Santorini (Thera) Volcano

Santorini archipelago viewed from summit of Profitis Ilias (Stitched Panorama Image)

The picturesque archipelago of Santorini is the most prominent member of the South Aegean Volcanic Arc, formed as a result of subduction of the African under the Eurasian plate. Santorini is largely volcanic in origin and consists of several islands arranged in a ring around a flooded caldera. This shape is the result of a series of at least four massive caldera-forming eruptions. The last and most famous of these was the VEI 7 Minoan Eruption (also referred to as Late Bronze Age (LBA) eruption) that occurred some time between 1627 and 1600 BC. This eruption may be of historical significance due to the impact of tsunamis generated by the eruption on mediterranean coastal communities. In particular, some authors argue that the demise of the Minoan culture on the island of Crete, lying 120 km south of Santorini, was linked to the Minoan eruption, this in turn possibly being a source of the legend of Atlantis. Both impact and exact chronology of the eruption remain subjects of debate.

Aerial image of Santorini from SSE. Right of center is non-volcanic part of island.	Aerial image of Santorini Caldera from south. Kameni islands are visible in center.

Ferry passing in front of Nea Kameni island	Imerovigli village above caldera wall

Geological Background

The South Aegean (or Hellenic) Volcanic Arc extends almost 500 km from mainland Greece to Turkey. The trench formed by subduction of the African plate lies about 250 km south of the arc, the latter lying in a zone with a crustal thickness of about 30 km. The subduction process is responsible for the magmagenesis which has resulted in the formation of the arc. Further volcanic centers on the arc include Aegina, Methana, Poros, Milos, Kos, Yali and Nysiros and are indicated by red Triangles. The red lines dissecting the active arc indicate fault lines associated with volcanic activity since they allow magma easier access to the surface. Grey arrows indicate directions of movement of the Aegean Plate (north of trench) and African Plate (south of the trench), the latter of which is subducting under the Aegean Plate. The map represents an extreme simplification of the complex tectonic structures in the region. The interested reader could view e.g. Vassilakis et al. 2011 (Earth Planetary Sci. Lett. 303, p.108-120) for a far more detailed analysis.

Santorini is located on the edge of a NE-SW-trending basement horst (formed due to a local extensional regime resulting from southward migration of the subduction zone) called the Santorini-Amorgos Ridge. The islands were shaped by volcanic activity but a pre-volcanic part of the ridge embedded in volcanic material forms the SE corner of the main island of Thera. First activity is estimated at 650000 years ago. Early lavas and tuffs can be found on the Akrotiri Peninsula but were largely the result of submarine eruptions and are today above sea level due to regional uplifting of the southern part of Santorini. Since 530000 years ago, activity has been largely concentrated around a graben adjacent to the basement horst on its NW side. The first subaerial volcanism at Santorini occurred between 530 and 430 thousand years ago and most significantly resulted in formation of several small cinder cones on Akrotiri Peninsula and the small andesitic Peristeria Volcano further north. More explosive activity commenced about 360 thousand years ago and at least 12 Plinian eruptions have occurred since, which are divided equally into two cycles based on magma compositional trends from largely andesite / dacite to predominantly rhyodacite, with the first cycle ending with the Lower Pumice 2 eruption (see Table 1) and the second with the Minoan eruption, both of which were caldera-forming. The dacite lavas of the Kameni islands suggest that a new cycle may have just commenced. For a detailed analysis of Santorini's geology see e.g. Druitt et al. 1999 (Santorini Volcano. Geol. Soc. Lond., Memoirs 19, 165pp).

Lying seven kilometers NE of Santorini along the Columbos Line is a further historically active volcano. The submarine Columbo volcano briefly emerged from the sea during its 1649-50 eruption. As it emerged in 1650, a massive explosion formed massive pyroclastic flows which reached the coast of Santorini, killing about 70 inhabitants.

Fig.1 Map showing the strong regional influence of regional tectonics on the development of the volcanic field. Extracted from "Santorini Volcano". Geol. Soc. Lond., Memoirs 19, 165pp, Fig.3.57 d.o.i.:10.1144/GSL.MEM.1999.019.01.12; With kind permission of T.H. Druitt.	Fig.2 Reconstruction of the four calderas of Santorini for which field evidence is preserved. Contours in meters. Extracted from "Santorini Volcano". Geol. Soc. Lond., Memoirs 19, 165pp, Fig.3.58; d.o.i.:10.1144/GSL.MEM.1999.019.01.12; With kind permission of T.H. Druitt.

