Nat Hazards DOI 10.1007/s11069-012-0405-0 ORIGINAL PAPER
Tsunami hazard risk of a future volcanic eruption of Kolumbo submarine volcano, NE of Santorini Caldera, Greece P. Nomikou • S. Carey • K. L. C. Bell • D. Papanikolaou K. Bejelou • K. Cantner • D. Sakellariou • I. Perros
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Received: 30 April 2012 / Accepted: 12 September 2012 Ó Springer Science+Business Media Dordrecht 2012
Abstract Kolumbo submarine volcano, located NE of Santorini caldera in the Aegean Sea, has only had one recorded eruption during historic times (1650 AD). Tsunamis from this event severely impacted the east coast of Santorini with extensive flooding and loss of buildings. Recent seismic studies in the area indicate a highly active region beneath Kolumbo suggesting the potential for future eruptive activity. Multibeam mapping and remotely operated vehicle explorations of Kolumbo have led to new insights into the eruptive processes of the 1650 AD eruption and improved assessments of the mechanisms by which tsunamis were generated and how they may be produced in future events. Principal mechanisms for tsunami generation at Kolumbo include shallow submarine explosions, entrance of pyroclastic flows into the sea, collapse of rapidly accumulated pyroclastic material, and intense eruption-related seismicity that may trigger submarine slope collapse. Compared with Santorini, the magnitude of explosive eruptions from Kolumbo is likely to be much smaller but the proximity of the volcano to the eastern coast of Santorini presents significant risks even for lower magnitude events. Keywords
Tsunami Hazard and risk 1650 AD eruption Santorini Aegean Sea
P. Nomikou (&) D. Papanikolaou K. Bejelou Faculty of Geology and Geoenvironment, University of Athens, Athens, Greece e-mail:
[email protected] S. Carey K. L. C. Bell K. Cantner Graduate School of Oceanography, University of Rhode Island, Narragansett, RI, USA D. Sakellariou Institute of Oceanography, Hellenic Centre for Marine Research, Anavyssos, Greece I. Perros Pedagogical Department of Primary Education, University of Athens, Athens, Greece
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1 Introduction The seismotectonic setting of the eastern Mediterranean, where ongoing subduction of the oceanic crust of the African plate beneath the overriding part of the Eurasian plate occurs, results in a high tsunami potential. Strong submarine earthquakes, seismically induced submarine mass movements, and volcanic eruptions occur frequently and contribute to significant tsunami risk for the coasts of southeastern Europe, northern Africa, and the Near East (Vott et al. 2008), with a significant impact on coastal communities, infrastructures, and ecosystems. Despite the fact that the most frequent tsunami generation mechanisms in the eastern Mediterranean region have been associated with co-seismic, sea-bottom dipslip faulting, two of the largest tsunamis known in the Mediterranean Sea were generated by strong eruptions in the Thera volcanic complex: the seventieth century BC Minoan tsunami and the 1650 AD Kolumbo tsunami. Seismic reflection survey in 2006 aboard R/V ‘‘AEGAEO’’ showed that a prominent NE–SW trending, rift zone, the Anydhros basin, extends to the north-east of Santorini and hosts Kolumbo submarine volcano and numerous other smaller cones (Nomikou et al. 2012a). The fault pattern has been interpreted as ‘‘flower type’’ structure developed above a single strike-slip fault, running NE–SW below the Kolumbo volcanic line (Sakellariou et al. 2010). It has been therefore named Kolumbo strike-slip fault zone, which has provided pathways for subduction generated magma to reach the surface and form the Kolumbo Volcanic Chain (Nomikou et al. 2012b). Pyroclastic gravity flows around Kolumbo volcano can be recognized in seismic sections collected on the flanks of the edifice and in surrounding basins. The tsunami associated with the volcanic eruption of Kolumbo in 1650 AD had significant impact at the surrounding areas. A powerful submarine eruption breached the surface and resulted in several months of intense explosive activity (Fouque 1879). Tsunamis generated during the event caused significant damage along the east coast of Santorini and especially the area of Perissa and Kamari (Dominey-Howes et al. 2000). Recent marine exploration of the Kolumbo submarine volcano has led to important new information about the structure of the volcano and the pyroclastic deposits associated with the 1650 AD eruption. Interpretation of the deposits provides insights into the evolution of the eruptive activity and the likely mechanisms by which the destructive tsunamis were generated. Recognition of the tsunami generation mechanisms is critical to an assessment of the hazards that Kolumbo may pose should it erupt again in the future. In this paper, we discuss the new findings regarding the 1650 AD eruption and describe the tsunami-generating mechanisms.
