Living life on the edge is not always just a metaphor. The people of Santorini take this saying to a whole new level as their cities are built upon the caldera rim. This provides for some amazing views and stunning landscapes but it is also subject to devastating mass wasting. A resistant metamorphic rock at the base of stacks of volcanic rocks creates an intense slip surface. These mass wasting events take place and will continue to occur because of the specific type of volcanic rock and their locations throughout the island.
There are several triggers that lead to mass wasting. Being that this is an island, the rocks are susceptible to the waves from the ocean. This leads to overtseepening of the rock ledges past the angle of repose (the highest angle a pile of material can be without falling). Pumice and ash are easily affected by rain and wind as the water works its way through the space with little resistance. Water saturation adds to the weight of the rocks; too much weight will trigger a landslide. This is due to the vesicularity of the rocks. Earthquakes are also triggers for mass wasting as they can shake the loose rocks and can crack the ground surface. Triggers are necessary for any form of mass wasting to occur.
There are many different rock types around Santorini, all reacting to things such as rain, wind, and additional weight from buildings, leading to mass wasting. Pyroclastic surge and flow deposits blanket the island, therefore it is exposed to the most wind and rain. It is a very weak layer of rock and is easily penetrated. The surge and flow deposits are mixtures of ash, pumice and lithic fragments (some up to 2 m). The large lithic fragments are dense lava rock and they sit in a matrix of ash and pumice that is easily eroded. As a result, rock fall from the cliffs is one of the most common hazards on the island. At Caldera beach, we witnessed a very large block sag in the layer of pumice. As that is slowly eroded away by rain and wind, that huge rock is going to come tumbling down and destroy anything in its path. It is not necessarily the ash that is dangerous but what can be stuck in it.
Scoria is much tougher than pumice and ash because it is less vesicular and is a less gaseous rock but is still subject to mass wasting. Scoria is a very coarse rock and not nearly as fine as ash or pumice, giving it a much higher angle of repose. This type of mass wasting is more specific to rockfall. As shown in figure 1, the slope of the rocks is almost completely vertical giving it a high potential for rockfall. As we traveled down to Ammoudi Bay, I witnessed many rocks at the base of the nets set up to catch the rocks. The scoria and other rocks can fall for several reasons. There could have been erosion from water or wind beneath them, they could have themselves been pulled out by the wind, or they were laying at angles higher than their angle of repose. Scoria rockfalls are very hazardous as they are part of the most common form of mass wasting on the island: rockfalls.
Before the Minoan eruption Santorini was composed of a core of metamorphic rock and deposits from the last caldera forming eruptions. Because metamorphic rock is so strong and resilient due to lack of gases and its cooling habits, it can still be found all over the southern peninsula. Santorini has had 4 caldera-forming eruptions in the last 200,000 years and as a result many layers of volcanic rocks were deposited on top of this metamorphic rock. This means that when enough rain saturates the ground, it pounds on the metamorphic rock and creates a slip surface upon which all the rocks above it can slump or slide downward when triggered. One such landslide is located at the base of the switchbacks at Athinios Port. The metamorphic rocks and previous deposits on the southern peninsula are still assisting in the current mass wasting seen today.
Despite the high potential for mass wasting on Santorini, theresidents here build upon the ledges as they are essentially forced to. With the increased weight of the buildings and roadways, they are adding to the instability of this already unstable ground. Many buildings have been subject to rockfalls and landslides throughout Santorini’s history and as a result, measures have been taken to prevent as much damage as possible but it is very difficult to completely avoid it.
In order to decrease the damage that is done to the various infrastructures, vehicles, and people in Santorini, preventative actions are taken with regards to mass wasting. To support rocks on oversteepened walls and the loose material in them, nets are set up to both hold them in and catch them at the base as they fall. There is also building into the various levels of rocks. This helps with supporting the rock above and around along with preventing the dangers of rock fall. When speaking of landslides and slumping, it is almost impossible to prevent this but to learn from mistakes in the past. Figure 2 shows a town built upon a hill that was destroyed by a terrible landslide. Learning from past mass wasting is very important in order to have a thriving civilization that is not always trying to rebuild.
Mass wasting is an inevitable part of life on the edge of a caldera rim. It is important to not only understand the rock types being built on but also what is below because of the weak rocks sitting on top of a strong slip surface. With the development of technologies that allow us to understand what is under the Earth’s surface, it is easier for us to discover what areas are safe to build on, but these luxuries were not available to preexisting civilizations in Santorini. The only way to know was to learn from their mistakes. That is a common occurrence when you live your life on the edge.