Eruptive History

Geological records suggest that at least 12 Plinian eruptions have occurred at Santorini volcano (Druitt et al. 1999. Geol. Soc. Lond. Memoirs 19, 165pp). Eruptive deposits are exposed in the steep caldera walls which effectively provide a cross-section of various parts of the island. Most of the flatter surfaces of the islands are covered in thick deposits from the Minoan eruption. The deposits of all Plinian eruptions are sandwiched between smaller interplinian deposits, indicating that the volcano always had periods of less violent activity between the major eruptions. These however only represent larger interplinian episodes since no deposits from the Kameni complex can be identified on the islands surrounding the caldera, inspite of numerous phases of activity including several explosive events. Hence, the total number of eruptions at Santorini is likely to be far higher than those preserved in the geological records.

Table 1 - Summary of major eruptions of Santorini. * Dates vary considerably depending on source. Accuracy varies (standard deviation not shown). Based largely on Druitt et al. 1999.

Studies of the deposits from the 12 Plinian events reveal that at least 11 of these caused extensive pyroclastic flows. A detailed analysis of the deposits and inferred sequence of each eruption goes beyond the intended scope of this webpage, yet the extensively studied Minoan eruption is discussed in some detail below. Flow deposits of in places over 20 meters thickness can be attributed to Lower Pumice I, Middle Pumice, Cape Riva and Minoan eruptions. The Lower Pumice 2 eruption that ended the first cycle of activity has also been studied in detail and appears to have proceeded through 4 main phases, similar to those of the Minoan eruption but of smaller volumes (Gertisser et al. 2009. J. Volc. Geotherm. Res. 186, p.387-406). The magma is however thought to have accumulated at a greater depth (about 16 km) prior to the eruption, which was eventually triggered following repeated intrusions of hotter mafic magma from below.

The interplinian eruptive activity is also of interest since it is the most common form of activity at Santorini and the only form we may observe in the near future. Deposits from such activity, named M1-M12 according to Druitt (see Table 1) are generally several meters thick. These consist of various layers of e.g. scoria, ash, thin fine-grained base-surge layers, soils. The soil layers were generally found at the top of the interplinian deposits, suggesting extensive periods of dormancy prior to Plinian eruptions. The interplinian phases of activity between Middle Plinian and Minoan eruptions have been studied in detail (Vespa et al. 2006. J. Volc. Geotherm. Res. 153, p.262-286). These interplinian deposits (IPDs) had from 50.7-69.4% silicate, placing them in the basaltic to dacite range and are composed of scoriaceous lapilli layers with some surge deposit layers. Often, the IPDs begin with minor pumice and ash deposits. The scoria-fall and pumice deposits appear to be virtually all derived from subplinian eruptions based on their dispersal characteristics. The M10 IPD is the most prominent, being up to 30 meters thick and containing 30 distinct layers. The number of strata-forming eruptions is possibly less than this since it cannot usually be determined if adjoining layers are the result of seperate eruptions or different phases of a single eruption. It is clear from the fact that meter-thick layers are found in many IPDs, that such eruptions would have eliminate most life on the islands. It is important to realise that risks are not only associated with the Plinian eruptions.

	Cliffs of Cape Aspronisi - Cycle 1 deposits visible under lighter Minoan deposits

	Beautifully stratified pre-Minoan deposits in W corner of Mavromatis pumice quarry

	"Red Beach" - Remains of cinder cone on Akrotiri Peninsula. Resulted from basaltic-andesitic strombolian activity.

Minoan Eruption

The Minoan eruption of Santorini is one of the most studied eruptions world-wide. Absent historical records, the eruption has been reconstructed entirely based on analysis of its deposits both on land and underwater. Whilst the exact sequence of events and in particular their timing may never be fully known, a remarkable amount of information has been assembled, allowing relatively detailed, although in places controversial, analyses of events.

