I’m at the base of the Fira Quarry, staring up at 20+ meters of pumice and ash from the Minoan eruption. All I see is the grey and tan hue of the outcrop towering over me. It is a sight to behold, but it means nothing to me. I don’t see patterns, or clues, or any indication of how the rocks got there. I may be staring, but I’m certainly not seeing anything.
Coming into this trip as an English major, I expected a certain amount of confusion; I did not, however, expect the amount of confusion I felt when making our first stratigraphic columns. Stratigraphic columns are used to describe rock layers to scale. They are composed of a graphic representation as well as written notes. Geologists use stratigraphic columns to analyze outcrops and compare their appearance and composition in a variety of locations.
We are at the Fira Quarry to make a stratigraphic column, and as an English major, I feel like I’m in way over my head. But with the reassurance of Lisa and my classmates, I begin the process of compiling a stratigraphic column.
My field partners, Alex and Carly, and I get to work. First identifying and measuring the beds of the deposit, then analyzing and describing what we saw, and translating that to our columns. With the help of my partners and the infinite wisdom of both Lisa and Conner, I slowly start to recognize patterns in the deposit: a double band of fine ash, lithic fragments grouped together, bedding among the ash fall. I’m not just staring anymore, I’m finally seeing. By the time our column is complete, I feel a deep connection with the land–almost as if I had witnessed the Minoan eruption myself.
My experience at Fira Quarry taught me that it doesn’t take a geologist or geology major to appreciate the landscape and how it originated, all it takes is a little know-how. So here is my comprehensive guide to creating a stratigraphic column.
A Field Notebook – preferably with graph paper
A Pencil and Eraser
A Grain Size Card
First, set up your stratigraphic column. As you can see from Figure 1, you’ll need to include a column for scale, bed, thickness, graphic representation, observations, and interpretations. At the bottom of your graphic representation column add another scale. This scale will be for the composition of the materials that make up the bed that you are examining. Once you have a rough outline, pack a water bottle and head into the field!
Once you’re in the field, you’ll need to find a good outcrop–like Figure 2 below. This outcrop is such an ideal one because the cliff is exposed and the different points of contact are accessible and identifiable.
Next, try and identify the different points of contact–these will be your beds. Points of contact may be recognized by a systemic change in composition of the bed. For example, you can see in Figure 2 a massive deposit of pumice under a layered deposit of ash. The systemic change in composition between these two layers indicates two separate beds and two separate phases of the Minoan eruption. Using your meter stick, measure the beds and record their thicknesses in the thickness column of your stratigraphic section. Also, don’t forget to label them in the bed column too.
Now that you’ve identified your beds, take a closer look. Be sure to jot down general observations like what color the bed is and what it is made of in your description column of your stratigraphic column. Then try to make more critical observations. How is the bed supported? How is it organized? What is the percent of lithic fragments in it? Here is where the grain size card comes in. Use the charts and measurements on it to help you quantify what you see. Again, be sure to write down all of these observations on your chart, sparing no detail. Use your rock hammer to get fresh samples of the material that makes up your bed. Does a fresh surface look different than the weathered one? What does it look like under your hand lens? All of these observations are crucial to a good stratigraphic column, as they will aid you in interpreting the deposit.
The next step is to turn all of that information into a graphic representation. Sketch a rough outline using both the scale for the height of the bed and for the composition of the bed. Once you have that, fill in the outline with symbols for what composes it. As you can see from Figure 3, a little artistic interpretation is not only allowed but encouraged for this part. You want to capture the outcrop as a whole, so be sure to include all of the details that you feel best personify the section you examined. Add notes in the margin for your future self and anyone else who you may pass along your stratigraphic column to.
Repeat the above steps for every bed you can identify. Once you have comprehensively documented your outcrop, the next step is to analyze your observations. From the patterns that you identified before, what can you tell about the outcrop and how the rocks got there?
For our stratigraphic section at Fira Quarry, we were able to decipher a few things about the origin of the outcrop we examined. Firstly, the bed of pumice with a few lithic fragments was likely from phase 1 of the Minoan eruption (1). The thick layer of pumice is indicative of an extremely explosive phase where water was not yet in contact with the vent of the volcano and pumice was raining from the sky(2). The second bed, which had layers of coarse and fine ash with some pumice, seems to originate from the second phase of the Minoan eruption (1). The characteristic layering of the ash is indicative of pyroclastic surges and it seems that during this phase water invaded the vent, causing these violent pyroclastic surges (2).
The night after we finished our stratigraphic columns of Fira quarry, I couldn’t help but read and reread my column. If a picture is worth a thousand words, a stratigraphic column is worth a million. I could clearly picture the outcrop: grey and tan color, the double band of ash, the bedding in the second bed. More importantly though, I could clearly picture what created it: pumice ejected into the air and then deposited on the island three meters thick, followed by pyroclastic surges destroying everything in their path. Chaos captured in stone and recorded in my field notebook, my little piece of Santorini, my glimpse into the Minoan eruption.
It seems a strange thing, but my stratigraphic column is without a doubt my favorite souvenir of the trip so far. It is a piece of Santorini that I will cherish forever. More than the jewelry, the clothes, or the food that I will bring home with me when we leave, my stratigraphic column captures my time here best of all.
(1) Druitt, T.H. 2014, New insights into the initiation and venting of the Bronze-Age eruption of Santorini (Greece), from component analysis. Bull Volcanology, 76:794
(2) Friedrich W.L., 2009, Santorini: Volcano, Natural History, and Mythology, Denmark: Aarhus University Press, 312 P.