As I began the hike to Ancient Thera, I saw before me an immense mountain of metamorphic and sedimentary rocks. Due to the rocks being millions of years old, I felt as if every step I took was another leap back in time and a look into the origins of Santorini. The tectonic setting in the Aegean Sea is rare due to the several processes that are constantly in motion. A subduction zone, normal faults, and a transform plate boundary come together to make a unique geologic setting.
For me, learning tectonism for the first time was like learning how to ride a bike, in the sense of it took me several tries to fully grasp it. Plate tectonics is the theory that the Earth’s crust is broken into several different plates, some larger than others. which move independently of one another. The movement of plates is driven by the circulation beneath the Earth’s crust. These plates can either converge, diverge or slide past one another which, in the case of the Aegean, can create an interesting geological area.
The figure below shows the convergent plate boundary between the African plate (South of the yellow line) and Aegean plate (North of the yellow line). These plates are converging or coming together which means one plate is falling underneath the other due to differences in density.
With the various movement along each of these geological settings, they are turning the Aegean plate at about a 23 degree rotation . This happens due to several components, the first being the movement of the Northern Anatolian Fault which is moving the northern part of the plate to the West. Also the the extension that is occurring due to the normal faulting that is taking place slightly north of the subduction zone, is pulling apart or thinning the crust of the Earth. This thinning process is due to the extension of normal faulting as a result of the diving pulling back on the overriding plate.
Subduction rollback is a process that occurs when the diving plate gets stuck with the lower part of the overriding plate due to heating. As the lower end of the subducting plate moves further into the Earth, forces pull it back in the opposite direction of its original movement. This in turn pulls the bottom of the overriding plate as well which in turn creates extension on this plate and propels the movement of normal faults.
Faulting is an important concept to help grasp the concept of the Aegean and how some of the islands originated. Faulting is the process of having movement along a crack, in sections of rock, that move in independent directions of each other. There are three types of faults: the first one is reverse which is associated with compression,the second is a normal fault which is more of an extension and finally there are transform faults. The main fault types that are present in the Aegean are normal faults and transform faults. Through the process of extension, normal faults have a thinning effect on the Earth’s crust.
On either side of a normal fault there is a hanging wall and a foot wall. In a normal fault the foot wall will be the section that is rising relative to the hanging wall. The figure below shows the movement of this fault along with how the preexisting rock for Santorini rose to the surface. The movement of the pre-existing metamorphic rocks, mainly phyllite, from lower part of the Earth’s crust to the surface is associated with the hanging walls and foot walls.
The picture below is Mount Profitis Ilias which is an enormous foot wall that had risen out of the Earth millions of years ago. If you can imagine this huge metamorphic, which is 50 to 23 million years in age, rose relative to the hanging wall and made its ascent to the surface of the ocean. This foot wall is the oldest on Santorini and it helps geologists uncover the history behind the beauty of the island. This same process was used to create the other metamorphic islands in the Aegean Sea due to the propelling of the foot wall towards the sea surface.
Once the pre-existing rocks reach the surface, the extension of the seafloor allows for spurts of volcanism to take place. As this magma accumulates towards the surface, domes, shields, and cinder volcanoes begin to form around the ancient rocks. This slowly build up to create both volcanic and non-volcanic islands.
As the years go on, these geological processes will continue to further change the position of the Aegean. The rate of the subduction rollback along with the increase in rotation of the Aegean block may create a completely new geological area. Also movement along the normal faults could potentially propel more pre-existing rocks through the surface of the sea. The future of Santorini and the islands that follow now lies with mother Earth and her mysterious ways.
 Piper, D.J.W, and Perissoratis, C., 2003, Quaternary neotectonics of the South Aegean arc, Marine Geology, v.198, p 259-288.