The volume of erupted material was recently estimated at 60 cubic kilometers of magma, based on marine geological surveys of deposits on the sea floor surrounding Santorini (Sigurdsson et al. 2006. EOS Trans. AGU, 87(34), p.337-). This is similar to the volume of the largest historical eruption at Tambora in 1815 and several fold larger than the volume erupted during the famous 1883 eruption of Krakatau. It is noted that the volume of deposits is far higher than that of the cource magma due to the lower density of the vesicular erupted material. The volume of sulphur released into the atmosphere, based on ice-core studies, was less remarkable with similar amounts being released about every 50 years by volcanic events according to ice core analysis. Hence, it seems unlikely that the eruption would have had a wide climatic impact (Pyle, 1997. Envir. Geol. 30(1/2), p.59-61).

In the run-up to the eruption, a significant body of magma must have accumulated under the volcanic edifice. Based on intricate analyses of trace element diffusion between different layers of crystals found in pumice samples, it has been proposed that process took place over a period of less than 100 years (Druitt et al. 2012. Nature 482, p.77-82). Several cubic km of dacitic magma appear to have risen into large bodies of rhyolitic magma at a depth of several kilometers. At a later stage, small amounts of andesite and basaltic andesite also entered the magma body and the components were extensively mixed before the eruption occurred. To accommodate such a large body of magma, significant inflation of the volcano would have occurred but this is likely to have been moderated somewhat by down-sagging of the area beneath the magma body. Earlier petrological studies suggest that the process may have taken longer and occurred in two stages (Cottrell et al. 1999. Contrib. Mineral. Petrol. 135, p.315-331). It was suggested that mafic injections into an about 8 km deep rhyodacite magma chamber led to its partial migration to a shallow storage chamber at a depth of under 2 km. Here, over several hundred years, further mafic intrusions continued to heat and pressurize the magma, ultimately resulting in the eruption.

Numerous studies analyse the tephra deposits of the eruption and suggest how the eruption evolved based thereon (see summary and references in e.g. Druitt et al. 1999. Geol. Soc. Lond. Memoirs 19, 165pp; Taddeuci and Wohletz, 2001. J. Volc. Geotherm. Res. 109, p.299-317). Whether the volcano was showing minor activity in the months or years leading up to the eruption can not be determined as such activity would not necessarily result in recognizable deposits. Short-term precursor activity included phreatic and phreatomagmatic explosions resulting in ash-fall in SE Thera. It is generally agreed that the following Plinian eruption can be divided into 4 main phases:

Phase 1 created a white to pink deposit, largely consisting of pumice, which was thickest (6 meters) SE of the assumed vent location to the W of Thira village. Ash flow layers up to 70 cm thick were found in the top sections of this deposit. Analysis of the size and density of pumice in the deposit suggests a rapid increase in eruption intensity forming a towering over 30 km high Plinian column, after which activity gradually declined resulting in probably collapse of the column to form the so-called flow-break (a layer of ash embedded in the deposit caused by at least 3 ash flows). The flow beak is attributed in part to vent obstruction. Following the flow break, activity intensified again as the blockage was cleared and the eruption intensity increased again (Taddeuci and Wohletz, 2001. J. Volc. Geotherm. Res. 109, p.299-317). Phase 1 deposits also contain small amounts of basaltic-andesitic scoriae, probably traces of mafic magmas which are acted as a trigger for the eruption by pressurizing the rhyodacitic magma body.

Phase 2 created a deposit consisting of numerous ash and lapilli layers with cross-stratification, mega-ripples and dune-like morphologies which are characteristic of pyroclastic surge deposits associated with hydrovolcanic activity resulting from increasing access of water to the vent. The phase 2 deposits are up to 12 meters thick near to the vent, thinning rapidly as distance therefrom increases. Since thin pumice fall layers are found between surge deposits, it appears that a large eruption column was still present. First lithic ballistics are found in Phase 2 deposits.

Phase 3 deposits have been attributed to a variety of different forms of activity by different authors, including debris flows and pyroclastic flows or surges. The deposits are up to 35 meters thick at the caldera rim and become increasingly thick, coarse-grained and rich in lithics, towards the top of the deposit (i.e. as the eruption progressed). The embedded lithics include dark glassy dacite and dacitic hyaloclastite up to several meters in diammeter which can be interpreted as ballistically emplaced parts of the intracaldera volcano ("Pre-Kameni") thought to have been present in the caldera at the time of the eruption (much like present day Kameni islands). Phase 3 is thus considered to encompass onset of caldera formation. It is noted that significant Phase 3 deposits are not found all over Santorini.

