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.
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).
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.
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.
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).
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.