Milos island group

Milos, Kimolos, Polygeos and Anitmilos islands and the Annaes islets were constructed starting 3.5 Ma.  Activity continued to recent time with phreatic explosions on Milos.  The group is the third field on the Aegean volcanic arc without a large central volcano.  Only Antimilos can be considered to be a composite volcano, built by lava flows, pyroclastics, domes and associated flows.

Milos, Kimolos and Polyegos islands are compound structures built by multiple volcanic edifices, mainly domes which intrude through thick volcaniclastic deposits.  The oldest volcanic rocks are found on the SW coast of Milos.  These date 3.5 Ma.  There are layers of older volcanic ash between sediment layers offshore that date 6.0 – 4.5 Ma.  It is unclear if these ash layers are early events from Milos or ashfall from neighboring volcanic eruptions (Santorini or Nisyros). 

Explosive and effusive activity built Kimolos, Polyegos and most of the present Milos 3.5 – 1.6 Ma.  The Ananes islets are probably one of the earliest in the area.  Extensive hydrothermal alteration of volcanic rocks does not allow dating or stratigraphic correlation between Milos Island group outcrops. 

Kimolos – Poliegos are two islets off the NE tip of Milos.  They are nearly completely built of volcanic rocks with only a few basement metamorphic and sedimentary rocks.

Antimilos it a polygenetic composite volcano erupting lavas and minor pyroclastics.  There are domes along the S flank of the island.  It is a similar age to Trachilas volcano on the NE Milos. 

Deep sea surveys NE of Antimilos discovered volcanic domes immediately offshore.  These were grouped with submarine volcanic debris, debris flows, domes or dikes, similar in shape to structured on the islands above sea level.  The domes are 30 – 350 m tall and 1 – 3 km in diameter.  Activity at these volcanoes is undated.

There is submarine hydrothermal activity SE of Milos.  While the last volcanic eruption on the island took place 80 ka at Fryiplaka volcano, the seabed around the island is quite active, especially offshore the SE margin of Fyriplaka volcano.  Venting was observed near Paleochori Bay at a depth of 90 – 350 m.  The focal point for a M 4.8 in 1992 was at a NE-SW trending fault in the area. 

Antimilos

The island of Antimilos is located 8 km NW of Milos.  It is considered to be the only composite volcano in the field.  The 8.5 km2 has two main topographic highs.  Sterna is the main rounded 650 m peak on the N part of the island.  Agriokastro is a 413 m peak on the S.  The entire edifice rises over 900 m from the surrounding seafloor.  It sits at the E end of a horst graben extensional structure of the South Aegean Arc.

Most of the island are steep dacite domes.  Some andesite is found only around the crater below the N summit and in blocky flows around a smaller crater on the S peak.  Minor dacite flows are found around the N crater.  Sterna erupted mainly basaltic andesites similar to flows on the E slopes of the island.  Rocks on Antimilos are not extensively dated, with 320 ka being the best available date. 

Milos construction

The Milos basement existed before volcanic activity began.  It is a metamorphic basement covered with ocean sediments, volcanics and finally erosive products.  The ocean sediments are marine limestones, some gypsums, and marine carbonates.  These have variable thicknesses 150 – 185 m.  Volcanic rocks are more than 70% of all outcrops on the island. 

Activity at the Milos archipelago over the last 3.3 Ma took place from at least 20 discrete submarine and aerial vents.  It is divided into three periods.

Two tectonic stress changes occurred in the region 4.0 – 2.0 Ma and 0.9 – 0.5 Ma.  Overall stress changed from compression to extension for a time, opening up pathways that enabled higher magma fluxes 2.1 and 0.6 Ma.  Crustal blocks near Milos rotated a bit 1.5 – 0.6 Ma, creating local compression in the crust that inhibited magma from rising to the surface.

