Living on a volcano has both its advantages and disadvantages. On the sheer cliffs of the caldera rim, hundreds of meters above the ocean, one experiences the breath taking views of Santorini and the vastness of the deep blue Mediterranean Sea. The vibrant sunsets, lively culture, and white structured churches and villas built into the edge of the caldera rim are just a few of the treasures that set this island apart from any other. Santorini is one of the most active volcanoes in the Mediterranean region and is responsible for Thera’s striking sheer cliff faces of the caldera rim. However, with all of its beauty there lies an unpredictable hazard that could give way at any time, landslides.
Landslides are mass movements of earth material or rock caused by slope failure. Every slope has an angle of repose, that is, the angle between 0 to 90 degrees at which a slope is stable before failure. Factors that influence the angle of repose are moisture content and material angularity and size. Slopes with larger angles of repose are also at higher risk of failure due to gravity (1).
Moisture content typically generates a greater angle of repose by increasing the cohesiveness. Though there are circumstances where water can lower the angle of repose by decreasing slope stability if the material is already water-saturated. Another way to think of this is building a sand castle. On a hot summer day you decide to spend your afternoon at the beach with the refreshing ocean breeze, and all the tools needed to build the greatest sand castle. Water is required in order for the sand to hold as you stack your castle higher and higher. Due to the molecular properties of water, the addition of moisture to your sand castle increases the cohesiveness resulting in a larger angle of repose.
Angularity and size greatly influence the steepness of a slope. When I was a child my Yaiyai (grandmother in Greek) had cardboard blocks with the red brick pattern at her house that my sisters and I would play with. Some were rectangular, square, and even round. I would always see how high I could stack the rectangular and square blocks before my sisters attacked my tower. Being as little as I was I knew that stacking the ball-shaped pieces wouldn’t work because they would roll off from one another. The same principle applies to the angularity of clasts influencing the angle of repose. Clasts that are angular are better able to interlock to one another and accumulate or build upward resulting in a higher angle of repose. Less steep slopes and lower angles of repose are associated with rounded clasts because they lack the ability to interlock.
At first, I didn’t quite understand how the size of the material would affect the angle of repose but after further research I made sense of it. Porosity, the voided spaces in material, decreases slope stability. The less pore space there is the more stable a slope will be. This is due to clasts, or other material, filling those voided spaces which increases the slope’s ability to interlock. Another way to think of this is filling up a mason jar with rocks. Depending on the size of the rocks you can only fit so many in the jar and still be able to put the lid on. Now, imagine you fill a second mason jar with sand. Obviously the jar with sand will be able to fit more material in it. So now you have a jar of sand and a jar of rocks. Sounds like a band name, doesn’t it? If you tap your fingers on the lid of the jar of rocks it will make somewhat of a hallow sound. This is because of the voided spaces. But if you tap your fingers on the lid of the jar of sand it sounds dense, in lack of terms. The reasoning for this is the lack of pore space for sound to travel through. Now, let’s reel this back in to our slopes. The more porosity in a slope, the easier it is for vibrations to resonate through it. This means that something as subtle as walking could cause slope failure due to the vibrations and weight of the person. Therefore, the angle of repose is larger for more granular material than it is for coarser material.
As one approaches Thera, Santorini by ferry they are presented a showcase of Santorini’s volcanic history. Nearly horizontal layers stacked on top of one another from shield volcanoes, basalt flows draping over the sheer cliffs of the rim, and a white pumice layer frosting the top of Thera. The appearance of Santorini is like non other. However, scars of landslides and rock falls are also easily noted while approaching the island. Out of the many things I’ve learned so far from my parents it is to be aware of my surroundings. I grew up in a small mountain town in California and always envied the people whose houses were amongst the towering pine trees. But after a few cases of trees failing during vicious snowstorms and crashing into people’s houses or fires tearing through houses as if they weren’t even there I began to realize why my parents built our home where it is. The aesthetically pleasing homes with luscious green trees and shrubs creating a fortress are lurking with hazards. As breath taking as Santorini is with its display of volcanic history and a frosted pumice surface topped with white churches and villas, it is an island prone to catastrophic and unforgiving disasters such as landslides and rockfalls.
