The geological nature of the archipelago is the main reason for its fabulous variety of colors and landscapes.
Milos and the nearby islands belong to an important volcanoes alignment called 'Aegean Volcanic Arc' formed by the subduction of the African plate under the Aegean area. This arc goes from the golf of Korinthos to the west cost of Turkey.
The main volcanic districts are represented by: Methana-Poros, Milos, Nysiros and Santorini , this latter being the only one showing a remarkable volcanic activity over known history..
The Milos volcanic district is a wide volcanic archipelago comprising the islands of Milos, Kimolos, Antimilos, Poliegos.
From the geological point of view, here the volcanic activity started about 5 millions of years ago and it is now considered to be extinct. Sarakiniko and Mutakas. As a result of the rhyolitic volcanism the obsidian was produced and exploited since Neolithic age (7000 years ago). Obsidian from Milos has been found in most of the Mediterranean countries.
The andesitic rocks, usually dark in color, outcrop in the area of Fylakopi and Pollonia and in the southwest portion of Milos. It is possible recognize both lava domes and dike set with spectacular columnar jointing originated during the thermal cooling of the lava bodies (Glaronissia, Kalojero).
Following their emplacement, volcanic rocks were involved by an intense hydrothermal activity. This kind of physical-chemical processes are responsible of remarkable changes in the chemical composition of the original rocks, with the formation of huge ore deposit of clay minerals. Their exploitation represents a considerable source in the Milos economy. During the 80's Greece was the 2nd world producer in bentonite , a valuable variety of clay deposit. This post-volcanic alteration increases the extreme variety of color and landscape of Milos Island (beaches of Fyriplaka, Fyropotamos and Plathiena). The volcanic nature of Milos is responsible for an anomalous heat-flux in the earth crust. In geology these areas are called 'geothermal fields' and could be exploited to produce energy. Unfortunately this favorable geological situation of Milos has not been utilized yet.
Lovers of geology are warmly invited to visit to the 'Mining museum of Milos' located less then 1 km southward from the Adamas port.

Glossary
· Andesite: Very fine crystalline extrusive rock of volcanic origin composed largely of plagioclase crystals with smaller amounts of ferromagnesian minerals.
· Clay: A natural, earthy, fine-grained material, which develops plasticity when mixed with a limited amount of water; composed primarily of silica, alumina and water.
· Columnar jointing: Parallel, prismatic columns that are formed as a result of contraction during cooling in lava bodies like domes, lava flow and dikes.
· Dike: A tabular body of igneous or lava rock that cuts across adjacent rocks or cuts massive rock.
· Explosive volcanism: Volcanic activity characterized by magma explosivity. It is the result of the violent expansion of magmatic gases within the rising magma because of the decrease of confining pressure.
· Fumarole: A hole, usually found in volcanic areas, from which vapors or gases escape.
· Greenschist: A schistose metamorphic rock with abundant crystals of chlorite and epidote, giving it a green color.
· Heat flux: Movement of the heat from the earth interior to the surface.
· Hydrothermal activity: Circulation of hot, residual watery fluids derived from magmas during the stages of their crystallization and containing large amounts of dissolved metals, which are deposited as ore in fissures along which the solutions often move.
· Lava: The rock mass formed by consolidation of molten rock issuing from volcanic vents and fissures.
· Lava dome: A large igneous or lava intrusion whose surface is convex upward.
· Metamorphic rock: A rock formed from preexisting solid rocks by mineralogical, structural, and chemical changes, in response to extreme changes in temperature, pressure, and shearing stress.
· Obsidian: A jet-black volcanic glass, usually of rhyolitic composition, formed by rapid cooling of viscous lava.
· Pumice: A rock froth, formed by the extreme puffing up of liquid lava by expanding gases liberated from solution in the lava prior to and during the solidification.
· Pyroclastic rock: A rock that is composed of fragmented volcanic products ejected from volcanoes in explosive events.
· Rhyolite: A light-colored, aphanitic volcanic rock composed largely of alkali feldspar crystals and free silica with minor amounts of ferromagnesian minerals.
· Sedimentary rock: A rock formed by consolidated sediment deposited in layer.
· Subduction: The process by which one crustal block descends beneath another, such as descent of the Pacific plate beneath the Andean plate along the Andean Trench.
· Volcanic rock: Finely crystalline or glassy igneous rock resulting from volcanic activity at or near the surface of the earth.

Mirko Rinaldi is a geologist of the University of Parma (Italy).
E-mail: mirkorinaldi@hotmail.com