The Geology of Santorini: The island of Santorini, popularly referred to as Thera, is located in the extreme southernmost part of the Cyclades islands. It is a unique island in the Mediterranean Sea, as it is the place of several remarkable volcanic explosions that have erupted here over the past thousands of years. In a way, Santorini is a massive lagoon, which is conspicuous by its rectangular shape and measures around 12km by 7km, encircled by 300 meters-high precipices and rocky promontories.
The island of Santorini slants downhill from the high cliffs to the Aegean Sea. There is also a comparatively smaller island, Thirassia, that forms the boundary of the Santorini lagoon. Initially, Santorini was a small non-volcanic island. Mount Profitis Ilias, located in the southeastern part of Santorini, bears testament to that, as its non-volcanic limestone would suggest. Scientists believe that about two million years ago, volcanoes located underneath the Aegean Sea to the west of Santorini began generating volcanic emissions that resulted in the formation of numerous small islands. Ultimately, there came into being two colossal shield volcanoes, which are nothing but conically shaped mountains. Those conical mountains got amalgamated with the non-volcanic islands to make way for one large-sized island - Santorini. Although there is no trace of these mountains nowadays, geological scientists have appropriately named them. The mountain to the north was named Mount Peristeria, while the one located to the south was named Mount Thera.
Geologists think that 200,000 years ago, Mount Thera began to release immeasurable quantities of volcanic emissions that got deposited under Mount Thera in the form of a chamber. The eruptions were so massive that the mountain was unable to sustain itself and collapsed downwards into the vacant volcanic chamber, forming a caldera, which is nothing but an extensive yawning hole in the basement.
That explosive phenomenon of the volcano occurred continuously for the next 200,000 years and, every time an explosion occurred, it would further deepen the caldera, which ultimately shaped Santorini as it is today. Today, an archipelago has been formed, consisting of the main island (Thera), Thirassia, Aspronisi, Palea Kameni, and the island of Nea Kameni (or the volcano as it is called by the locals). The volcanic zone of Santorini is the most dynamic in the South Aegean Volcanic Arc. The volcanic arc of Santorini is approximately 500 km long and extends from the Greek mainland to the Turkish Bodrum peninsula.