Initial eruptions on Milos produced 300 m thick layers of pumice breccias.  These are interlayered with mudstone sediments meaning they were relatively shallow submarine events.  Wave action modified the layered pyroclastics.  Initial eruptions deposited dense lithics.  Later ones deposited pumice indicating open vent explosions, rapid water quenching of hot, pumice lapilli.

Rhyolitic and dacitic cryptodomes and sills erupted through the original pumice pile (Kalogeros dacite).  These disrupted the layered material and locally domed the seafloor.  Parts of the original volcanic centers near sea level were easily eroded.  Some historical shallow water rhyolitic eruptions formed ephemeral islands that went through several cycles of construction and erosion.

Explosive rhyolite activity was followed by widespread effusive volcanism that emplaced numerous dacite and andesite lavas and domes on the shallow seafloor.  Hyaloclastites are interlayered with sandstones and mudstones.  Domes formed local volcanic islands.  Weakly explosive strombolian eruptions built a small, wet scoria cone that collapsed into deeper water.  The volcanic centers were separated by low lying volcanic sand bars. 

Period I 3.3 – 2.1 Ma

Period I was relatively low volume, long term volcanic output.  It erupted thick layers of submarine pyroclastics including pumice flows, tuffs and pyroclastic flows.  There are also lavas and hyaloclastites.  Vents today are occupied by dome complexes.  Early eruptions constructed the Profitis Illias and Filakopi cryptodome – pumice cone volcanoes on the SW and NE end of Milos respectively.  These two volcanoes produced dacite – rhyolite pumice breccias in a submarine environment.  Mavros Kavos and Mavro Vouni andesite and dacite domes were extruded on the SW of Milos in a shallow submarine environment. 

Period II 2.1 – 1.5 Ma

Magma output substantially increased during Period II.  Volcanic activity was concentrated in central Milos and in Antimilos.  Eruptions were primarily effusive.  It emplaced domes, plugs and lava flows along local fault systems.  Effusive activity was accompanied by explosive activity from the domes and tuff cones.  Eruptions emplaced pyroclastic flows and breccias on submarine and emerging structures.  Toward the end of this phase around 1.4 Ma, tuffs erupted on the W part of the island and spread to the E part.  During this period, a small number of islands (domes, plugs, tuff rings) existed offshore the N part of the island.

Substantial volume of magma erupted at Triades lava dome, Bombarda and Dhemenghaki cone volcanoes.  They were originally formed under shallow water.  They deposited dacite – rhyolite on the NW, N and E parts of the island.  Two more andesite – dacite lava domes, Kantaro and Korakia erupted underwater and grew above the surface on the NW and NE parts of the island. 

Period III 1.5 Ma – Present

The most recent Period III was the lowest volume eruptive cycle of the island.  By 1.44 Ma, eruptions of numerous dacite and andesite lavas and domes established much of the outline of the current island above sea level.  Volcanic centers erupted near sea level and built above sea level.  Halepa and Plakes were dacite and rhyolite volcanoes.  Trachilas and Fyriplaka were rhyolite tuff cones up to 2.5 km in diameter on the N and S parts of Milos.  These were located along a graben structure across the island.

Phase III volcanism was active on the E and NE parts of the present island.  At the time, it was covered by shallow sea water.  Volcanic activity was intense pyroclastic activity with rhyolitic domes, pumice cones and phreatic explosions.  Multiple layers of pyroclastics are interlayered in places with hyaloclastites and pillows.  Effusive eruptions in the S and N middle of the island created a large complex of domes and lava flows.  Submarine activity in the NE part of the island progressed above water erupting domes and lava flows. 

Over much of the N and S parts of Milos, widespread phreatic activity took place at the same time as the larger rhyolitic centers.  Small, steam-driven explosions formed numerous overlapping craters in the N part of the island.  Some explosions were caused by pressure variations in the upper 300 m of an active hydrothermal system.  Phreatic activity continued into recent times, 200 BC – 200 AD.  The island was inhabited at this time.  Recent sediments indicate a lagoon-like environment.  Sediments of clay, sand and gypsum are about 80 m thick.