Phase 4 deposits were emplaced all over the islands by a number of massive pyroclastic flows. Individual flows resulted in deposits up to a few meters thick, with the total thickness of Phase 4 deposits reaching as much as 40 meters on the outer shoreline of southern Santorini, but being less than 2 meters thick near the caldera rim.

The deposits are further overlayed in places by up to several meters of deposits formed by remobilization of erupted materials following rainfall (or possibly tsunami inundation) after the eruption.

Layer 3 and Layer 4 deposits near Akrotiri	Cliff formed by deposits on south coast by Akrotiri	Large boulder embedded in layer 4 near Akrotiri

	Massive Minoan deposits on south coast, 1km east of Akrotiri

Minoan deposits, Mavromatis pumice quarry	Minoan deposits, Mavromatis pumice quarry

Probable ballistic with impact sag, Mavromatis pumice quarry	Minoan deposits, Mavromatis pumice quarry

Tsunamigenesis is likely to have been associated with at least phases 3 and 4. Caldera collapse may have contributed to the process but it could have been a gradual process and tsunamis would have been somewhat channeled by the remaining islands around the caldera. It is however unlikely that the caldera formed without any tsunamigenesis, since it covers an area of about 65 square km and is approximately 1.6 km deep (It is largely filled with sedimented eruptive fallback materials, so that the actual sea floor in the caldera is only 400-500m deep). Viewing the thickness of Phase 4 deposits on the outer coasts of the islands, it is clear that massive pyroclastic flows must have entered the sea at these locations. There is little doubt that such voluminous pyroclastic flows will lead to the generation of tsunamis.

The vent location of the Minoan eruption is generally considered to have been on the Pre-Kameni intracalderal volcanic island, probably north of present day Nea Kameni. However, based on the sites of impact of ballistic blocks thrown from the vents in phases 2 and 3 of the eruption, and taking previous studies into account, a more detailed analysis of vent location has been attempted (Pfeiffer 2001. J. Volc. Geotherm. Res. 106, p.229-242). Based on the assumption of even distribution of heavy ballistics around the vent(s), the author proposes that the during Phase 2 the initial vent evolved southwards to the Kameni Line, forming an approximately 2 km long fissure which reached the southern coastline of the Pre-Kameni island, allowing water to interact with the magma at this location. In Phase 3 it is proposed that a further fissure opened NW of the primary vent location along the Columbos Line (see Fig.3).

Fig.3 Ballistic blocks (triangles) from Phase 2 of the Minoan eruption with median diammeters in cm. The asterisk marks the center of a circle enclosing all ballistic blocks and is interpreted as the most probably source area of the blocks. The vent from the first phase probably widened into a SE-NW trending fracture allowing sea-water contact with magma at the southern rim of Pre-Kameni Island. From Pfeiffer 2001. J. Volc. Geotherm. Res. 106, p.229-242), with kind permission of the author.

Impact of Eruption on Minoan Civilisation

Whilst the small Minoan settlement of Akrotiri on the south of Santorini was completely buried and ash and pumice, unlike in Pompeii, no bodies were found, suggesting that the inhabitants may have had sufficient warning to leave the site prior to the cataclysmic part of the eruption. Some buildings in Akrotiri apparently show signs of damage due to earthquakes. This damage had at least partially been repaired and it is unclear if the earthquakes had any link to the eruption.

