The Acropolis in Athens sits high above the city on a peculiar limestone mound. The Acropolis has had many uses since its installment: it provided a vantage point for defending the city, and fresh water springs around the base of the mound provided a place to wash and bathe. But most importantly, because of the immense size of the towering limestone outcrop, the Acropolis sat high above the city for all to see. It was a point of pride for the ancient Athenians, as well as a place to worship the gods. The Acropolis is unique because of its placement on the limestone mound, but what makes this mound special?
Limestone was not deposited in this area, it was deposited about 120 km away and was moved to Athens by tectonic activity. The limestone is about 100 million years old and lies on top of Athenian “Schist” which is only 70 million years old. So why is the older limestone on top of a younger schist?
The limestone was deposited as a regionally extensive body. Limestone is formed when calcium carbonate precipitates out of solution in a shallow marine environment. The calcium carbonate is deposited on the seafloor where it accumulates over a long period of time and is then lithified. Because of its composition, limestone is a very strong rock, it is not easily broken. It would require a great deal of force to move it even an inch. (Pictured here is a limestone pedestal from the Ancient Agora of Athens).
About 35 million years ago limestone was thrust over top of the Athenian Schist during a tectonic event known as the Alpine Orogeny which was responsible for building the Swiss Alps. In order to move a layer of rock such as limestone and emplace it on top of another existing layer requires a great amount of sheer force. This type of force is called a thrust fault. The thrust fault is responsible for the peculiar placement of the limestone in this area. (This is a diagram of a thrust fault. The limestone is thrust over top of the schist).
There are other limestone mounds around Athens from this same faulting event. The Acropolis, though it is built on limestone is actually composed of marble from the surrounding area. Marble is metamorphosed limestone. When the thrust fault occurred, the sheer stress of forcing the limestone over top of what was then shale, heated and pressurized the rock turning some of the limestone into marble, and turning the shale into schist along the fault plane. Heat produced by fault movement can cause metamorphism along the fault plane. This is called contact metamorphism. (In this photo a marble footpath on the right sits next to an outcrop of schist on the path up to the Acropolis).
This is a sketch of what the limestone mound looks like in cross section. The limestone sits atop a layer called cataclasite. Cataclasite is broken up pieces of the limestone and some shale, it represents the fault plane of the thrust fault. Below the cataclasite is the Athenian Schist. Normal faults cut through all of the layers of rock present, indicating that the faulting happened after all the rock was in place. This normal faulting is a result of extensional forces that have been ongoing since the Pleistocene epoch.
The Athenian Acropolis still stands today due to its impervious marble constitution and significant placement atop the limestone hill. The hill provides a vantage point for defense due to it’s towering height. In addition fresh water springs come out of the base of the limestone hill due to water permeating the limestone and being flushed out through the cataclastite. The Acropolis has been under attack many times, it has been destroyed, abandoned, but never forgotten through the course of history because of its installment on the limestone hill. If this bold feature had not been built atop such a prominent geologic outcrop, it surely would have been destroyed and forgotten with time, but it still stands strong today.