I am currently studying geology on the island of Santorini. One of the concepts that is really important in doing that is how the islands in the Cyclades form. For most of the islands, plate tectonics hold the answer.
The idea of plate tectonics is that the Earth’s crust is made up of plates. These plates move around and crash into each other which is what causes most of Earth’s volcanism and earthquakes.
At the edges of each of these plates are smaller plates that are called microplates. These are usually grouped with the larger plates because they behave in a similar fashion. However, in the Aegean, it is important to understand that these plates are separated from the main plate by fault lines and sometimes act separate from the main plate.
In the Mediterranean, the African plate in moving under the Eurasian plate and the Aegean microplate. When one plate moves under another plate. It is called subduction. This creates a trench in the sea floor at the plate boundary which is marked in red in the above map. Parallel to subduction zones are volcanic arcs, including one within the Aegean Sea.
As the subducting plate sinks into the Earth, it pulls the overriding plate with it. This is known as subduction rollback and it makes the overriding plate move faster near the plate boundary.
The pulling from the subduction rollback creates uneven movement within the overriding plate. This leads to extension and thinning of the crust within the plate or microplate.
In order to thin the crust, normal faults form. Faults are breaks in the earth. Normal faults have a hanging wall and a footwall. The hanging wall is on top of the footwall. In a normal fault the hanging wall moves down relative to the footwall. The faults in the Aegean tend to point in a line from northeast to southwest.
As the hanging wall moves down relative to the footwall in a normal fault, the footwall also moves up relative to the hanging wall. After enough time has passed and the footwall has moved enough, the footwall will emerge above the water as an island. This is how most of the islands in the Cyclades are formed.
However, a few of the islands were constructed by volcanism and are either active or dormant volcanoes. These volcanoes are located on a different type of fault called a strike-slip fault. Strike-slip faults do not have a hanging wall and a footwall, but move horizontally whereas normal faults move vertically.
The strike-slip faults that are located with the volcanoes are special in that they are oblique strike-slip faults, so they move predominately horizontally but also move vertically. This type of motion creates a gap where the magma can move to the surface. Which is why there are volcanoes on these strike-slip faults, but no volcanoes along a strike-slip fault such as the San Andreas.
There are only four volcanic centers within the Aegean Sea. The rest of the islands all formed from normal faults. The four centers in order from West to East are Methana, Milos, Santorini, and Kos/Nysiros. These are circled in pink on the map of the Aegean above.
Plate tectonics created faulting, which then created islands and allowed magma to flow through the crust and make volcanic islands. I think this is a very important concept to understand in order to truly comprehend why there are so many islands within the Aegean Sea.
1. Walter Friedrich, Santorini, Aarhus University Press, 2009