Before taking any type of geology class, the only thought I had about volcanoes was how terrifyingly huge and explosive they were. I never would’ve thought that almost a year later, I’d be in Greece studying the eruption of the Santorini Volcano. Before getting to understand the development of Santorini, I had to first understand how a volcanic eruption occurs.
Beneath the surface of the earth, a magma chamber is formed. A magma chamber is an enclosed space that holds magma and dissolved gases. There are three different types of magma: mafic, intermediate, and felsic. All magma starts out as mafic. Mafic magma has the greatest temperature and the lowest percentage of silica content. As the magma gets closer to the surface, the temperature decreases, but the silica content increases. If you put these two together, you get viscosity. Viscosity is how resistant the magma is to flow. For example, low viscosity is like oil, extremely runny. While high viscosity is like honey, very thick. In this case, mafic magma has low viscosity.(Figure 1)
Figure 1, (Chart of Magma Types, the silica content, viscosity, temperature and volatiles.)
The longer the magma sits in the crust, the more volatiles (gases) are added making the magma more felsic. The magma becomes so felsic and highly viscous that it starts to stack creating a dome also known as what our professor calls the “cap.” The “cap” seals the magma chamber which increases the pressure inside. When the cap is removed or cracked, usually from earthquakes, the pressure is released and the gas leaves solution so rapidly the chamber produces a volcanic eruption, having the lava spread across landscapes.
In order to understand this concept, let’s look at this process in a different context. What really helped me was putting this concept into a geologic analogy. In order to see this work. Let’s use a soda bottle.Figure 2, the comparison of a soda bottle and the process of an eruption.
Inside a soda bottle lies liquid soda mixed with carbon dioxide (CO2). The soda bottle is like the magma chamber (Figure 2) holding dissolved gases in an enclosed space. The magma is sealed by a cap just like the soda is closed with a cap.
If the soda bottle is shaken, the pressure increases because the cap is holding all of the liquid and gases inside. When removing the cap, the gases expand which explodes the liquid apart creating a messy, informational way of understanding a volcanic eruption.Figure 3, the soda bottle being shaken and the pressure building up inside the magma chamber.
After the magma chamber empties, it starts to build up again. Again? I know. I was confused too. This is called periodicity. Periodicity is the eruptive frequency of a volcano. Periodicity of a volcano can depend on many things such as the composition of the magma or how the magma rises.
To understand periodicity, let’s look at the Santorini eruption. The islands of Santorini did not just form from one single event. The twelve eruptive phases and the four caldera collapses is what made the shape of Santorini. Each volcano eruption erupts at its own time and way. For a clearer understanding of the different layers refer to Jenna Chaffeur’s “Karavolades Stairs: A 600 step geologic wonderland.”
As I face the orange-yellow Santorini sky, overlooking the caldera, I now think about the gases building up under me. Beginning to understand how the earth is not just what I see. A magma chamber is like a shaken up soda bottle. As the magma sits in the magma chamber, it goes from mafic to felsic magma increasing the silica content and the addition of volatiles. The pressure creates a cap and once that cap gets slightly opened all the pressure is let out, the gases expand, leading up to a massive, scorching volcano. With the magma chamber empty, mafic magma begins to build again and it starts all back up. With all this information now embedded in my brain, I can never look at a volcano the same again. I will never just think of a volcano being terrifying and huge. I will think of how the volcano occurs, what really goes on underneath us and how it can affect the people, culture and the famous white and blue buildings this city is known for.
Friedrich, W, 2009, SANTORINI – Volcano – Natural History – Mythology; Narayana Press, DanmarkThe Author and Aarhus University Press, 300 pages.