Akrotiri was however not a major population center of the Minoan civilisation, so its fate, although of interest, was unlikely to have had much impact on the Minoan civilisation as a whole. Historical analysis of the impact of the eruption is rather focussed on the possible impact of tsunamis resulting from the eruption on the coastline of Crete, lying south of Santorini. In 1939, Marinatos (Antiquity 13, p.425-439) proposed that Minoan Crete was devastated by tsunamis generated from the LBA eruption of Santorini. The theory was based on (i) comparison to the effects of the famous Krakatau eruption of 1883, (ii) destruction of coastal sites by fire (due to e.g. overthrown oil lamps), and (iii) excavations at Amnisos on Crete showing putative tsunami damage. Further authors presented evidence supporting the theory in the following decades. However, the supporting evidence for inundated of the coastline of Crete by LBA eruption generated tsunamis was questionable. No significant deposits attributable to a LBA tsunami could be found at 41 sites analysed on Crete, and critical reexamination of the original hypothesis and supporting evidence suggested that it was flawed in several ways (Dominey-Howes, 2004. J. Volc. Geotherm. Res. 130, p.107-132). Regarding destruction of coastal sites by fire, it is noted that sites far inland were affected by fire too, suggesting an earthquake as a more likely cause (or indeed civil unrest). Further, the excavations at Amnisos may have merely reflected damage from a storm rather than a tsunami. The eruption of Krakatau was not comparable and although the caldera formed by the LBA eruption was larger, caldera formation was not necessarily a rapid enough process to be significantly tsunamigenic. It is noted that the eruptive center at Krakatau was largely in open water, whereas the LBA eruptive center was largely contained in the pre-existing caldera. This would have channelled any tsunami generated within the caldera westwards. Indeed analysis of marine sediments suggest that tsunamis may have been propagated in this direction (Cita et al. 1984. Marine Geol. 55, p.47-62).

Several years after this publication, convincing evidence for tsunami deposits on Crete was however provided following detailed geoarchaeological studies at Palaikastro, a large Minoan coastal settlement in the NE of the island (Bruins et al. 2008, J. Archaeol. Sci. 35, p.191-212). Chaotic layers of material including volcanic ash, building material, pottery shards, marine shells, bones and beach pebbles were noted at numerous locations along the coast. The material was extensively "imbricated", i.e. the materials overlay eachother similar to roof tiles. Imbrication is generally the result of strong water currents. Further, the layer showed erosional contact with the underlying strata, another signature for a tsunami deposit. Further, carbon dating of several bones and sea-shells from the deposit dated these largely at the time of the Minoan eruption. Based on the location of the deposits, the authors modelled the tsunami responsible, concluding that it had an initial amplitude of from +35 to -15 meters with a crest-length of about 15 km. Since the layer contained ash from the eruption, it appears that the main tsunami reached the coast of Crete after ash-fall from the initial phase of the eruption. It is likely to have been triggered by massive pyroclastic flows entering the sea during phases 3 and 4 of the eruption and/or by caldera collapse.

Historical Activity and Outlook

The recently active volcanic center of Santorini is located at Nea Kameni island, in the center of the flooded caldera. Nea Kameni and the smaller nearby island of Palea Kameni are the products of post-Minoan volcanism at Santorini. The islands represent the peak of an intra-caldera volcanic edifice which is about 4 km wide at its base and has a volume of about 2.5 cubic kilometers, most of which is under water (rising from the base of the caldera at a depth of about 300 m to a summit 126 m above sea level). They were formed by calc-alkaline dacitic lavas with a remarkably consistent composition (64-68% SiO), considering their emplacement over a period of over 2000 years. Historical records suggest that the island of Lera first emerged from the sea at the present location in 197 BC but was subsequently eroded. Eruptions in 46 and 726 produced Palea Kameni island, whilst activity subsequently shifted slightly northeastwards, constructing the Nea Kameni island during eruptive episodes in 1570, 1707-1710, 1866-1870, 1925-1926, 1939-1941 and 1950. Most of these eruptions were largely effusive, yet the 197 BC and 726 eruptions were more explosive, the latter producing significant amounts of pumice (Higgins, 1996. J. Volc. Geotherm. Res. 70, p.37-48; references therein).

Kameni Islands Geological Maps (numbers indicate year(s) in which lavas emplaced). Red stars show known vent locations. 1950 vent not indicated as directly under small deposit. Based on Druitt et al. 1999. With kind permission.	Aerial image of Kameni islands in 2003. Taken from west with Palea Kameni Island in front of Nea Kameni. Thira town on caldera rim in background.

