Staying Active… Tectonically Speaking

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.

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Figure 1. A map showing the major plates boundaries around the world and the relative plate movement. Solid green lines represent divergent boundaries and arrowed green lines represent subduction zones. Also shown is our path of travel from the U.S. 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.

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Figure 2. Section detail of large scale tectonics beneath Santorini showing subduction zone, rollback, and creation of magma/ volcanism.

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.

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Figure 3. Diagram showing Aegean Sea and surrounding area plate tectonics. 

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.

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Figure 4. GIS representation of subduction zone and its relation to volcanism in the Southern Aegean Sea. This map also shows the ferry route from Athens to Santorini. Santorini lies along an arc where earthquakes about 170 km deep allow magma to rise to the surface. 

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.

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8 thoughts on “Staying Active… Tectonically Speaking”

  1. As we talked about, I think you picked one of the most difficult topics to blog about. I am happy you were up to the challenge! To clarify just a bit, subduction rollback is a process that happens at most subduction zones, where the descending plate becomes compressed and “sticks” or “suctions” to the lithosphere it is diving under. As a result of this, extension occurs in the overriding plate. This extension, or thinning of the crust in the overriding plate, provides pathways for magma to ascend. In the Aegean, the extension is complicated by the surrounding strike-slip motion. As a result of some pretty complex interactions, the ascent of magma is controlled by the position of the normal faulting (in fact…oblique normal faulting).

    The figures you used were excellent and the post was very well organized. The map of the Aegean is well done.

    1. Thanks for the clarification and thank you for the compliment on my figures! The topic was too broad to stuff all in one blog post but I did get a great review for the test while writing about it 🙂 so it was good for me in the end!

  2. Dang. This is a better description of tectonics than in most textbooks. Framing the volcanism via tectonic setting is perfect. Now, I’m embarrassed about my writing in college.

    1. Thank you Jim! Sorry for the late response, I must have missed the send button. But I appreciate the high compliment and I’m striving to be as successful with such an awesome career path as you have done 🙂

  3. Hi Naya,

    Breaking down the macro subject of geology down to basic movement of the tectonic plates was a great way to make sure your reader understood what your post was going to talking about. It was clear, concise, and very well done. Also with your explanation that pressure is released while the tectonic plates are moving helps me better understand how earthquakes occur and what truly an epicenter is. The step-by-step explanation of the overall process of earthquakes was easy to follow and well organized.

    I also liked how you reinforced what you learned before to give your post greater credibility with your readers. The entire post was formatted in a way that a person with any level of interest in geology would be able to comprehend the information you give them on this topic of tectonic movement.
    Keep it up.

    1. I appreciate your kind critique! I’m glad you were able to follow it. For a second, I was nervous it would be too much information and too jumbled, but I’m happy I got at least the main point across. Thanks again!

  4. To a complete volcanology novice, this article was both densely informative for me while maintaining an approachable and conversational tone. Thank you for striking a compelling balance! Enjoy your Grecian adeventure…

    1. Glad to hear that Kate! It wasn’t easy but I learned more by writing about the Aegean tectonics and I’m happy I was able to teach readers new things as well 🙂

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