June 8th, 2016: Today, NAU in Greece visited the moon…in a Fiat van. With my head out the window and my hair not-so-elegantly-wind blown, we had arrived to Vlychada beach. Upon arrival, my ears were overwhelmed by the sound of the waves gently picking up stones and dropping them and my eyes didn’t know whether to take in the deep blue of the ocean, or the rigid outline of distant islands, or the massive moon-like rock wall to the left of me. All of this pleasant thinking was quickly interrupted by a hefty gust of wind that dusted my eyes with a uninvited layer of ash. At that moment, I directed my interest to the origin of my pain: the moonscape.
Before giving the class any answers, our professor, professor Skinner, asked us to sit and observe. I first noticed the unique crevices within the rock that made me think of the moon. The crevices threw me off because almost all of the other rock layers we had observed on this trip had been solid and maybe a little rough, but certainly not covered in holes like this one. Figure A, below, shows the unique texture of the rock wall bordering Vlychada beach.
As I was intently looking at the holes in rock, another gust of wind came through. This time, I observed the aftermath of the wind on this rock. As each gust came, it swept up grains of ash and pumice up and away from the layer.
After that observation, professor Skinner had informed me that these holes in the rock are a process of weathering. This process is called ‘taphony’, a geological term for which material is carved away by wind patterns to create a pocket-like texture. In this process, less dense materials (ash) are most likely to be swept away by wind, leaving more dense materials behind (pumice). Hence, the culprit for the ash in my eye.
I then observed that this layer of rock was a darker, more tan color than the one under it. Figure B, below, highlights the color change from the lower bed to the upper bed. To me, this was unusual because we were studying the same eruption, the Minoan eruption, so I expected there to be certain similarities. This layer was indeed from the Minoan eruption, however, the temperature of the deposited ash and pumice was different…it was hotter.
In my first blog post (link here: https://nauingreece.wordpress.com/2016/06/06/karavolades-stairs-a-600-step-geologic-wonderland/), I talk about an orange layer in a bed of ash and pumice on the caldera rim as a result of oxidization due to heat. The same process was in play at Vlychada. The ash and pumice deposited here was so hot, that the layer oxidized and became a tan/red color.
The reason why this mix of ash and pumice was so hot, was because the magma making the deposit did not interact with water to cool it down. Hypotheses have been juggled around that state that the water in the caldera was either 1. used up by other phases or 2. blocked from the volcano’s vent. A volcano’s vent is the location in which magma and other volcanic material is ejected from.
Next, I noticed how massive this layer was. Compared to the other layers, which were maybe 12 meters maximum, this layer was HUGE. It was impossible to measure the height of the layer with the mere 2 meter measuring stick we had. Figure C, below, shows in relation to the layer beneath, how massive the bed of rock is.
A fourth observation that I had made was that there were no lithic fragments in this layer. Lithic fragments are rock pieces that were a part of the volcanic vent and were erupted from the volcano upon explosion. In other layers of the eruption that we studied on the island, there was a plethora of lithic fragments, which led me to wonder why they were not present here.
Material in the magma chamber is geologically referred to as primary magma. When primary magma is exposed to atmospheric pressure, a vacuum effect will take place and essentially suck all of the magma out of the chamber like a straw and throw it onto land as ash and pumice. Figure D, below, outlines the situation in which magma is ‘vacuumed’ from the chamber.
The presence of ash and pumice in any eruption is because of the gasses in the magma chamber expanding rapidly. This process that breaks up the magma is called magma fragmentation. The fact that this thick layer contains solely ash and pumice says that the eruption at this point had an efficient magma fragmentation process.
This layer that I was so fascinated by, represented the first part of Phase 4 of the Minoan eruption in 1613 (+/- 13 years) B.C. Due to events within previous phases, the vent was widened and the water was blocked, allowing for the magma to be efficiently ‘vacuumed’ from the 6 kilometer deep chamber at extremely high temperatures.
This first part of Phase 4 was a hot pyroclastic flow. A pyroclastic flow is a result of the collapse of volcanic material that was shot up into the atmosphere by a volcanic event. The flow is bound by gasses and is very hot. Pyroclastic flows move quickly and destroy everything in its path with no mercy.
Together, the thickness of the layer, the absence of lithic fragments and the color difference in the bed of Phase 4 all serve as evidence for the Santorini volcano preparing for it’s historic collapse (To learn more about the Minoan collapse, visit Erin Kaplan’s post: The Ultimate Collapse).
The Minoan eruption was a devastatingly beautiful event. After learning in depth about the first part of Phase 4, I feel as if I can take a step back in time and admire the magnitude of this eruption. After a period of violent gas expansion, this rock layer was layered on the land and remains here for people to observe and learn from today.
As both a tourist and a geologist-in-the-making, I am confident in saying that I have finished reading a mere chapter in the extensive, historical, and beautiful story of Santorini.