Ballistic Blocks, Steam Explosions, and Turbulent Flows?

From the point of view of a fictional Minoan character-

Sitting in our boats, miles from home, I look back to where we came from. I see my beloved island sitting on the horizon as life as I know it dissappeared. Everything happened so fast these past couple days that the events are starting to blur together into a swarm of panic, packing, and leaving. I yearn to go back to my beloved Santorini, back to before my life was interrupted by the ominous signs of our once-peaceful volcano…

I am shaken awake from my restless sleep by a sound I have never heard before. It’s like rolling thunder, but much louder and stronger. I can feel it in my chest, ringing in my ears, and pulsing through my veins as if the source is all around me. Sitting up, I take in my surroundings outside of the boat. In the water all around me, I see white rocks floating on the surface. They seem to surround my boat like a layer of mud on the surface. None of the other boats with my family and friends are in sight, the rope connecting the train of boats ripped to shreds at the stern.

Coming to terms with my solidarity, I turn around to see a magnificent cloud covering the sun. Where am I? What is the huge cloud, and why does it look like it’s raining white rocks?

As the rumble attacks my senses for a second time, I see an island below the terrifying scene. Immediately I recognize the shape of the coast: it is my Santorini. Panic sets in as I realize that everything is wrong. The cloud I see above me is the same as the cloud that poured ash on our town two days ago. Could this really be what the elders were talking about?

As I drift closer the rumbling gets louder and the hot air smells like sulfur. The elders had called the white sand that rained on us ‘ash’. Maybe that is the same white rocks raining on the island right now. But it looks different. I am too far away to really tell, but it looks like rocks falling from the cloud.

Suddenly a giant mushroom shaped cloud of smoke rises above island. I see a wave of air coming towards my boat, moving clouds out of the way, but I hear nothing. All of the sudden a crack of sound shakes my boat and knocks me off my seat. My ear drums pop and I’m left sitting in my boat, staring at my island with my ears ringing.

——————————————————————————————————————

Witnessing a massive volcanic eruption, like the one described in the story above, is not a reality that most people will be faced with in their lifetime. These enormous eruptions are so rare that first hand accounts of the events are hard to come by. In the case of the Minoan eruption on Santorini from about 3,600 years ago [3], there are no witnesses and we are left to put together the story left behind in the geology of the island.

The puzzle pieces we can use to see the whole event of the eruption are the rocks deposited across the island of Santorini. Eruptions can change and shift mid-explosion causing differences in the volcanic material that is ejected from the volcano. The material is deposited in layers that help us destinguish between the phases of the eruption (Fig. 1).

     

Figure 1: Image of 3 seperate phases at Ancient Akroteri.

While exploring Santorini, our group has seen up close each phase of the eruption preserved in the rock record. My favorite one to study has been Phase 2 because I can imagine the power it possessed and the way it was created.

About a week ago, my class walked to the Fira Quarry to created our very first stratigraphic column. While cleaning up some of the trash on our way, I immediately noticed the color of the cliffs above us. Phase 2 shines bright white in the sunshine and looks almost peaceful with wisping layers and soft texture. Up close, Phase 2 is made out of soft, microscopic ash the width of the lines in your fingerprint and some small pumice pieces made from cooling magma [1]. When I touched the wall, the ash crumbled and I was left with a soft, white powder all over my hands (Fig. 2).

Figure 2: A handful of ash from Fira Quarry.

The next characteristic I noticed were small lines that look like layers, some like waves, that run the length of the cliff (Fig. 3). The lines are very thin, but still observable from a distance. These lines are called cross bedding and are a result of pyroclastic surges that occur in Phase 2 [2] .

Figure 3: Picture of cross bedding at Vlychada Beach on th outer rim of the island. Crossbedding lines are aproximately 2m wide and 1m tall.

When a volcano erupts, magma is exploded from a vent and shot straight into the air. Santorini is interesting because there is water surrounding the volcanic vent, creating a certain set of circumstances that produce pyroclastic surges. Eventually, enough of the volcanic vent is broken off that the vent becomes level with the sea surrounding it.

When the summit drops below sea level, water rushes into the vent. Because of the extreme heat generated by the molten magma,  the water immediately flashes to steam. The steam then shoots out carrying bits of rock and ash suspended in the gaseous steam, creating a pyroclastic surge. These surges are considered to be relatively cool, about 100-250 degrees Celsius [2], because of the water that invaded the vent and cooled down the magma [2].

These pyroclastic surges are a cloud of gas and suspended material like the cloud described in the story of the Minoan watching the eruption. These surges are shot laterally across the landscape at incredible speed of 200km per hour, tumbling rocks across the topography (Fig. 4). This tumbling motion, sort of like how a dryer tumbles clothes, is called turbulent flow because it is extremely bumpy and violent. As the surge of moves across the land, it deposits the ash that was suspended in the steam in wet ash layers (the same layers that I saw in the cross bedding).

 

My favorite characteristic of the Phase 2 rock record that we have seen on Santorini is called a block sag (Fig. 5).  When a surge occurs, the power of the expanding steam breaks some of the rocks off of walls. The pieces of rock are shot ballistically with immense force and land on the layers of pumice and ash of Phase 2. If the layers were dry there would be no effect, but because the layers are wet, the blocks create huge sags in the ash layers as they sink down from the force of ejection [2].

Figure 5: Recreation of block sag in Phase 2.

Volcanic eruptions can be hard to piece together without any first-hand accounts of the event (like the fictional story of the Minoin bystander). With the help of the rock record and our passionate professor and TA, our class has been able to put together the whole story of the Minoan Eruption that occurred on Santorini circa 1613 BC [3]. The story of the Minoan describes what Phase 2 might have looked like to someone watching the chaos from a distance. Cross bedding, turbulent flow, and rock sags can found in the rock record all over Santorini and help tell the story of the powerful Minoan Eruption.

[1] Druitt T. H.,  2014, New insights into the initiation and venting of the Bronze-Age eruption of Santorini (Greece), from component analysis, v. 794, p. 1-21, doi: 10.1007/s00445-014-0794-x

[2] Druitt, T. H., Edwards, L., Mellors, R., Pyle, D. M., Sparks R. S. J., Lamphire, M., Davies, M., Barriero, B., 1999, Santorini volcano. Geol Soc Lond Mem 19:165, pp 13-56.

[3] Freidrich, W., Kromer, B., Freidrich, M., Heinemeier J., Pfeiffer, T., Talamo, S., 2006, Santorini eruption radiocarbon dated to 1627-1600 B.C., Science, 312:548