The Milos volcanic district is part of the Aegean Volcanic Arc stretching from Korinthos to the W coast of Turkey.  There are multiple volcanic districts along the arc.  These include Methana – Poros, Milos, Nysiros and Santorini.  There are many islands in the Aegean Sea between Greece and Turkey.  Four groups of them are volcanic in nature.

For its part, the Milos volcanic district is listed as the SW-most island of the Cyclades, 120 km from the coast of Laconia.  Milos itself is roughly horseshoe shaped, with an area of 151 km2, and a highest point of 748 m.  Other neighboring islands include Antimelos (Antimilos), 21 km NW, Erimomilos (Kimolos, Argentiera) , 1.6 km NW, Polyaigos (Polinos, Polybos, Polivo, Polyaegos), 2 km SE.  There are multiple smaller islands, all uninhabited.  Combined area of all islands is just over 160 km2 with a population of just under 5,200.

Volcanic activity began here 3.3 Ma, erupting andesites to basalts.  Over time, magma evolved into rhyolites leaving pyroclastics and obsidian deposits on the island.  Although magmatic volcanism mostly ended 90 ka, phreatic eruptions took place as recently as 140 AD.  Milos hosts an active and vigorous hydrothermal system that has altered eruptive products into a variety of clays mined and traded for millennia.

Milos is located some 155 km SSE from Athens, 91 km WNW from Santorini, and 131 km NNE from Crete.  It is close enough to Santorini to have shared in the volcanic impact from the 1600 BC eruption of Santorini.  We have two posts on the Aegean (Hellenic) volcanic arc.  The first was a 2018 post on Nysos.  We briefly touched on Santorini in a 2022 post about Exceptionally Violent Volcanic Eruptions. 

This part of the Mediterranean Sea has a Mediterranean climate, with mild, rainy winters and warm to hot, dry summers.  Average highs are around 20° C, though record highs of 41° C were recorded.  Average low is around 15° C.  Average rainfall is 41 cm/year. 

Volcanic activity here is handled via the Greek Geodynamic Institute, one of the three institutes of the National Observatory of Athens.  This was originally set up for national seismic monitoring and has at least two volcanic observatories, one at Santorini and the other at Nysos.  Seismographs were installed at Methana, 50 km from Athens in 2019.  There are commercial webcams available on Santorini and Milos. 

History

Humanity has been active in this part of the Mediterranean for a long time, with obsidian being mined (or collected) as early as 15 ka.  This was widely transported well before farming became widespread in the region.  Milos location midway between Greece and Crete made it an important early Aegean civilization center.  Obsidian lost its importance when bronze became the preferred material for weapons.

First settlers arrived during the Bronze Age as obsidian mining declined.  They were fishermen, likely from Crete based on their architecture and carvings.  The Thera eruption 1600 BC disrupted these settlements for a time, but they were rebuilt. 

The first Dorian (Spartan) settlement was established no earlier than the First Millennium BC.  They were independent from Sparta and used a variant of the ancient Greek script.  The Melians made some small contributions to the Spartan was effort during the Peloponnesian War 431 – 404 BC.  Athens raided them in 426 BC.  Ownership of the islands changed multiple times until 197 BC when the Romans forced the Macedonians out. 

Rome was interested in various volcanic clays created by hydrothermal alteration of volcanic rocks.  They were mined for use in the greater empire.  These include bentonite used as soap, a base for cosmetics, and some medical uses, sulfur used in medicine, calonite used by artists to create whites, pumice dust used for polishing, and alunite also used in medicine. 

Following the fall of Rome, Milos was important to the Byzantines and in the center of the ebb and flow of Islamic spread in this part of the world.  It became part of the Ottoman Empire in 1566 and joined the Greek War of Independence in 1821.  Throughout all this upheaval, mining continued to be a (the?) major economic engine, today being replaced by tourism.

Region

Volcanic activity in the Aegean area is concentrated along the semicircular Aegean volcanic arc.  The main tectonic driver is the collision between the African Plate and European Plate.  There are additional complexities we will describe in more detail in our tectonics section.  For the regional volcanism portion, we will be discussing the five volcanic systems along the arc.  All distances will be measured from the middle of Milos. 

