Look Closely: Every Layer is an Event

Layer upon layer of history, tragedy, and misfortune built up every wall that ascended as we hiked deeper into Cape Plaka on Friday (3 June). Each wall, exposed and vulnerable, waits for someone to hear the stories inscribed in every grain. As I gaze up at the colossal remnants of the countless catastrophes that took place here, I am no longer in my body. I can see history being made before my eyes, eruption after eruption, construction and erosion, life and death. Every layer is an event. I feel my feet slip on the loose rock beneath me and I snap back to 2016.

 

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Figure 1. (Cape Plaka, Santorini, Greece)

Located on the inside of the caldera, Cape Plaka is a prime spot to study/admire the many layers that make up Santorini (See figure 1). Each layer is unique and has a different story to express. It doesn’t take a geology major to notice the overlapping layers of history that created this paradise. Looking at rocks may not sound interesting, but studying the leftovers of devastating eruptions makes this fair skinned, elementary education major want to suffer more sun burns out in the field (not to mention all the ash inhalation) (See figure 2).

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Figure 2. (Measuring clast sizes)

We composed our first stratigraphic columns at Cape Plaka on Friday (see Becca’s Stratigraphic Columns and How to Keep Your Cool for a complete tutorial). We started at the beginning of the Minoan eruption(s) and went forwards in time. The Minoan starts where the Cape Riva tuff ends.

First, we have Phase Zero: The warning layer. This layer is only a few centimeters thick and consists of fine ash and was only deposited on certain parts of Santorini. It is called the warning layer because it happened before the eruption buried the ancient city of Akrotiri therefore the Minoans left and didn’t perish in the eruption (that we know of). Tune in next week for more about Akrotiri.

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Figure 3. (Phases one & two photographed at Cape Plaka)

Next we have Phase One: Pumice fall. This layer is much thicker than phase zero, about seven meters thick in the town of Fira, and tells a new story. It is thickest in Fira and thins toward the north, west, and south, which indicates that the wind during the eruption phase pushed the deposits to the east. Phase one consists of pumice, a highly vesicular (containing many holes) volcanic rock created when the pressurized rock is ejected from the vent of the volcano.

You have to look closely to see an important piece of the story. Phase one has a reverse grading, which means the smaller pieces of pumice are on the bottom and the larger pieces are at the top. How? Why is it that smaller pieces are falling to the ground faster than bigger pieces (it should be the opposite…right?)? Allow me to explain. When magma fragmentation is efficient (gasses expanding rapidly), it creates smaller pieces and vice versa. This means that smaller pieces were erupted first and were followed by big pieces because the magma fragmentation was more efficient at the beginning of the eruption and less efficient after.

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Figure 4. (Drawing of pyroclastic surges and block being ejected from the vent of the volcano)

On to Phase Two: Surges. Phase two consists of alternating layers of ash and pumice. This is slightly difficult to see but is essential to the story. Within one bed were multiple layers of ash and pumice. How did this come to be, I thought. The alternating layers within one bed is called cross bedding. When magma is shot from the vent of the volcano up into the air, it excavates the surrounding rock and the volcano will eventually drop below sea level. When this happens, seawater pours into the vent and causes steam explosions (because the water is dropping down onto hot magma).

This results in pyroclastic surges. These surges are relatively cold (200-300 degrees Celsius) gaseous clouds filled with ash and other rock bits that shoot laterally across the landscape. These surges are turbulent, meaning the gas and materials spin within the cloud, similar to tornadoes. These surges are erosive, they cause cross bedding and erosive contacts between separate layers (See figure 4).

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Figure 5. (Drawing of block sag)

Another element that we observed of phase two is called block sag. This is when a pyroclastic surge breaks off pieces of the vent and it is shot ballistically into the air. When it lands on top of other layers, the impact causes the wet layers to sag, hence the name block sag (See figure 5). We climbed a large hill with ash and pumice sliding out from under our feet in order to see a block sag. (See figure 5)

We measured a little over twenty-seven meters of history and every layer told us a separate chapter of the story of Santorini; the warning layer, the pumice fall and the pyroclastic surges (and this is only the beginning of the Minoan eruption). While Santorini’s excavational eruptions were grand, the archipelago offers its many layers to all who wish to travel back in time (See figure 6). Who knows, in a few million years the iconic blue and white islands may be buried once again and geologists will be here to discover the story of our layer.

