The island of Milos in Greece is well known for its beautiful scenery. One of its prominent features is the Sarakiniko Beach, whose thick white rocks formed through highly explosive volcanic eruptions as almost the entire island is volcanic in origin. However, the volcano which caused the creation of this unusual landscape is by no means extinct but rather is still active.
Looking around the island there are a number of spots in the landscape where volcanic gasses rise out of the ground, or bubbles rise to the surface just off the coast from submarine fumaroles. And, in 140 CE, a volcanic eruption was witnessed by Roman residents, leading to the destruction of a small town near the coastline. Although the island of Milos is less volcanically active than the island of Santorini, it could still produce a similar repeat of that Roman era disaster.
Where is the Milos volcano located?
The volcanic island of Milos is located in south central Greece, where it is 150 kilometers south-southeast of the city of Athens. While it might initially look like this island contains the remains of an ancient caldera, this is not the case. Rather, the water filled “U” shaped section represents a tectonic graben, aka a low point caused by two large faults which run adjacent to the edge of the bay. Volcanic activity over a timespan of several million years formed vents which were centered to the east and west of this bay, thus creating the island’s modern shape.
The island of Milos is part of the Aegean volcanic arc which also contains the volcanoes Methana, Santorini, and Nisyros. It exists due to a tectonic plate collision more than 150 kilometers southwest of the island where the African plate is subducting underneath the Aegean Sea plate. The melted material from this collision then migrates upwards until it erupts onto the seafloor, forming a chain of shallow submarine volcanoes.
Volcanic activity at Milos began 3.3 million years ago when a variety of unusually viscous rhyolite magma erupted onto the seafloor at depths of approximately 300 meters. Although some explosive activity occurred, it was largely suppressed by water pressure at this depth. Over tens of thousands of years a series of rhyolite lava domes grew on the ocean floor and overlapped one another, forming the beginnings of the Kimlos island which is located northeast of Milos.
The construction of the island of Milos began 3.08 million years ago when a similar series of submarine vents erupted onto the seafloor. Over the next several hundred thousand years several dozen vents erupted in the same region, forming much of what is now the western half of Milos. Around 2.6 million years ago, a cluster of highly explosive eruptions occurred at shallow depths to the east, ejecting large volumes of white tephra into the air.
Much of this tephra settled back to the slopes of the existing volcanic edifice, forming a thick bed of material which eventually became Sarakiniko Beach. For context, large rhyolitic eruptions often leave behind large volumes of white tephra that is primarily composed of pumice. Take for example the similarities the tephra layer has to the deposits left from the caldera forming rhyolite eruptions of Mount Churchill in Alaska. Due to a combination of volcanic activity and magmatic uplift, the volcano permanently broke the ocean surface around 1.4 million years ago. The two islands later connected, forming the modern U shaped Milos.
Milos volcano last eruption
In the last 1 million years, eruptions have almost universally occurred on the eastern side of the island. One of these eruptions formed a beautiful cluster of polygonal columns shown here. Since then, volcanic activity has been largely phreatic in nature, constructing a series of tuff rings.
One such 1700 meter wide tuff ring on the south of the island formed in a series of highly explosive eruptions between 110,000 and 60,000 years ago. The only historical eruption occurred in 140 CE when a volume of magma caused a large chunk of groundwater to flash to steam and explode.
This created a pyroclastic flow which raced towards the coastline, burning several buildings. These buildings would later be buried by lahar deposits up to 4 meters deep. Although an eruption is unlikely for the immediate future, if one were to occur it would almost certainly be phreatic and occur in the region which I have outlined on screen.