The most recent volcanic event on Milos took place 90 – 19 ka (depending on dating technique used).  It built the Fyriplaka tuff ring and associated rhyolite flows on the S edge of the active region.

A huge geothermal field with widespread fumaroles, hot springs, hot grounds and submarine gas emissions remains after magmatic activity ended.  The field experienced multiple large hydrothermal (phreatic) explosions before and after the last magmatic eruption.  Events in central – E Milos covered the island with a thick mantle of debris and mud flows.  These are mapped as the Green Lahar formation.  Hydrothermal explosions continued into historic time, 80 – 300 AD.  Active fumaroles around 100° C are still present. 

Volcanic structures on Milos

Four large submarine cryptodome – pumice cone volcanoes cover more than 85% of total area on the island.  The vent areas are centered on Profitis Illias, Bombarda (more on Bombarda later), Filakopi and Dhemeneghaki.  The volcanic centers are all similar. They are roughly circular.  One covers tens of km2.  The layered pumice breccias are typically 300 – 350 m thick, though one is 450 m.  Edges are much thinner, less than two meters.  Volume of these centers individually are 4-6 km3.  These are similar to other rhyolite dome intrusion tuff volcanoes in other parts of the world.

The center of each apron has small cryptodome – pumice cone volcanoes / dikes that erupted through the center.  Single intrusions are generally less than 1.2 km wide, though multiple intrusions at Profitis Illias cover several km2.  Contacts between intrusions and the pyroclastic aprons are defined by hyaloclastites, indicating eruptions through wet, poorly consolidated debris. 

Initial eruptions were explosive and formed thick submarine deposits of pumice breccias.  Later degassed magmas extruded into the pile, forming cryptodomes, sills and dikes in and around the vent areas.  While initial activity was entirely under water, later activity was not necessarily under water. 

Submarine dacite and andesite lava domes cover 10% of the island.  They range 2.5 – 10 km in diameter and are 250 – 350 m thick.  There are several of these.  The Triades area of NW Milos is best exposed.  They were effusive eruptions in a submarine environment.  The domes typically have a massive core, flow banded outer zone, brecciated margin and a shell of hyaloclastites. 

There is a scoria cone covering 18 km2 near Pollonia.  Eruptions were weakly Strombolian and erupted through shallow water.  The underwater rhyolitic lava pumice cone volcanoes have thick (up to 200 m) pyroclastic deposits and small volume rhyolite lavas up to 100 m thick.  They all cover several square kilometers.  Internal structure is similar to the dome complexes, with a massive core, an outer flow-banded zone and a brecciated margin.  The pyroclastic aprons are tens of meters thick lapilli ash / pyroclastic surge deposits and thin layers of bedded ash (fall) deposits.  Toward the periphery, lavas are thinner at 15 – 20 m as are the pyroclastics.  The edges are thin, bedded ash fall deposits.

Bombarda

The Bombarda volcano is a 1.7 Ma rhyolite eruptive center located off a peninsula N of the middle of the main island of Milos.  It consists of a 50 m thick pumice apron and a dome.  The dome is 0.5 km W of Adamas Villate.  It is elliptical, with the long axis NW-SE.  Maximum thickness of the dome is 150 m, observed as cliffs on the E side.  The interior is jointed.  The exterior of the dome is a 2-3 m thick shell of obsidian.  The boundary between the dome and apron is sharp and nearly vertical.  The entire structure is covered by sandstone deposited after activity stopped. 

The apron is layered with marine sediments, meaning the vent initially erupted below sea level.  Each pyroclastic layer was rapidly deposited followed by ocean sediments.  The layers appear to have erupted at water depths less than 200 m.  Wave action is visible.  The apron first erupted on ocean floor less than 200 m deep.  It stopped erupting above the wave base. 