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Figure 6. (Oia, Santorini, Greece)

 

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6 thoughts on “Look Closely: Every Layer is an Event”

  1. Hi Lizzie – this blog starts off strong with your impressions of doing strat columns and the Minoan phases 0-1. Phase 2 gets the reader a little astray and I suspect that is because that was the hardest to explain because we have only described it one time. Wait for this week and you will understand that phase much, much better. Whats figure 6 about? I don’t see where it is referenced in the blog. I’m looking forward to what you come up with next!

  2. Hi Lizzie,

    Great work with this post. I enjoyed your creative flair in the first paragraph, and I think you could strengthen it by adding a picture of the scene you are describing. You created a vivid image with the first few lines, and the post could definitely benefit by seeing that image clarified. You provided the emotion of the moment, so it could be interesting to see that emotion grounded in something visually powerful like a picture of the volcano itself.

    Throughout the post you deal with unfamiliar terms very well by defining them for the audience. I may have missed the definition of “vesicular” or that’s another one for defining. The reason defining these terms is so essential to a post like this is because it prepares you for writing in which your audience does not know what you know; you’re the expert and we’re learning from you. So defining unfamiliar terms clarifies unfamiliar concepts and makes the whole blog that much more readable to anyone outside the class.

    Most of your images contributed to the overall post very well by highlighting what you were writing about. The hand-drawn diagrams give the post a sort of field-note tone to them which works nicely. There was one image and corresponding line that stood out, and I think you could replace it without too much work. The sentence before the citation for figure 6 “While most of the events that happened here were devastating, Santorini is world-famous for its panoramic views, wine, and SantoNuts (See figure 6)” seemed out of place only because you spent so much time going into great detail about the different ashfalls, then there is this sentence that seems to be about the tourist attractions. You could substitute this line with something that describes the volcano as a whole, and that way you could keep the picture because it’s a really good picture and a good note to end on. For future posts, I would just make sure all pictures and figures correspond directly with the ideas you are writing about in the post, that they stick to the main ideas. Great work on this one.

    Justin

    1. Justin, thank you for your feedback! I appreciate your specific comments. I made a few changes based on your suggestions. I unfortunately couldn’t get a great photograph to pair with the first paragraph, but I will definitely keep that in mind for future posts. Thanks again! x Lizzy

  3. Hi Lizzie,

    Sounds like you’re having quite an adventure in Santorini! Like you, my field is not geology, but by reminding the reader of how the rocks tell a story, you’ve managed to make this sound interesting for someone with a different field. Well done!

    I enjoy your use of visuals. The diagrams help only explain certain phases, so you could also mix in a picture or two of the real life area you’re exploring. For example, what does a block sag look like in real life? If possible, try adding both the diagram and the photo so a viewer can better visualize what you’re talking about.

    By defining the terms you again helped make the post more understandable to readers without extensive geology knowledge. It’s always important to keep in mind who will be reading your post, and this certainly makes this an easy read for anyone with any level of geologic knowledge. Something else you did well was the mixing of a formal tone and a creative one. As I stated above, I love how you introduced the post, but you switched to more technical writing right afterwards and kept it formal.

    Can’t wait to read about your next adventure!

  4. Hi Lizzie,
    Sounds like you’re having quite an adventure in Santorini! Like you, my field is not geology, but by reminding the reader of how the rocks tell a story, you’ve managed to make this sound interesting for someone with a different field. Well done!

    I enjoy your use of visuals. The diagrams help only explain certain phases, so you could also mix in a picture or two of the real life area you’re exploring. For example, what does a block sag look like in real life? If possible, try adding both the diagram and the photo so a viewer can better visualize what you’re talking about.
    By defining the terms you again helped make the post more understandable to readers without extensive geology knowledge. It’s always important to keep in mind who will be reading your post, and this certainly makes this an easy read for anyone with any level of geologic knowledge. Someone else you did well was the mixing of a formal tone and a creative one. As I stated above, I love how you introduced the post, but you switched to more technical writing right afterwards and kept it formal.
    Can’t wait to read about your next adventure!

  5. Hey Lizzy,

    This is a good introduction to the eruption your class is focused on. These three phases increase in complexity and can be difficult to fully capture. The combination of your visual descriptions with your diagrams successfully link what you are seeing to the eruptive process behind them.
    I like that you made a point to explain the reverse grading in Phase 1 and you implied normal grading has big pieces on the bottom and small on top, but why is that normal? Maybe explaining why that’s considered ‘normal’ can help the reader to understand why the reverse grading is so important and how it led you to the conclusion of a change in efficiency.
    Speaking of efficiency, what are your theories on why the magmatic fragmentation efficiency changed?

    Alex

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