Our first day of class on Santorini was nerve wracking for me. I had no clue what information Professor Skinner would be telling us, how fast paced the class would be, or what knowledge I would have to remember from when I took Geologic Disasters two semesters ago. It helped to see that everyone was in the same position I was in, all nerves with a slight bit of excitement for the unknown information.
Professor Skinner first began by saying that Santorini was made up of different volcanic layers, which was evident to me when we arrived on the ferry the day before. I saw the red scoria from cinder cones, the layers of basaltic lava flows from shield volcanoes and the thick layers of ash and pumice. I knew about cinder cones and shield volcanoes since I had taken Geologic Disasters but I wanted to know more about the volcanic events that created this breathtakingly beautiful island. This led into the class which we spent talking about the volcanic eruptions that formed the island, and what the impacts of each of the eruptions were, from forming the island to breaking it apart. I was immediately fascinated with the fact that Santorini was created by old lava flows and many other types of volcanic rocks.
Before I get into the information about how the island formed, some background knowledge is needed. Magma is formed by the melting of the asthenosphere under the earth’s crust. As magma sits over time, it becomes more explosive causing larger and more violent eruptions. [If you want to read more on this, visit Briana Zives blog Its Poppin’ ]
To begin, this area of the Aegean sea was two large pieces of basement rock called Profitis Ilias, composed of mostly limestone and some metamorphic rock aging between 50-20 million years (ma), brought up to the surface by normal faulting that began 20-10 ma and is still continuing today. Because magma comes up to the surface in the easiest way possible, it came up that same normal fault that pushed the limestone up previously. This created the earliest volcanoes that started to shape Santorini.
Santorini has had 12 major volcanic eruptions in its history which have resulted in four different caldera collapses. In between these caldera collapses there were many smaller eruptions that built the island back up again. I am going to focus on the first and last caldera collapses and some of the smaller events that built them back up. These were the ones that caught my interest the most in class that first day on the island.
The first volcanoes to erupt were the Akrotiri domes that formed the southwestern part of the island at around 650-550 thousand years ago (ka) . There is evidence of marine conglomerate that is a red color on the southwestern tip of the island today which is evidence that not only were the Akrotiri domes the first volcanic activity on the island, but that the volcano began underwater and built up over time to create that part of the island .
Then around 500 ka, the Peristeria composite cone began to form and erupt to the north of the basement rock and the Akrotiri dome complex. At around the same time, the Thera-Volcano was thought to be erupting as well and began connecting the space between the previous dome complex and the basement rock . The actual span of how large that volcano was is unknown from lack of geological evidence [this is shown by the dashed line].
The next major event in the history of Santorini are the lower pumice eruptions that happened around 200-180 ka. This was a major explosive series of eruptions that blanketed the existing islands with pumice and ash that is on average 30 m thick, though there are places with thinner pumice deposits. It was thought to have filled in the space between the existing islands and create a larger island.
The lower pumice eruptions led to the collapse of the volcano. It began to be built up again by many shield volcanoes and domes.
Fast forward to around 15 ka after the third Cape Riva caldera collapse. Stromatolites begin forming. If you aren’t a geologist, like me, and have no clue what stromatolites are, here is a quick definition. Wikipedia explains stromatolites as being “layered bio-chemical accretionary structures formed in shallow water by the trapping, binding and cementation of sedimentary grains”, so in other words, it is a very unique rock type and it gives us much information about what the island looked like at that time . With this geologic evidence, we can know that there was water inside the caldera at this time. This also means that at least the northern bay area was shallow. There is also evidence that there was a dome in the center because dome fragments from the vent have been found inside the Minoan pumice and ash deposits on the rest of the island.
The dome in the center of the island capped the volcano and built up the pressure. When the pressure built up and was finally released, it was the start of the Minoan eruption and caldera collapse. The Minoan tuff is the most recent caldera eruption in Santorini’s history. During this eruption, thick beds of pumice and ash blanketed the landscape covering everything.
In around 197 BC new volcanos began to resurface in the center of the caldera once more. First the island of Palea Kameni was formed. The island Nea Kameni which is also an active dome volcano is still forming to this day.
Over the past 180 thousand years the shape of the Santorini archipelago has changed many times from the first building up of the caldera with the lower pumice eruptions to the most recent Minoan eruption and caldera collapse. Now the process is starting over again with the domes, Palea Kameni and Nea Kameni, in the center of the island now.
From the second I saw Santorini from the ferry on our way through the caldera I was captivated by the striking view of the unique layers of the island. I had to know more. Luckily our first class on the island answered my questions and I found out that it was shield volcanoes, domes, and layers of pumice and ash deposited by the 12 major explosive eruptions. I can’t wait to learn more about the eruptions that formed the island and get an image in my mind of what it looked like while it was forming.
- Friedrich, W, 2009, Santorini Volcano Natural History Mythology: Denmark, Aarhus University Press, 312.
- “Stromatolite.” Wikipedia. Wikimedia Foundation, n.d. Web. 06 June 2016. <https://en.wikipedia.org/wiki/Stromatolite>.