2 Historical record of the 1650 AD eruption The last recorded activity of Kolumbo submarine volcano was a series of explosive eruptions that took place in 1650 AD. Fouque (1879) summarized many of the eyewitness accounts and provided an excellent interpretive summary of the activity. The first signs of impending activity took place in March of 1650 when violent earthquakes were felt in the area. Seismic activity became more frequent as the eruption approached, and the sea surface was observed to be discolored. Discoloration of the sea surface is often associated with submarine volcanic activity as magma interacts with seawater and causes hot turbid water to ascend to the surface. Thus, it is likely that the eruption had been proceeding underwater for several days before it was energetic enough to break the surface of the
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ocean. Earthquakes and subterranean roaring continued intermittently during the next 6 months until on September 27 a climax was reached and dense ash clouds were seen rising from the sea about 4 miles northeast of Santorini. Noxious odors were recorded on the island and a ‘‘snow-white’’ ledge emerged from the sea (Fig. 1). Additional columns of ash rose and quickly disappeared, and it was observed that the sea was covered with pumice. On September 29, 1650, the most violent eruptive activity occurred at Kolumbo. Father Richard’s account notes ‘‘this was the most terrible day. The earth quivered and the air was afire. Thick sulphurous steam billowed out of the depths. Then suddenly the clouds caught fire, lightning rent the sky, thunder burst forth and strange forms moved before one’s eyes: flying snakes shining spears and lances and whirling blazing torches. All that day the clouds hung low and the wild elements met in such raging combat that their clamour could be heard a hundred leagues off.’’ These descriptions captured the spectacular displays of lighting that commonly accompany highly explosive eruptions. The lightning within the eruption plume was also associated with tremendous explosions that were heard as far away as 400 km in the Dardanelles, and earthquakes at this time were felt as far away as Crete, about 100 km to the south. Prevailing winds carried fine-grained volcanic ash from the eruptions to the east where it was deposited as a thin, white powdery layer in parts of Turkey. Closer to the volcano, pumice was floating on the sea surface. Pumice can float for several days and travel long distances from the source volcano before its immersion and deposition on the seafloor. Though the activity ceased around 11 p.m. on the 29th, it began again at midnight and continued throughout the 30th. After a few days of decreased activity, the eruption diminished. One of the main hazardous effects of this volcanic eruption on the local communities was the clouds of poisonous gases that were released. The gas caused eye pain, blindness, and cerebral congestion and many inhabitants temporarily lost consciousness for several hours. There were also reports that the gases discolored coins, sacred vessels in the churches, paintings, and walls in many buildings. This suggests that the gases were both acidic and poisonous as a result of sulfur, chlorine, and carbon dioxide release during the explosive eruptions. Pyroclastic surges which represent one of the most lethal effects of explosive volcanic eruptions are known to be able to travel great distances over water (Fig. 2).
