Evolution of the Santorini Volcano

Imagine being on the island of Santorini around the time of 1613 BC. Before the power of the Minoan eruption altered the landscape forever, you would be able to see this unstable volcanic vent surrounded by a landscape that had been reworked many times before by the forces of volcanism. As you look across the island you would be surrounded by the destructive beauty of hundreds of thousands of years of volcanic activity. You might feel safe and comforted because the last eruption was over 17,000 years ago. Although this time it is different, and there is a feeling that something may change, something may occur that will truly shape the island for the future. It is only a matter of time before this volcano begins to roar again, and present Santorini with an eruption that has never been seen within the Aegean Sea or the Mediterranean for that matter.

The Minoan eruption changed the island’s topography forever (see Ray’s blog, Shaping the Landscape: A Topographical Change). It was an explosion so powerful that traces of it have been found as far as Greenland [1]. This event was the fourth caldera forming eruption that had occurred on Santorini and it left its mark. The cataclysmic event had five phases with their own unique characteristics that can be seen in the layers of rock that cover the island. Each of these layers tells a little story about the how volcanic eruption evolved.

The shield volcano, that was present in the middle of the caldera, built up a volcanic vent which rose out of the sea gradually. The vent was exposed due to the movement of magma from magma chambers to the surface of the Earth. This vent, at this specific time, was producing mafic lava that flows out of the vent creating a steady slope. This mafic volcano can be compared to the present day Nea Kameni volcano, which produces non-explosive, hot temperature lava. This vent is strategically placed inside the caldera which offers a sort of barrier from the huge amounts of seawater.

(Figure 1) Possible location for Pre Kemani volcanic vent. This vent was more to the north than modern day vent.

Another important concept to understand this evolution of the volcanic eruption is the effect of water in the phases. When seawater pours into the vent area, it is instantly turned into steam due to the intense heat of the lava. I like to think of it as dropping water on to a hot stove: once the water comes into contact with the hot stove surface, it fizzles and evaporates. This conversion from water to steam promotes small powerful explosions that do not reach deep into the magma conduit. A conduit is a tunnel that funnels magma from within the Earth to the top of the vent. These explosions although small, promote widening of the vent due to their explosive nature. When these explosions occur, the vent is excavated and further widened, in the instance a northerly direction, which allows for additional water to enter and accelerate the process [2].

If water (via the sea or a lake) is not present and the lava is left to stay hot, a different process occurs. When the conduit is exposed to the air in the troposphere, the gases in the lava react and in turn pull the magma deep from the conduit. It is almost like a straw, where the conduit is acting like the straw and the magma is being suctioned out due to the movement of volatiles (gases). When this occurs, deep explosions occur and have little to no widening effect on the vent.

Starting from the beginning, Phase 0 is the smallest of the layers present in the rock record with only a few centimeters of ash being preserved in a few places across the island. This phase is known as the warning layer, as it could have warned the then prominent Minoan population of the volcano’s intentions. Some observations by other geologists have been recorded and found that this phase had some type of water involved which is seen in the way it was deposited [1]. No definite answer has been given for the source of the water but some speculate that a rainstorm could have been taking place or a lake was present near the top of the vent. Although this phase was not quite as powerful as the following phases, it still was the push the volcano needed to set events in motion.

The next phase of the eruption, Phase 1, was pumice fall which is hot and dry compared to Phase 0. This phase had little widening effect on the vent due to the lack of water. Although the caldera was full of water at the time, the vent was still above the water level, which led to an explosive eruptions but not a vent excavating eruption. Few lithic fragments, or broken pieces of the vent, are located in this deposited layer which indicates that very small amounts of the vent were removed.

(Figure 2) Sharp contact between Phase 1 and Phase 2. Phase 1 being the hot, dry layer while Phase 2 being cold and wet. Phase 2 contains large lithic fragments from vent due to the powerful eruptions.

The next phase marks the transition from hot and dry to cold and wet. It is thought that seawater invaded the vent created pyroclastic surges (more information on these surges can be found in Holly Buban’s blog post Ballistic Blocks, Steam Explosions, and Turbulent Flows?). This presence of water in the vent creates shallow but powerful explosions that excavates part of the vent and, in the case of the Pre Kameni shield volcano, widened it further to the north [2]. Imagine this eruption starting at Nea Kameni and being widened more north with each of these cold eruptions. The pre-existing rocks that made up the volcano are broken and ejected ballistically and deposited in several locations across the island. I observed this in the layers at Fira quarry where these blocks were shot into the wet ash layers which in turn left a sag in the sediments. This sag looked like silly putty that has been stretched in this perfect symmetrical “U” shape. This phase is special in that no other layers seen in the record have these blocks sags or this putty like behavior.

(Figure 3) Block sag located in Phase 2 bed. Block was ejected from the vent and landed in the wet sediment deposited at that time.

Phase 3 then transitioned into another cold, wet phase but instead of having surges, it had pyroclastic flows.  This distinction was visually clear to me as I looked upon this putty looking layer with faint lines carved into it and then up to this massive chaotic layer of unsorted material. I felt as if the layers were telling me the story of this transition and I was able to visualize the landscape at that time. This huge cloud of hot gases, sediments, and rock pieces moving in one direction across the landscape leaving these big piles of sediments as they moved. Since seawater was still pouring in the vent at the time, widening continued to occur. The process of widening has slowly been making the vent unstable as it lacks a solid structure. Phase 3 is not the last time pyroclastic flows would roll across the landscape and create these massive deposits.
The fourth and final phase of this eruption has been found to have two parts to it. Both of these parts were pyroclastic flows like the one seen in Phase 3, but instead of both of parts being the same temperature, the first part was hot and the second was cold. After the eruption in Phase 3, rocks and sediment had been building up and protected the vent from the invading seawater. This barrier is postulated to be a tuff ring and the best way to picture what it looks like would be to think of a giant donut that has been laid right over the vent. This allows for a hot and dry explosion that hardly removes any fragments from the vent and produces hot pyroclastic flows that roll across the land in different directions.

(Figure 4) Sharp contact between Phase 3 and the cold part of Phase 4 at Cape Mavropetra. Hundreds of lithic fragments present in this Phase 4 layer due to collapse of the caldera. Represents the difference between the hot pyroclastic flow of Phase 3 and the cold pyroclastic flow of Phase 4.

The second part of Phase 4 gets much more intense because water is again added to the equation. This creates complete and utter chaos because as the eruption occurs, the tuff ring breaks and water floods into the vent [2]. As the water is added, the vent begins to create small powerful explosions which are continuously emptying the magma chamber [2]. Except this time the explosions are too powerful and the vent collapses into itself. This causes an enormous amount of rock and lava to be excavated from the vent and thrown across the island. This process is clearly scene in the layer at Cape Mavropetra. The chaos that ensued is beautifully captured in the layer and it looks as if all hell had broken loose those thousands of years ago. Many different types of rocks are encased in this layer and they range from igneous all the way to metamorphic. All these rock are present due to the mass excavation of the caldera and all the layers that have been deposited over time. This phase was the final phase of the Minoan eruption and it surely delivered a thrilling end to the volcanic story.

Now if you can picture all these layers and slowly put them together, it would look like a complex cake that has several layers and flavors. This cake has all the layers, some better looking and better prepared, while some look like they were created quite hastily. These layers display the evolution of the vent throughout this eruption and tell the story of the long forgotten volcanic history. It is a reminder that lets people know the true power that this caldera once had and also a gaze into what may come if it decides to erupt again.

References

1. Friedrich, W. L., 2009, Santorini – volcano, natural history, mythology: Denmark, Narayana Press, 79-94 p.

2. 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