Moving Megatons: The Excavational Eruptions of Calderas

I’ve just learned the full story of the Minoan eruption. In my mind, I’m imagining 60 cubic kilometers of earth. This is about the size of a block of the Los Angeles basin, by volume; a massive amount of land. I can see all this land being ejected by the volcano miles into the sky, over a span of 24 hours. Such is the case of Santorini’s last caldera forming eruption. 

Many people understand a caldera as one giant volcano that has a big eruption which excavates the magma underneath, leading to a collapse. This collapse is what forms a caldera. This is how a caldera is commonly taught in general ed geology classes. While not an incorrect definition, calderas are a little more complex than that. Calderas are not simply just one volcano. They are composed of many types of volcanoes that are found on Earth. Each caldera contains volcanic domes, cinder cones, composite cones, shields, and tuff rings- hence named caldera complexes. They contain rocks from all types of compositions, ranging mafic to felsic. All of these different volcanics form in one area. When the main vent of magma in a caldera finally blows, massive amounts of material is ejected in such a rapid and violent manner that it completely empties. The land under it collapses, forming a caldera (Figure A).

Figure A: A simple diagram of how a caldera forms. (Volcanoes and Volcanic Eruptions, Tulane University, 2012)

As mentioned before, the Santorini’s latest eruption, the Minoan eruption, deposited 60 cubic meters of material (Druitt, 2014). This is mainly in the form of ash and pumice from the magma, but many other surrounding materials from the earth were ejected along with it. The Minoan eruption can be categorized into 0-4 phases, each having their own distinct characteristics. Each phase is found in one or multiple parts of the island, as displayed by the map below.

Figure B: A geologic map and stratigraphic column with each phase described on it. (Druitt, 2014)

In order to find all these layers, we had to travel all around the island to get the best exposures. As seen in figure B, phase 4 has two different layers: a phase A and a phase B. While each part is an important phase of this cataclysmic eruption, phase A is the most interesting. This part of the eruption is beautifully exposed in the northern part of the island, at a place called Cape Mavropetra. We traveled a very dusty road in a well less traveled part of the island to get here. Phase 4A can be described as a pyroclastic flow layer of pumice with a very high amount of lithic fragments, which are pieces of rock not original to the magma chamber from which it is erupting. In other words, they’re pieces of rock surrounding the magma chamber which were excavated by the eruption. Each layer is seen in Figure C.1 (picture) and Figure C.2 (hand drawing). As this phase has the largest and highest concentration of lithic fragments within compared to the others, this says a lot on how the volcano is erupting.

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Figure C.1: A picture of the outcrop at Cape Mavropetra. There are 2 phases present here- 3 and 4A.
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Figure C.2: Drawing of Cape Mavropetra outcrop with annotations

The presence of all these dark lithic fragments indicate that the volcano vent is widening. This means more material is being ejected at this point in the eruption. Because the amount of lithic fragments in this layer is the highest of all the phases, it can be interpreted that this was the most excavational phase of the entire eruption- the climax.  Tons of rock were being blasted into the sky along with the pumice and ash. As a result of this ejection and emptying of the magma chamber, this part of phase 4 is where the caldera collapse occurred. It is the reason present day Santorini is shaped how it is today. Due to its identification as a pyroclastic flow, all of this material at this phase dropped from the sky when it became too large to be suspended in the air and landed here. Walking around the outcrop, which is right on the beach, I noticed many different types of rocks. I collected as many different types of rocks I could find and noticed; at Cape Mavropetra, every single type of rock found on the entire island cluster of Santorini can be found here (Figure D).

Figure D: A picture of every single rock type found on Santorini.

To understand the nature of how every rock ended up in the same place, an excavational eruption must be understood. Figure E shows exactly how this happens; as phase 4A was the most excavational (and also the last part of the eruption), it brought the deepest magma up to the surface to erupt. The first layers to come up are the first to fall, and the last layer to be excavated is the last to deposit.

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Figure E: Layer C, once at the top, ends at the bottom of the deposit and vive versa for layer A. Layer C can be thought of as phase 1 and 2, and layers B and A can be thought of as phases 3 and 4, respectively.

We can trace each rock back to a part of the island in which they originate. Each rock will be traced to their original locations and the distance will be measured (using Google Earth). Each picture below refers to a letter that references to figure F.

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Figure F: A map labeling the origin of where each rock may have come from.


Every rock in Santorini was violently excavated by this phase. Although only marked in 4 places, this area represents a very large area in which the caldera eruption excavated. The actual size of the caldera is essentially bounded by the shape of the island, near the center. The power it takes to launch these rocks many kilometers, which can range in size from a pebble to the size of a small car, is rated to be thousands of times more powerful than the atomic bomb dropped on Hiroshima and Nagasaki in WWII. The eruption also excavated all the way down to the basement rock, the lowest layer of rock present in the lithosphere. Figure G shows one of the largest lithic fragments found on the island. The boulder is the size of a car and it launched ~16 km from the vent. This rock weighs literal tonnes and flew about 10 miles across the sky.

Figure G: A car sized lithic fragment

Caldera eruptions are the one of the most powerful forces that we can observe on this planet. They excavate massive amounts of rock bigger than cities in volume in incredibly short amounts of time. The Minoan eruption excavated all 60 cubic km of its rock in less than 24 hours. It flung rock that was rested many kilometers down all the way to the top. An entire landscape was terraformed, changed forever, in less than a day. Rocks the size of small cars were flung miles from their original position. Coming to Cape Mavropetra and seeing the evidence for an eruption like this preserved in the rocks there is truly amazing. Most geologic processes are slow and steady processes that take millions of years to change a landscape, but all it takes is one major volcanic eruption to change everything you see forever.


  1. New Insights into the Initiation and Venting of the Bronze-Age eruption of Santorini (Greece), from component analysis. T. H. Druitt. 25 January 2014. Springer-Verlag Berlin Heidelberg 2014.
  2. Volcanoes and Volcanic Eruptions. Tulane University. 27 Janaury 2012.
  3. Ch. 1-3 Research. H. T. Druitt. 1999

5 thoughts on “Moving Megatons: The Excavational Eruptions of Calderas”

  1. This truly was an excellent post. You were very knowledgeable in your topic and displayed information that truly intrigued the reader. This was very well done.

  2. You explain everything really well, it was clear and easy to follow. Also your figures were relevant and helped with your explanation. Your drawing could have been a little more detailed but it was a good simple explanation.

  3. You did a really good job on this. Your writing style is intriguing and the explanations are well done. Good job!

  4. Hi Ryan,
    I especially liked your picture of what happens during a caldera eruption and how it collapses within itself after releasing all the magma within its chamber. AS well as how you took multiple different rocks and mapped out their original region before the explosion was also exceedingly interesting to me.
    I did note that there were some things that can be improved upon. For instance, your second sentence says that you imagine 60 cubic kilometers of earth but do not elaborate on why or what is happening to all that earth very clearly. Explaining why you imagined all that earth will cause less confusion among your readers. The greater the detail, the less likely it is for the reader to become confused since not all of your readers may have the same knowledge on your topic as you do.

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