Α. Creation of Western Kos and Pyrgousa – Pahia

1. • The first eruptions in the Kos – Nisyros volcanic field – the eastern extremity of the Southern Aegean volcanic arc – date back to around 3.4 million years ago and the rocks generated by these processes are currently present in western Kos. The first period of volcanic activity lasted up to 1.6 million years ago. Molten rock commonly outflows at moderate speed; only in a few cases it violently bursts out of the earth core, creating several minor volcanic centers in western Kos. The Pyrgousa and Pahia islets, as well as a series of other volcanic centres now located below sea level east of the Kandaleoussa islet, all appear during this period.

1. • Volcanoes located in the area remained dormant for around a million years and then abruptly returned to activity, through successive eruptions in the current Kefalos bay, creating the local tuff ring of the same name

1. • The subsequent moderate volcanic activity produced Zini hill, also in Kefalos bay, approximately 500,000 years ago.

1. • The most prominent volcanic event in the region occurred 161,000 years ago. A severe eruption broke from a massive volcano, located at that time in close proximity to the area occupied today by northern Yali. The eruption blasted 240 billion tons of molten rock and covered the largest part of Kos with layers of pumice stone and volcanic ash, amounting to several tens of meters. Thick clouds of blazing ash would run off to all directions. These would quickly travel on the surface οf the sea to reach Kalymnos, the Turkish coast, Pyrgousa, Pahia and Tilos, covering a total area in excess of 3,000 sq. km. The fine ash, blasted into the air, is capable of traveling thousands of kilometers away. Layers with deep sea sediments containing this type of ash have been discovered 300 km away from the south of Kos. Yali and Strongyli did not exist at the time, while Nisyros was only an underwater volcano whose top was probably just above the sea level.