Rockfalls are exactly what they sound like, an avalanche of loose rocks. Ways in which these falls can occur are by free falling, toppling, bouncing, or rolling (1). Rockfalls that are not free falling ballistically tumble down steep slopes knocking other fragments loose. Unfortunately the road to Athinios port, the main port on Thera, is built on an old landslide scar that is now prone to rockfalls. Before I forget to mention this, locations where rockfalls and landslides occur are more susceptible to slope failure because they are zones of weakness. It is probably better that I didn’t know the road up from Athinios port is at such risk of rockfall.
Pyroclastic material, ash and lava fragments from an eruption, comprise the inner walls of the caldera rim (2). The layer of white pumice blanketing Santorini was deposited from the Minoan eruption and is the base layer for Thera’s cities. Here is a review lesson for you. Do you remember what I said about porosity? The more pore space in a material allows vibrations to travel through readily. Pumice is a highly vesicular volcanic rock. Vesicularity refers to how much pore space is in a rock. Are the pieces beginning to fit? Santorini’s landscape is prone to landslides and rockfalls!
The longer I stay on Santorini the more appreciation and enjoyment I get from its geologic history, its cool ocean breeze in the hot sun, and the refreshing scoops of vanilla ice cream with freshly baked baklava. However, the more I learn about the geology of Santorini and its hazards the more worrisome and paranoid I become. Whenever I go for a walk I often call it a hike to make myself feel more productive. My class and I went for a “hike” to Oia recently and all I could think about was how the city was decimated in the 1956 earthquake. As I mentioned before, areas where landslides and rockfalls occur are zones of weakness. There are stable locations on Santorini such as Imerovigli, Thera. Though it is built on pumice, a highly vesicular volcanic rock, the pumice rests on successive basalt flows from the Megala Vouno shield volcano. The volcanic material beneath the pumice Oia resides on is fragmented and unconsolidated upper scoria that has undergone multiple volcanic eruptions. The 1956 earthquake was so devastating due to the lack of structural support beneath the highly vesicular pumice. Fira, the main city of Thera, Santorini sits on the caldera rim overlooking the sheer cliffs. After the 1956 earthquake residents resorted to cutting into the caldera wall creating cave-like structures due to the lack of building materials. Though the volcanic material supporting Fira is stable, human development has decreased slope stability by weakening the strength of the slope through excavation for buildings and exerting more stress on the slope through the development of Fira.
Spending a little over two days researching the hazards of landslides and rockfalls on Santorini has given me a similar insight that my parent’s had when they were building our home. I am more aware of my surrounding, though I still go to Fira almost everyday to enjoy its vast views and vibrant sunsets. What has me concerned is the lack of awareness from locals and tourists. Since being in Greece I have taken note that the laws emplaced are more like suggestions. The building codes in Greece would come nowhere close to passing inspection in California. Based on the tourism and the revenue Santorini brings and provides for the economy I think it’s safe to say that the phrase, “ignorance is bliss” can be a motto for Santorini. The warning signs are all around but I am afraid they will be ignored until it is too late.
(1) Antoniou, A. A., Lekkas, E., 2010, Rockfall susceptibility map for Athinios port, Santorini Island, Greece, Geomorphology, v. 118, p. 152-166
(2) Friedrich, W. L., 2009, Santorini: Volcano, Natural History, and Mythology, Aarhus University Press, Denmark, p. 232-241.
(3) H., C. “How to Island Hop to Popular Santorini.” TripSavvy, Tripsavvy, 4 July 2017, http://www.tripsavvy.com/santorini-map-and-travel-guide-4135202.