Volcanic activity occurs along a belt stretching from Susaki – South Kos Nisyros Island group just SW of Turkey.  The subducting plate under the arc is 110 – 130 km.  While volcanism along this arc produced large volumes in four of the five fields, up to 100 km3 below sea level, the systems are not closely spaced, nor are there many of them.  Individual volcanic centers / islands in each group tend to align perpendicular to the arc at that point.  Activity generally moves along those lineaments away from subduction, from outside the arc to inside it.  There are large portions of the arc without volcanic fields. 

The central and E sectors have large composite volcanoes with calderas (Santorini and Nisyros).  Activity in the W sector is small, often monogenetic eruptive centers.  Eruptive products evolved from basalts to rhyolites, similar to those of island arcs on thin continental margins. 

 Susaki 

Susaki (Sousaki) may not be large enough to earn the description of a volcanic field.  Instead, it is a group of small volume volcanic outcrops scattered over a large area 181 km NW from Milos.  Total volume of all eruptive products is less than 1 km3.  Volcanic centers are monogenetic, dacite domes and associated lava flows.  Oldest lavas dating 4.05 – 3.6 Ma are found in the W portion of the field.  Youngest lavas dating 2.7 – 2.3 Ma are found in the E.  Vent distribution and shape are controlled by E-W and NW-SE tectonic lineaments in the area.  There are NW trending extensional lines that seem to control the location of the E centers.  There are a few layers of tuffs found among sediment layers.  These are likely from explosive emplacement of the domes.

Today, the Susaki area has a low heat flow geothermal field at 200 m.  There is also a weak hydrothermal field powering weak fumaroles.  Volcanic products are dacites and rhyolites.  There are outstanding warnings for packs of dogs roaming the area and attacking visitors. 

Egina (Aegena) – Poros Methana 

Egina – Poros – Methana are part of the Saronic Gulf Volcanic Group, a volcanic field located in the Gulf of Saronikos (Saronic Gulf), 143 km NW from Milos.  The complex was created by eruptions from effusive monogenetic centers with low eruption rates.  It created several islands with domes and lava flows.  The group is located in a tectonic depression that started forming some 5 Ma, about the same time volcanism started. 

Activity at Egina Island started around 5 Ma and eventually built an 80 km2 structure.  55 km2 of it is currently above sea level.  4.7 – 4.3 ka activity produced andesitic pillows and hyaloclastites in a shallow submarine environment.  There were also rhyodacite pumice flows and tuffs from multiple eruptive centers.  The main phase of activity 3.9 – 3.0 Ma built the central and S Egina Island with dacite and andesite magmas building domes, dome complexes and lava flows.  Most of the erupted magmas were basaltic andesites and andesites. 

Poros Island is a small 1 km2 outcrop of a lava dome and flows that date 3.1 – 2.6 Ma.  The rocks are evolved andesites.

Methana erupted the youngest volcanic products of the entire area.  The oldest dated products are 900 ka, though there are probably older rocks around.  The youngest event built an andesitic dome and lava flows 230 BC.  The island is constructed of domes, dome complexes, lava flows, block and ash and related debris flows.  It appears there was a nearly continuous low-rate production for the entire active period. 

There are submarine volcanics a few kilometers NW of the Kamelo Cuno center on the NW tip of the island.  These are estimated at 1.0 Ma based on thickness of sediments on top of them.  Volcanic rocks for Methana and the offshore volcanics are mainly andesites and dacites with rare basaltic andesites.   

Paphsanias submarine volcano was discovered in 1987.  It is located about 2 km NW of the Methana peninsula and covers some 12 km2.  The summit is crescent shaped and tops out within 143 m of sea level.  The cone is 300 m deep and 2-3 km wide.  Sedimentation on the cone is 300 – 350 m.  There are volcanic deposits in places on the sediments.  Volcanic activity in this general location was described in the second century AD and three centuries ago. 

Christiana – Santorini Kolumbo 

Santorini is located 91 km ESE from Milos.  We touched on its 1600 BC Thera caldera forming eruption in a 2022 post on Exceptionally Violent Volcanic eruptions. 

The larger Santorini volcanic field includes the Christiana islets, Santorini island group and the Kolumbo submarine volcano.  It is built on the thinnest (25 km) continental crust along the arc.  The field is built along a NE-SW trending horst that is tectonically active with major earthquakes.