Kameni Islands viewed from southwest (Stitched Panorama Image)

Landing site for tourist boats on Nea Kameni - Dark lava from 1866-1870 eruption in background	View to Palea Kameni from center of Nea Kameni

Recent crater on Nea Kameni	Crater with dark lava dome over one of vents of 1866-1870 eruption behind

Studies of the composition of the lavas erupted on the Nea Kameni provide a possible understanding of the underlying magma chamber(s). The main chamber is thought to lie at a depth of 2-4 km. Studies on microlite and megacryst crystal size composition in erupted magmas suggest that eruptions are preceeded by introduction of fresh magma into the bottom of the magma chamber, from where it gradually mixes with magma remaining from the previous eruption, before an eruption takes place within a period of 6-13 years after the replenishment event (Higgins, 1996. J. Volc. Geotherm. Res. 70, p.37-48). The 6-13 years correspond to the assumed growth rate of the microlites following the start of mixing of the two magma batches. In a more recent study, magmatic enclaves (i.e. "blobs" of different magma type enclosed in the main magma body which represent replenishing magma which has not mixed with the resident magma prior to eruption) were studied in lavas erupted on Nea Kameni in order to learn more about the replenishing magma entering the magma chamber prior to eruptions (Martin et al. 2006. J. Volc. Geotherm. Res. 154, p.89-102). The enclaves had a significantly lower silicate content (51-57%) than the resident magma (65%), were highly crystalline, largely ellipsoid in shape and ranged in size up to 60cm in length, but were generally smaller. The largest enclaves were generally found nearer to the vents. The size and composition of the enclaves was interpreted as showing that the magma chamber is typically replenished with denser magma than the partially cooled dacite already residing in the chamber. The denser magma initially forms a layer at the base of the magma chamber before gradually mixing with the resident magma. The study also concluded that the volume of replenishing magma is directly proportional to the subsequently erupted lava volume, and that since each lava contained a single dominant enclave type (lavas from different eruptions had distinct enclave compositions), each batch of replenishing magma must be almost entirely removed during the subsequent eruption. In the more recent study by Martin et al. (Science 321, p.1178 (2008)), diffusion rates of Fe and Mg in olivine crystals from lavas of the 1925-28 eruption were assessed and in contrast to the Higgins study it was suggested that basaltic andesite entered the magma chamber from 15-75 days before the eruption.

The course of any new eruption of the Santorini volcanic complex is of great interest due to the fact that the small island may host around 50000 people at peak tourist periods in the summer months. An eruption of the Nea Kameni volcanic center could be a touristic spectacle in what is effectively an amphitheater-like setting. However, it is to be feared that a sensationalist press coupled with risk-averse authorities will lead to a significant decline in regular tourism or even evacuation of the island with the associated dire economic consequences. Indeed, islanders fear the economic impact of a possible eruption more than the eruption itself (Dominey-Howes and Minos-Minopoulos, 2004. J. Volc. Geotherm Res. 137, p.285-310). A massive Minoan-style eruption is highly unlikely in the near future based on past patterns of activity, yet gas and ash are likely to cause respiratory problems. Vulcanian explosions powerful enough to propel ballistics into populated areas could possibly occur. Historical accounts of previous eruptions on Nea Kameni show that whilst predominantly effusive, most eruptions were accompanied by explosive activity. The activity in 726 was particularly violent and produced copious amounts of pumice. The intensity of this eruption may relate to the 679 years of inactivity preceding it, unusually long for the Kameni center.

Certainly, in the short-run, a Plinian eruption can be almost certainly ruled out. Based on studies of the Minoan eruption, it is clear that such an eruption would be accompanied by significant inflation of the edifice (Druitt et al. 2012. Nature 482, p.77-82), which has not been noted at present. Further, extrapolating from previous intervals between Plinian eruptions of Santorini (17-45 thousand years), the next such eruption should only be due in at least 13.5 thousand years time. Additionally, it is noted that periods of inactivity are thought necessary to allow evolution of magma composition to the type associated with the violent Plinian episodes (Vespa et al. 2006. J. Volc. Geotherm. Res. 153, p.262-286). Nevertheless, the deposits noted between Plinian eruptions were probably largely emplaced by sub-plinian events which could also be fatal for anyone on the island at the time.

Further, renewed activity of the submarine Columbo volcano could represent a hazard to the inhabitants in lower-lying parts of Santorini.