Fig. 1 Ash and gas plume from a submarine eruption in the Tonga Islands in 2009. Initial stages of the Kolumbo 1650 AD event likely resembled this type of activity
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Fig. 2 Pyroclastic surge traveling over the sea surface off the east of Montserrat Island in the Lesser Antilles
The second major impact from the eruption was the generation of tsunamis. At least one tsunami inundated Thera, carried away livestock, destroyed buildings, and eroded the roadways and 500 acres of the eastern coastline. Two churches in Perissa and Kamari were swept into the sea, and the foundation of old churches and ruins was revealed. These newly exposed areas were likely buildings of the Hellenistic Period on the island (e.g., Ancient Oia and Ancient Eleusis). It appears that the initial sign of the oncoming wave was a large retreat of the sea. Dohiaritis (1947) notes that ‘‘Then 20 men went to the beach to gather fish, which were floundering on the exposed bottom of the sea, and they lost their lives. Suddenly earthquakes and solicitations, and thunder have stopped. After that the sea swelled and climbed upon it to the island as 2 miles. The sea destroyed boats and vineyards and everything.’’ The impacts of the tsunamis were observed not only in Santorini but on many of the neighboring islands as well. It was reported that boats of the Turkish fleet that were tied up along the coast of Dia island (north of Heraklion, Crete) were carried away by the tsunami. Waves that came ashore in Ios reached a level of 50 ft, while in Sikinos, they advanced inland up to 350 ft and covered the fields. A few isolated explosions occurred on November 4th and 5th with associated gas release and minor earthquakes. After an increase in earthquake intensity and submarine disturbances in the early days of December, the 1650 AD eruption of Kolumbo ended. Small shocks and high water temperatures around Kolumbo continued for a number of years but the small island eroded beneath the waves within a few months leaving an 18-m deep cone. The summary of the 1650 AD submarine eruption of Kolumbo by Fouque (1879) contains key observations that are critical to interpreting the eruptive processes of this event. The first is that the activity clearly breached the sea surface and was able to produce subaerial eruption columns of substantial height. Fallout of tephra in western Turkey suggests eruption column heights in excess of 10 km based on some preliminary modeling we have carried out using the PUFF ash transport model (Searcy et al. 1998). The development of such high eruption columns indicates high mass eruption rates and substantial degassing of primary volatiles in addition to the likely role of phreatomagmatic fragmentation. Second, observations of the extensive development of floating pumice on the sea surface support the existence of a volatile-rich silicic magma. The very rapid
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disappearance of the island once the activity stopped indicates that the emergent part of the island consisted of easily erodible pyroclastic material. Finally, a certain type of deposits (angular non-volcanic clasts) was discovered at St George’ s small church near Kamari village, at a distance of 80 m from the shoreline, reinforcing the historical accounts regarding the tsunami (Papadopoulos 2009). According to Dominey-Howes et al. (2000), all the sites investigated at the eastern coast of Santorini (Kamari, Perissa) show that the tsunami, which accompanied the 1650 AD eruption, probably had a run up of less than 2 m a.s.l. and penetrated less than 500 m from the shore.
3 Morphological analysis and ROV exploration of the study area 3.1 Morphological analysis The bathymetric data presented in this paper were collected during oceanographic surveys in 2001 and 2006 aboard the R/V Aegaeo, using the multibeam systems SEABEAM 2120 and SEABEAM 1180. The collected multibeam data have been extensively processed by means of data editing, cleaning of erroneous beams, filtering of noise, processing of navigation data, and interpolation of missing beams. The resulting bathymetric map was originally compiled at 1/50 000 scale which was greatly reduced for publication with 11
Fig. 3 Swath bathymetric map of Kolumbo volcano using 5-m isobaths
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different colors corresponding to every 50 m depth and with additional isobaths of 5 m (Fig. 3). The Kolumbo volcanic chain extends 7 km to the northeast of Santorini. It is composed of more than 19 volcanic cones of varying sizes, which are aligned along two distinct linear trends (N 29°E and N 428E) that converge at the point of the Kolumbo volcano (Nomikou et al. 2012a). Kolumbo is the largest of the submarine volcanic cones along the volcanic rift, having an ellipsoidal shape with the major axis trending NE–SW. The diameter of the volcanic cone is approximately 3 km, whereas the crater diameter is 1.7 km. The average depth of the caldera rim lies at about 150 m depth forming a submarine circular cliff with the shallowest point at only 18 m below sea level (Fig. 3). Within the cone is a relatively flat-lying crater floor with an average depth of about 505 m (Nomikou 2003). The interior crater