, of course, that is not what’s taking place here in the Mediterranean. Instead, the African plate is hitting the Anatolian plate at an oblique angle, causing the interaction between colliding plates to hit on the west of the Aegean microplate first and then start to roll beneath the surface as it moves along east. It involves the African plate diving under the Aegean microplate, but is doing so in a wave-crashing fashion. Since I grew up living near the beach in California, I’d compare the rolling of the African plate to how waves create a barrel, blending and destroying anything that gets caught within the barrel. This is also known at subduction rollback. As this barrel or subduction rollback mellows out, it has to destroy crust to accommodate for all that is colliding. When the subducting plate begins to curl over itself, the wave of the crust gets pulled back. While the crust is pulling back, it is also tugging on the plate that overlies it, creating extension just before the trench. This is where volcanism comes in.
Notice how just north of where the plates are colliding, there are several volcanic islands. Before I understood what was happening tectonically in this area, I didn’t know why certain islands were volcanic while others were not. We took the ferry from the Piraeus port in Athens to Paros, to Naxos, to Santorini. After passing Paros and Naxos, two non-volcanic islands north of Santorini that formed by faulting, we had a lecture to go over the tectonics within the Aegean Sea. We talked about the map in Figure 3, and I soon began realizing that Santorini sits right above where the African plate subducts, creates rollback, and where the melting magma forms and tries to find its way to escape (naturally through the crust that has thinned by extension).
As mentioned above, the Aegean Plate is being twisted counter clockwise. With that motion, scraping against other plates creates stress and eventually the land has to flex. In this specific case, the stress is creating extension or stretching along the plate boundary near the Hellenic Trench. The extension furthermore creates earthquakes making it easier for magma to reach the surface, hence the island that I’m calling home for a month being very active volcanically.
When looking at where I am on a map with untrained eyes, it seems as though the island was randomly picked amongst many. But, when I was able to dive into the plate tectonics of the region and understand what is happening beneath the crust I walk on, I can now say that Santorini is a fantastic location to study basic volcanism and even challenging the amateur geologist to step back and look at the bigger picture of what’s happening in the area. As shown in the map, there are only a handful of islands that resulted from volcanism in the South Aegean, with Santorini displaying the most clear cut caldera and straightforward rock record out of the others. As the days go by, I’m learning more and more about how this island formed and I’m looking forward to leaving with a deeper understanding of how the plate tectonics have further influenced the geology of the island. And who knows… maybe I’ll enjoy the geological history of this island so much that I’ll end up specializing in volcanism. But I’ll get back to you on that!
D.J.W. Piper, C. 2003, Perissoratis: Marine Geology 198 page 285. Fig. 21. Schematic tectonic interpretation of the South Aegean arc.
Friedrich, Walter L. 2009, Santorini: Narayana Press, Denmark, The Author and Aarhus University Press. Pages 23-30.