In January 2011, seismic unrest was been noted at Santorini in 2011. The unrest has continued with fluctuating intensities into 2012. The seismicity is likely to signify influx of fresh magma into the chamber under the volcano, which according to Higgins (1996) would suggest an eruption in the next 6-13 years. However, based on Martin et al. 2008 an eruption could be expected 15-75 days after influx of fresh magma. The assumption that seismicity is linked to the influx of magma into the volcano is supported by GPS-based studies on deformation at the caldera (Newman et al. 2012. In Press). The deformation is best explained by accumulation of an approximately spherical body of magma at 4 km depth in a region about 2 km north of the Kameni islands under which most of the seismicity is localized. In spite of the spacial separation between centers of seismicity and inflation, both are temporally strongly correlated to eachother, and hence undoubtedly connected. During 2011, it is estimated that about 14 million cubic meters of magma have entered the inflating chamber. This has resulted in uplift and lateral expansion. The lateral extension of the volcano away from the center of inflation in the northern part of the caldera was about 14 mm in 2011.

Absent comparative seismic or inflation data from previous eruptions, due to a lack of modern monitoring equipment at the time, it is difficult to predict if and when an eruption can be expected. Reassuringly, the magma influx measured in 2011 is equivalent to only about 0.3% of the magma expelled during the catastrophic Minoan eruption.

Pumice Mining on Santorini

Pumice mining at Santorini has a long history. Indeed the the archaeological site of Akrotiri was discovered in the 1860s during excavation of pumice for export to Egypt where it was used for making cement by the Suez Canal Company during the construction of Port Said. Numerous abandoned quarries can be found on the E and SE rim of the caldera where pumicious deposits from the Minoan Eruption were thickest. Pumice from these quarries was excavated and passed down a variety of chutes to ramps from which it was loaded onto bulk carriers on the inner coast of the caldera. Much as at the nearby island of Milos, products of past volcanic activity have proved industrially valuable in recent times. Work at the quarries was stopped in 1986, in order to preserve the unique landscape of Santorini, also taking into account that the economy was becoming growingly dependent on tourism rather than pumice exportation.

Building at Fira quarry	Decaying ship loading ramps below Fira quarry	Massive deposits mined at Fira quarry

Deposits of previous eruptions buried by Minoan pumice exposed at Fira quarry	Top of sieve for size-sorting pumice, Fira quarry

	Decaying ship loading ramps below Fira quarry

Decaying ship loading ramp below Fira quarry	Quarry with ship-loading facility overlooking Athinios from north

Mavromatis pumice quarry, view from SW	Mavromatis pumice quarry with ship-loading facility	Mavromatis pumice quarry, Minoan deposits

Ruins of pumice storage facilities	Remains of conveyor belts

Fine pumice deposit at base of cliff	Cliff with ash / pumice deposits of Minoan eruption	Cliff with ash / pumice deposits of Minoan eruption

Visitor Information

The local economy on Santorini is primarily dependent on the approximately one million tourists that visit the island every year. Numerous lodgings are perched on the eastern crater rim with breathtaking views over the flooded caldera. The island has an airport with daily flight connections to Athens. Numerous charter companies also offer flights to the Island in the summer months. Further, a port connects the island to the mainland and neighbouring islands by ferry connection. Cruise ships are also seen in the caldera almost every day in the tourist season.

Main town of Fira perched on eastern caldera rim	Thira port with Fira town above

The small volcanic island of Nea Kameni can be reached by tours departing from the port area. Further, the archaeological excavations of Akrotiri on the south of Santorini can be visited. Unfortunately, as in Pompei, virtually all artifacts (and in Akrotiri ALL murals) have been removed from the excavation site, leaving it looking rather bare and removing artifacts from their proper context. Archaeological finds from the excavations at Akrotiri can be seen in the Archaeological Museum, or Museum of Prehistoric Thira, both located on Santorini. The National Archaeological Museum in Athens also harbours many artifacts from Santorini.

Broken staircase is evidence of earthquake damage, probably preceeding Minoan eruption	Broken staircase	Bedframes found outside house - suggest process of cleaning after first events, before complete abandonment

Various buildings	Building with roof guttering

Chamber with many storage vessels	Various buildings

Explanatory sign showing murals found at Akrotiri, now in museums	Explanatory sign showing murals found at Akrotiri, now in museums

Those wishing to have a detailed geological tour of the island may consider joining one of the Santorini Tours offered by Tom Pfeiffer, a trained geologist who performed research on Santorini in the past. For anybody wishing to do a mediterranean volcano tour, visits to the italian volcanoes Stromboli, Etna and Vesuvius would also be recommended.

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