I started out my day sweating more than I would like, staring up at a large wall of rocks. It was weird because I never thought I would be doing that, let alone in Santorini. The sun was sweltering hot, sweat was dripping from every part of my body, and I could feel the stinging of my burning skin. Against my instinct I turned my body away from the beautiful crystal blue water crashing against the rocks under my feet. I sat uncomfortably with my feet and legs falling asleep on the hot rocks. We were learning about part of phase 4 of the Minoan eruption at Cape Mavropetra.
There were 5 phases of the Minoan eruption, each differing by how the volcano was erupting. When looking up at the deposits from the Minoan eruption (around 1613 BC), there are distinct layers that you see; each of these is a phase of the eruption .
As I sat there staring up at the massive wall of rocks in front of me, I tried to make sense of what I saw. Lisa was having us draw outcrop sketches which meant we sat there staring up at the wall, studying it, to try and understand how everything got there. Above phase 3 was the part of phase 4 we were studying for the day (image 1). Looking up, there were thinner bands of pumice and ash that were a light tan color. At the bottom of the layer there were huge lithic fragments.
Overall this bed of phase 4 was full of lithic fragments. These were not the same lithic fragments that we had been seeing for the past two weeks. These lithic fragments are from every volcano (shield, cinder cone, composite cone, tuff ring, and dome) that had built up this island and even the basement rock that was here before the volcanoes. Not only are the lithic fragments every type of rock, there are also an incredible amount of them, unlike anything we had seen anywhere on the island (seen in the image below). They were red, grey, and many shades of black, scattered chaotically throughout, but also in bands of their own along the wall.
You might be wondering how all of these lithic fragments got there just like I was. These lithic fragments were carried by a mix of pyroclastic flows and surges. These pyroclastic flows deposited the heavier rocks before the lighter rocks which is why this part of phase 4 has so many lithic fragments.
The lithic fragments in the deposit are the same ones that make up the beach. The only difference is that the rocks on the beach have been rounded from years of water rushing over them from the ocean.
This part of phase 4 was a phreatomagmatic eruption. Phreatomagmatic eruptions are eruptions where water pours into the vent where it mixes with the magma and instantly turns to steam which causes shallow explosions and eruptions. It is like pouring water into a hot pan and watching it immediately turn to steam. The water also cools the temperature of the eruption to around 200-300 degrees celsius causing the pyroclastic flow to be wetter. In the case of this phase of the Minoan eruption, the vent was emptying, meaning it was releasing all of its stored up magma. When most of the stored magma had been ejected from the magma chamber, the dense rocks around the vent caused a collapse and then water poured into the vent. Instead of causing shallow eruptions, the seawater rushed in and caused the pyroclastic flows and surges.
The explosions and collapse would have blown the vent out along with the remainder of the magma in the chamber. Since the vent area had rock from all of the volcanoes making up the island, cinder cones, shield, composite cones, domes, and others, those rock fragments became part of the flows and surges.
While sitting in class that night after having stared at the massive wall of rock, and analyzing and imagining what the eruption could have looked like, Lisa told us something that would forever change my view of the island. She told us that with evidence from ash and pumice that was deposited in Crete and western Turkey, and deposits from the tsunami (a result of the caldera collapse), the whole Minoan eruption probably went down in around 24 hours. That left me speechless in class, and even now, two days later, I am having trouble grasping the fact that this beautiful island was changed so drastically in that short amount of time.
Sitting on the beach in the sweltering sun, drenched in more sweat than I care to admit, looking up at a wall of rock deposited there by pyroclastic flows and surges, was a helpful step in understanding just what happened during the Minoan eruption. I also learned that all of the lithic fragments, and rocks on the beach that came from the deposit, were from all of the volcanoes that helped build up the island. I solidified that knowledge while sitting in the breakfast room and finally learning that this massive caldera collapse happened in just around 24 hours. It amazes me that something that took thousands of years to build up was destroyed in such a short amount of time. As I sat with my classmates that night reflecting on what we just learned, I marveled in the wonder of this beautiful island and how the chaos turned into beauty.
- Druitt, T, 2014, New insights into the initiation and venting of the Bronze-Age eruption of Santorini (Greece), from component analysis: Bull Volcanol, vol. , 1-21.
- To learn more about pyroclastic flows and surges, refer to Jessica Aerts’s blog “The Forces that Drive Rock” !
- If you want to learn more about the first part of phase 4 of the Minoan eruption, go read Jenna Chaffeur’s blog “To The Moon, Pyroclastic Flows, and Beyond!”!