The Santorini island group consists of five islands.  Three of them ring a flooded caldera.  Two of them are resurgent, post-caldera islands.  Activity here started some 650 ka and continued to present.  Major explosive volcanism takes place 17 – 40 ka between episodes.  There are active hydrothermal vents in the caldera and on the active Kolumbo submarine volcano.

Christiana islet group are lava domes and flows with interlayered pyroclastics.  The islets are the remains of an eroded central volcano that has been submerged.  Activity here was among the youngest in the field.  There are older pyroclastic tuffs and ash layers.  Volcanic rocks here are andesites, dacites and rhyolites. 

Christiana Islands is a group of four small islets around a submarine volcanic cone.  The cone is built at the perpendicular intersection of two fault zones.  The Christiana cone is 10 x 13 km and rises 900 m from the sea floor.  Three new domes are located 10 km E of Christiana Islands.  Their bases are at an average depth of 500 – 600 m.  They are 200 – 300 m high.  Active faulting on the domes created cliffs.  There is hydrothermal activity that supports bacterial mats and abundant wildlife. 

The caldera is the result of at least four collapse events, the most recent of which was the Minoan (Thera) eruption of 1600 BC.  The caldera rim rises up to 300 m above the water.  Its floor drops over 390 m below sea level.  New islands Palea and Nea Kameni are surface expressions of a resurgent dacite shield around 2 km3 in volume. 

The 1600 BC eruption was one of the most violent eruptions worldwide over the last 4 ka.  The volcano grew in six recognizable phases.  These include the two largest eruption cycles by volume.  Together, they produced 12 major explosive eruptions, built at least five large lava shield volcanoes.  Deposits of the main pyroclastic eruptions are individually up to 200 m thick.  Eight of them are bimodal.  Accurate volume estimates are only available for the Minoan Tuff at 30 km3.  The other 11 eruptions were in the single km3 to tens of km3 range.  Each cycle started with intermediate magmas and ended with a major rhyodacite eruption and caldera collapse.

The Kolumbo submarine volcano outside the main caldera is the most recent active vent.  It was active above the water 1649 – 1650 AD.  The sub-Plinian explosion ejected around 1 km3 of pyroclastics and ash and triggered a tsunami.  Kolumbo erupts andesites and rhyolites.  There are at least nine submarine volcanoes NE of Kolumbo   

South Kos Nisyros island group

Nisyros is located 243 km E from Milos.  Granyia documented a visit to the volcanic island in her 2018 Nysyros – (Re)-Visited post.  Location of volcanic centers in this field are controlled by regional NW-SE lineaments.

Activity at South Kos started 3.4 – 1.2 Ma, building scattered dacite – rhyolite domes.  The domes and pyroclastics of Pyrgussa and Pahia islets are located W of Nisyros island.  They are covered by the Kos Plateau Tuff.  0.5 Ma near Kefalos, rhyolitic magmas fed hydrovolcanic explosive eruptions that settled down to extrusive eruptions.  These built the Kefalos tuff ring and Zini obsidian dome.  Activity triggered a small caldera collapse in the Kamari Bay area.  The final volcanic event 161 ka was the most powerful explosive eruption the arc, emplacing the Kos Plateau Tuff by ejecting over 100 km3 of rhyolite.  Pumice flows covered over 5,000 km2.  The vent for this was likely near the present day S Kos coast.  Actual location of the caldera is still in dispute.  There may have been multiple calderas from this vent.

Nisyros island is a 42 km2 composite volcano with a central caldera.  The island is an 8 km wide truncated cone with a central caldera 4 km in diameter.  The first eruptive cycle built the cone.  The second created the caldera.  There are two caldera building cycles.  Both began with low intensity phreatomagmatic explosions fed by rhyolitic magma.  This triggered caldera collapse followed by resurgent dacite – rhyolite dome building and lava flows.  These took place more recently than the Kos Plateau Tuff.  Best estimates for both phases are 66 ka and 24 ka, with wide error bars.

All historic explosions at Nisyros 1873 – 1887 are small, phreatic affairs.  They created small craters on the W caldera floor.  There are 8 older explosion craters on the caldera floor.  The island continues to be the site of intense hydrothermal activity that feeds fumaroles and hot springs.  Volcanic rocks here range from basaltic andesites to rhyolites. 

