The multitude of contradicting values resulting from radiometric dating of the Nisyros lavas, as well as from dating of the volcanic ash layers in deep sea sediments makes it difficult to establish any correlations between them and

prevents us from drawing definite conclusions regarding the precise age of the volcanic formations.

No radiometric dating is available for lavas constituting the volcanic edifices of Pahia, Pyrgousa and Strongyli. The existence of deposits of Kos Plateau Tuff above the Pahia and Pyrgoussa lavas, define their age as being older than 161 thousand years (Κa). Juxtaposing these occurrences to the ones of lavas of Western Kos, the age that could be attributed to them would be around 2 million years (Ma). As regards the Strongyli lavas, the Upper Pumice of Yali situated in its crater would define their age as being older than 31 Ka. The absence of deposits of the Kos Plateau Tuff on the subaerial part of the islets also permits us to estimate that they are earlier than 161 Ka.

As regards Nisyros, the lack of deposits of the Kos Plateau Tuff on its subaerial part allows us to estimate that they are earlier than 161 Ka.

The first K-Ar radiometric dating in the area (Fytikas et al., 1976) yielded an age of 200 Ka for rhyolitic lavas of Argos (formation 17). Nowadays, this age is considered to be unreliable.

An initial 14C dating on carbonized wood fragments contained in the surge deposits of the Upper Pumice of Nisyros (Limburg & Varekamp, 1990), has shown an age >55 Ka (i.e. beyond the limits of the method). Dating by fission tracks on the glass of the same pumice yielded 114 Ka (Vougioukalakis, 1984). A recent iteration of 14C dating, performed by a high precision method on fragments of carbonized wood picked from the ceiling of the paleosoil intercalated between the Lower and the Upper Pumice of Nisyros, (Keller et al. 2007) yielded an age of 35 Ka.

Dating by fission tracks on Yali obsidian yielded an age of 24 Ka (Wagner et al., 1976). Dating of ash layers on deep sea sediments gave age values of 31 Ka for the Upper Pumice of Yali (Federman & Karey, 1980) and 24 Ka for the Upper Pumice of Nisyros (Vinci, 1985). The last attribution is apparently incompatible with the area’s stratigraphy.

A series of radiometric dating K-Ar measurements has estimated ages between 66-38Κa for Argos and Pahia Ammos lavas (formations 18, 16) and one 14C dating yielded 24 Ka for the paleosoil under the Upper andesitic lavas and volcaniclastics (Keller et al., 1989).

By compiling all the above data, and taking into account the stratigraphy of the site and the outlook from hiatuses between volcanic activity periods (re-deposited formations, paleosoils, degree of erosion), we may now propose the following chronologic order for the volcanic activity in the area:

Pahia and Pyrgousa were formed approximately 2 Ma years ago, along with the respective structures of Western Kos.

Nisyros was submerged 161 ka ago, at the time of sedimentation of Kos Plateau Tuff. Between 150 and 60 Ka, the main volcanic structure of Nisyros was formed from Lower andesitic lavas up to the lavas of Argos.

Between 60 and 50 Ka, the eruptions that deposited the pyroclastic series of Kyra to Nisyros, Pyrgousa, Pahia, Tilos, Halki and the coast of Asia Minor followed.

During the same period, the underwater eruption that resulted in the deposits of the Lower Pumice of Yali occurred; the formation of Strongyli and the Agios Antonios islet followed. The first major explosive eruption on Nisyros occurred approximately 45 ka years ago and led to the deposit of Lower Pumice. Then, the Nikia lavas were extruded and the last catastrophic eruption event on Nisyros occurred 35 ka ago; it deposited the Upper Pumice and created the current caldera.

Subsequently, 31 ka ago, the explosive eruption that occurred led to the deposit of the Upper Pumice of Yali.

Kamara domes and tuff ring were formed on Yali, just like the North Yali obsidians and perlites, approximately 25-20 ka ago.