Pumice units of the base were deposited by water-supported mass flows from collapse of small-volume, underwater eruption columns / tephra jetting.  This has been seen on wet volcanic vents in Japan.  Pyroclastic debris around the vent and slumped during eruptions, forming gravity flows.  This debris was remobilized by seismic activity. 

Short underwater explosive eruptions expelled enough water from the eruption column to keep it dry.  Column collapse at the end of these allowed water-supported pyroclastic flows in multiple eruption pulses.  Increasing thickness of layers appears to be due to an increase in volcanic activity. 

Activity shifted to effusive, cutting through the apron beds.  The dome was emplaced very late during the eruption and may reflect degassing of source magma.  The apron and the final obsidian shell isolated the dome from the marine environment.  This is an example of a shallow water rhyolitic eruption. 

Geothermal

Milos has an active and vigorous hydrothermal system, mostly due to its low relief and the island being a collection of small eruptive centers rather than a single large composite cone.  Thermal waters are scattered island-wide and offshore.  Thermal waters and their clays have been used medicinally for as long as people have been on the island.

Geothermal exploration at Milos began in 1971.  The E part of the island has excellent geothermal potential with fluids 300 – 323° C at depths of 800 – 1,400 m below sea level.  Geothermal potential of the Island is estimated at 40 – 100 MWe.  Water from geothermal exploration can cover local water demand and provide energy for seawater desalination. 

A 2 MW geothermal pilot plant was installed on Milos in 1985, commissioned in 1987.  Waters in the aquifer are highly saline, and created scaling, disrupting operation.  Trials were interrupted several times for long periods to repair the problems.  The plant was retired due to public opposition triggered by excessive steam venting, silica in the steam, H2S release, and noise.  Following a blowout of a production well a few years later, all wells were plugged. 

A few houses and one hotel currently use shallow, low temperature ground water for space heating and a swimming pool.  Geothermal potential from the system should cover electrical and heating needs of the population and the mining industry.  Future geothermal development is problematic, in that it will run directly afoul of large protection zones erected across the island following past difficulties with geothermal wells.  Pushing geothermal development upon a skeptical local population with a long memory of past problems may be a bridge too far. 

Tectonics

Tectonic activity in this part of the world is mainly driven by the collision of the African Plate N into the Eurasian Plate.  Further complicating the collision is the Arabian Plate also moving N into the Eurasian Plate, though at a slightly more northerly angle.  This collision is squeezing the Anatolian Block (basically most of Turkey) to the W.  The W portion of the Anatolian Block under the Aegean Sea is described in some places as the Aegean Sea Plate, a microplate, in others as the Anatolian – Aegean Sea Microplate.

The main subduction between Africa and Eurasia is a curved line just W of Greece, curving S of Crete and NE into S Turkey and Cyprus.  There is no obvious subduction trench.  There are varying convergence rates between the NE subduction of Africa depending on location.  The motion is a bit faster in Greece than Cyprus – Anatolia.  There may be some extension going on in the Aegean.

The Aegean area is one of the most rapidly deforming parts of the Alpine – Himalayan mountain belt.  This is primarily demonstrated by high seismic activity along the belt.  Continental crust in the Aegean is thinned and has high heat flow.  Deformation is dominated by the W motion of the Anatolian block at 2 cm/yr, the SW motion of S Aegean at 3 – 3.5 cm/yr, and the vertical movements of large pieces of lithosphere. 

Conclusions

Milos was constructed by at least 20 coalescing eruptive centers active over the last 3.3 Ma.  Changes in regional stress from compression to extension to compression a few times drove pulses in volcanic activity.  Today, we are in a compressional regime, so new magma is not so readily available, though residual magma under the island does drive an extensive and vigorous hydrothermal system.  Recent activity has all been phreatic explosions, the last as recent as 1,200 – 2,000 years ago.  I would expect the threat of phreatic explosions to continue, though that of new magmatic eruptions to be decreasing. 

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