Yali (Gyali) islet is the youngest volcanic center of the group.  It consists of two small hills, up to 165 m high, connected by a narrow isthmus.  The SW part are pyroclastics.  The NE part is obsidian (rhyolite) domes with pyroclastics.  The dacite Agios Antonios islet is located between the two hills. 

Eruptions started with an initial explosive submarine eruption of rhyolite pumice fall followed with extrusion of obsidian domes and flows.  These eruptions are not precisely dated but are younger than the Kos Plateau Tuff.  The Yali Upper Pumice is thought to have erupted 31 ka and was followed by dome extrusion.  A more recent eruptive cycle took place following this one.  Yali is considered as a potentially active volcano. 

Strogyli is a steep andesite cone on a 500 m deep sea bottom.  Its top is 120 m below sea level.  It is covered by the Yali Upper Pumice, meaning it must be older than 31 ka.

There are multiple submarine volcanic centers around Nisyros Island.  They are mainly domes and appear to be very young.  It is a very busy submarine volcanic field. 

We will do a detailed description of Milos in Part II along with a discussion of regional tectonics and conclusions.

Additional information

Smithsonian GVP – Milos

Milos volcanic field – Photovolcanicia

Differentiation and storage conditions of mafic magmas in Milos volcanic field:  insights from crystal cargo in basaltic andesites and gabbroic nodules, Xydous, et al, Oct 2022

The significance of the Aegean region for earth-science conservation in Europe with emphasis on the geological heritage of Milos, FW Wiedenbein, 1993

Geological, geochemical and mineralogical studies of shallow submarine epithermal mineralization in an emergent volcanic edifice at Milos island (western side), Greece, D Alfieris, Hamburg, 2006

Epithermal gold mineralization in the active Aegean volcanic arc:  the Profitis Ilias deposit, Milos Island, Greece, Kilas, et al, Feb 2000

Deformation bands and alteration in porous-rich volcaniclastics:  insights from Milos, Greece, Lero, et al, Nov 2023

The volcanic relief withing the Kos-Nisyros-Tilos tectonic graben at the eastern edge of the Aegean volcanic arc, Greece and geohazard implications, Nomikou, et al, May 2021

Interpretation of the Bouguer anomaly of Milos island (Greece), GN Tsokas, Aug 1996

The submarine eruption of the Bombarda volcano, Milos island, Cyclades, Greece, Rinaldi & Campos-Venuti, May 2003

A west-east traverse along the magmatism of the south Aegean volcanic arc in the light of volcanological, chemical and isotope data, Francalanci, et al, 2005

Volcanic facies architecture and evolution of Milos, Greece, AL Steward, Jul 2003

Vertical motions in the Aegean volcanic arc:  evidence for rapid subsidence preceding volcanic activity on Milos and Aegina, van Hinsbergen, et al, 2004

Shear wave anisotropy across the geothermal field of Milos, Aegean volcanic arc, Sachpazi & Hirn, Dec 1990

Geology and geothermics of the Island of Milos (Greece), Fytikas & Marinelli, Jan 1976

Eruptive history and 40Ar/39Ar geochronology of the Milos volcanic field, Greece, Zhou, et al, Oct 2020

Exploration of soils developing on volcanic materials on the island of Milos, Greece, Georgoulias & Moustakas, Nov 2009

Volcanism of the south Aegean volcanic arc, Vougioukalakis, et al, Nov 2020

Facies architecture and late Pilocene – Pleistocene evolutioin of a felsic volcanic island, Milos, Greece, Stweart & McPhie, Nov 2005

Parameters controlling the eruption frequence of long-lived felsic magmatic systems:  an example from the Milos volcanic field (Greece), Zhou, et al, Feb 2022

Alternating diatomaceous and volcaniclastic deposits in Milos island, Greece:  a contribution to the upper Pliocene-lower Pleistocene stratigraphy of the Aegean Sea, Calvo, et al, Mar 2012

Compositional variations in Smectites:  part I.  Alteration of intermediate volcanic rocks.  A case study from Milos island, Greece, Christidis & Dunham, Jul 1992

The current geothermal exploration and development of the geothermal field of Milos island in Greece, Karytsas, et al, Jun 2004