Milos is a mainly volcanic island (like the neighbour islands of Kimolos, Polyaigis, Antimilos, Glaronisia and Akradia islands). Some older metamorphic rocks are exposed, that were formed many millions of years before the existence of the island (schists, prasinites, calc-schists, etc.). Later these layers were covered by sea-sediments. Volcanism on Milos started in upper Pliocene and continued until late Pleistocene. The last volcanic eruptions on Milos (90.000 B.C.) took place in the area of Tsingrado volcano. Volcanicm on Milos is similar to the other parts of the Hellenic Volcanic Arc comprising also Methana, Santorini and Nisyros.
It is caused by the geodynamic convergence of the African and Aegean plates. The collission and subduction of the African plate produces calcalkalines (andesitic, dacitic) magmas and rocks. The eruptive phases on Milos produced mainly acidic tuffs & pumice, pyroclastic flow and lahar deposits. During extrusive phases a number of andesitic volcanic domes were built and some lavaflows emplaced. Towards the "end" of the most recent volcanic activity many phreatic explosions produced small craters (like near Agia Kyriaki or Achivadolimni) visible still today. Most of the volcanic rocks on Milos were strongly hydrothermally altered, which makes Milos become an important supplier for mining minerals like baryte, silver, perlite, caolin, bentonite and in the future perhaps even gold (Hontrovouno). Studies show the presence of a high-temperature hydrothermal system (up to 310 deg. C) below the island, responsible for the abundance of active fumaroles (e.g. in the Kalamos area). This reservoir might be related to an active magma chamber and therefore, future volcanic activity on the island can not be excluded.