How it’s Made: The Formation of Santorini

As we arrived in Santorini, I wondered how this island was created. How did this island become this unique and geologically complex crescent shape? So, as a person who needs their questions answered, I investigated this topic, and the results were more than satisfying. From just a few limestone and metamorphic hills to massive caldera-forming eruptions, Santorini definitely had a more complex history than I thought. I am here to share this information with you. We will journey back into the past to discover how Santorini was made.

Figure 1. Shown in the royal blue, these are the very first rocks that makeup Santorini (1)

50-23 million years ago:

In the beginning and very slowly, limestones and mudstones are being deposited in the Tethys Sea (ancient Mediterranean Sea). If we just fast forward a few million years, these same rocks are being lithified, folded, metamorphosed (adding extreme heat and pressure to a rock), and uplifted (2). These are the same limestones, schists, and other grades of metamorphic rocks seen at Mt. Profitis Ilias, Platinamos, Monolithos, Pirgos, and the inner side of the caldera at Athinios, Cape Plaka, and Cape Thermia (Figure 1). This is what Santorini started as- just a few hills exposed above sea level. If you think this is complex, just wait until the volcanism starts.

Figure 2. Represented by the dark yellow, the Akrotiri Peninsula is the first place above ground volcanism started on Santorini (1).

650-550 thousand years ago:

Now, this does seem like quite a large time gap, but as shown on Figure 2, this is the next event that really started creating Santorini. On the Akrotiri Peninsula, the very first volcanic material on the island is exposed. During this time, early submarine centers are creating and pushing the domes to the surface. These domes are characterized by rhyodacitic lavas. Shortly after the appearance of domes, cinder cones of basaltic-andesitic composition formed (one is located at Cape Mavrochidi). These volcanic events, and some pyroclastic flows I will not discuss created the Akrotiri Peninsula (2).  

Figure 3. Shown by green, the composite cone called Peristeria is formed up north (1).

530-430 thousand years ago:

To the north, the Peristeria volcanic complex is being formed (Figure 3). This volcano is a composite cone created by alternating layers of tuff and basaltic-andesitic lavas (2). Just a few thousand years later, Peristeria will soon be covered with basaltic lava due to an overfilled shield volcano. How did the volcanic activity all of a sudden jump northward you ask? There are two major oblique strike-slip faults in Santorini: the Kameni and Koloumbo line. Magma wants to travel in the path of least resistance, so faults and fractures are excellent places for the magma to get to the surface. It just so happened that at this time, magma started coming out through the Koloumbo line in the north.


Figure 4. Represented by the black line and inward facing tick marks, this is where the suspected rim would have been. The yellow outline and hash marks represent the volcanic material from the eruption (1).


200-180  thousand years ago:

Boom! This is the first of four caldera-forming eruptions called the Lower Pumice (Figure 4). This is the first time the north and south part of the island are connected. This eruption is composed of rhyodacitic lavas and would’ve laid down tons of pumice all over present-day Santorini.  The eruption also blew up many of the volcanoes on the island, but not to worry there are more to come.

Figure 5. The light blue shade to the north represents the Megalo Shield volcano (1).

76 thousand years ago:

With the magma chamber starting to fill up again, the Megalo Shield volcano and surrounding cinder cones are starting to form. They are basaltic-andesitic in composition. The shield volcano is filled to the brim and is pouring over Peristeria (green) and surrounding areas. Not long after popcorn like eruptions, called Strombolian eruptions, are creating cinder cones on top of the shield volcano.

Figure 6. This is the Upper Scoria caldera-forming eruption the (black line with inward tick marks represent the caldera rim and the yellow outline and hash marks represent the deposits (1). 

76 thousand years ago:

Very shortly after the last caldera-forming eruption, the second caldera-forming eruption is occurring. This is a massive and dangerous hydrothermal eruption. It is known as the Upper Scoria and contains basaltic pyroclastic flows that created the fragmented tephra, scoria.  As shown in Figure 6, the caldera rim expanded and the deposits covered many parts of the main island. Future Therassia gained some deposits as well. Again, many of the volcanoes on the island were blown up.

Figure 7. In red, the Skaros shield volcano is formed on the northern part of the island (1). 

67 thousand years ago:

Just a few thousand years after the last shield volcano and very close to it, another shield volcano is forming. This is known as the Skaros Shield. Due to the magma chamber emptying and refilling again after Upper Scoria eruption, the lavas are basaltic in composition. It appears that the magmas really like to flow through the Koloumbo line besides some pyroclastic flows, not much is happening down south.

Figure 8. In purple, Therassia is created from domes (1). 

37 thousand years ago:

In the northeastern corner of Santorini Therassia (little Thera) is being formed (Figure 8). The island is primarily all rhyodactic-andesitic domes. So far most of the island is already formed from volcanic deposits. Try to imagine this island to just be an extremely hot, volcanically active place. There is so much volcanic activity that there is little to no life on the island right. Santorini is a dangerous barren land. After these next few eruptions, you’d be surprised that people actually managed to inhabit this island and become the way it looks today.

Figure 9. This is the Cape Riva eruption. The caldera is shown with a black line with inward tick marks and the deposits are shown as a yellow outline and hash marks (1). 

21 thousand years ago:

Finally, the semicircular island is connected after the Cape Riva eruption. This was the third caldera-forming eruption in Santorini. It was composed of rhyodacitic-andesitic lavas and pyroclastic. Today, it is well preserved on the island. After this eruption, life finally came to the island. Thousands of years later, after the island was not just hot lavas and ash, the Minoans settled here on top of the Cape Riva tuff. We are very close to seeing present-day Santorini, many smaller pyroclastic flows have occurred that filled in the gaps that I missed or failed to mention.

Figure 10. This is a depiction of the Minoan eruption. The caldera is represented by the black line with tick marks and the deposits are represented by the yellow line and hash marks (1). 

1613 B.C.

Boom! The fourth and largest caldera-forming eruption is occurring. The volcanoes on the island are being blown up and pumice is falling everywhere. Hopefully, the people and animals on the island were able to leave because nothing could survive this eruption known as the Minoan eruption. The whole island is pretty much covered by the rhyodacitic volcanic deposits from this eruption. It blew up everything in its path and formed the biggest caldera yet. This eruption is incredibly significant in the journey of knowing how Santorini was made. As shown in Figure 10, Santorini finally looks the way it does now (besides Palea Kameni and Nea Kameni in the middle).

197 thousand years ago:

Santorini still isn’t complete until Palea and Nea Kameni are formed. Both of these islands are resurgent domes in the middle of the caldera. In all of the figures above Nea and Palea Kameni are represented by the black/brown islands in the center of Santorini. these are the most recent volcanic activity in the island. The last lava happened on Nea Kameni in 1950 (2).

Finally, we are at the end of this journey. I took you through the 11 major events that created Santorini. To me, I find this incredibly fascinating that an island with only a few limestone and metamorphic hills turned into the island we see today. I am sitting here at Santorini just thinking about what the future holds because it is nowhere near done changing. I have so many unanswered questions about Santorini’s future. Unfortunately, I have to wait until next week to learn about it. Thank you for taking this blast to the past with me!


  1. Druitt, T.H., 1999, Field guide to Santorini Volcano: MeMo Smart Course, 1-48.
  2. Friedrich, W, 2009, Santorini-volcano, natural history, mythology: Denmark, Aarhus University Press, 34-39.

2 thoughts on “How it’s Made: The Formation of Santorini

  1. Saludos Sarah!

    Thank you for the information this week. You did a good job of telling the reader your main question at the beginning of your post as this guides us through the information you present later. I also liked the images you added with each informational blurb simce this allows visual learners like myself to better capture the information!

    Along the lines of visuals, it could be beneficial to include a little timeline with the name of the major event and the time it occurred in your conclusion. You talk about 11 major events and mention that the gap between certain events is large. Reading numbers is one thing, but seeing the distance on a time line can reinforce what you’re trying to say in the mind of a reader.

    I would also recommend having a friend look over your entry. You’re thoughts are there, but there are some occasional typos. Other than that, it was a good read! I look forward to your net post.

  2. Sarah – you did an excellent job with the maps. I think a few pictures of what these volcanoes actually looked like would have been good to include as well (or maybe a glossary at the end to help the reader with the terms you are using).

    Oldest deposits we have from Palea Kameni are from 197 BC (not 197 thousand years ago). Pereteria is a composite cone as you mentioned in the second sentence of that paragraph (not a volcanic complex).

    For your last post I challenge you to include more of your own personal experiences in the blog.

    Sentences such as “It appears that the magmas really like to flow through the Koloumbo line besides some pyroclastic flows, not much is happening down south,” don’t really provide a lot of information to the reader. Why? Why are there only pyroclastic flows in the south? You probably can’t answer these questions with your post (that’s ok) so try to avoid suggestive language that may leader the reader astray or feeling like you didn’t answer all the important questions.

    Also sentences like “If you think this is complex, just wait until the volcanism starts,” are more conversational and that makes it difficult to read because there is no actual conversation (only one way). Try to avoid statements like this in your next post.

    Good job on grasping the complex volcanic history here! Lisa

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