may have been created by the partial collapse of a pre-existing volcanic cone or have been produced largely as a constructional feature associated with growth of the edifice during the 1650 AD eruption. Further analysis of the bathymetry has led to the construction of a slope distribution map (Fig. 4). The distribution of slope values within the studied area has been subdivided into five categories in a way that clearly illustrates the zones where there is an abrupt change of slope, which reflects the steep volcanic slopes of the crater wall or the external flanks. The highest slope values (35–50 %) are observed within the internal walls of Kolumbo. Its northwestern external slopes gradually diminish from 25–35 % to 15–25 % and 5–15 %, whereas in the southeastern part, the external slopes gradually diminish from 15–25 % to 5–15 % and 0–5 %. Several linear-to-curvilinear scarps are observed at the west and northwest flanks with inward dipping faces and high slope values 25–35 %. The orientation of these features suggests that they may represent remnant crater rims and are likely to contain outcrops of volcanic rocks from the earlier stages of cone building. 3.2 Remotely operated vehicle exploration Remotely operated vehicle (ROV) exploration of Kolumbo submarine volcano took place during multiple cruises in 2006, 2010, and 2011. A number of interesting morphological features were observed at the inner slopes of Kolumbo volcano, which have likely been formed as a result of the evolution of the volcanic center. Vertical transects at the crater walls revealed a scalloped morphology formed by outcrops of a variety of volcanic products such as thick pumice layers, dikes (Fig. 5), intrusions, breccias, and mass wasting deposits (Fig. 6). Extensive deposits of pumiceous scree occur over large parts of the lower crater walls obscuring in situ outcrops. These morphological features have been created by mass wasting of unconsolidated pyroclastic deposits onto the crater floor. Vertical outcrops form cliffs that taper into promontories that either extend back into another vertical face or transform into talus slopes. However, at about 200 m above the crater floor, an unexpected discovery was the occurrence of spectacular outcrops of thick pyroclastic deposits in the southwest and northern walls of the crater (Carey et al. 2011). The deposits consist of well-bedded pumiceous tuffs, with relatively fresh collapse faces up to 20 meters in height. Preliminary examination suggests that there are at least two main types. The first are decimeter-thick units consisting of relatively coarse, well-sorted angular pumice lapilli and blocks with planar contacts and normal grading, suggestive of submarine pumice fallout (Fig. 7a, b). Maximum pumice clasts are up to 40 cm in diameter. A second type consists of lapilli and fine ash beds that are moderately to poorly sorted and usually exhibit mantle bedding. Some units in the second group show lateral variations in thickness and stratification,
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Fig. 4 Slope distribution map of Kolumbo volcano Classification of slope magnitudes (down right): (1) areas of mean morphological slope 0–5 %, (2) low slope areas of 5–15 %, (3) moderate-slope areas of 15–25 %, (4) high-slope areas of 15–25 %, and (5) very steep-slope areas of 35–50 %
Fig. 5 Lava dyke at the lower parts of the eastern wall of Kolumbo submarine crater
suggesting emplacement by sediment gravity flows (Fig. 8). The lack of erosional contacts within the sequence indicates rapid accumulation of the pumiceous units, from the last explosive eruption in 1650 AD. A majority of pumice samples from the upper, bedded sequence are fresh, highly vesicular, and crystal-poor. The phase assemblage includes plagioclase, orthopyroxene,
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Fig. 6 Undulating cliff faces created by mass wasting processes of unconsolidated pyroclastic deposits in the upper crater walls of Kolumbo submarine crater
Fig. 7 a, b Images of well-sorted, laterally continuous pumice block and lapilli beds that are interpreted as submarine pumice fallout units in the crater wall of Kolumbo submarine crater
Fig. 8 Image of a pumice deposits that show wedge-shaped bedforms that may be related to deposition by pyroclastic gravity flows
biotite, and Fe–Ti oxides. Bulk major element analyses by XRF and electron microprobe analyses of interstitial glass yield relatively homogeneous rhyolitic compositions indicating that the whole sequence was formed during the 1650 AD eruption (Cantner 2010; Cantner et al. 2010).
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4 Eruption model of 1650 AD Cantner et al. (2010) developed a model for the evolution of the 1650 AD eruption based on the lithology and stratigraphy of the submarine pyroclastic deposits at Kolumbo. The eruption was driven primarily by exsolution of volatile-rich (*5 % H2O) rhyolite magma that had been stored at a depth of approximately 5 km in a crustal magma chamber. Initially, the eruption appears to have been completely submarine as suggested by historical observations of a period of intense water discoloration at the surface and intense seismicity (Fig. 9a). For 13 days, the eruption was evolving underwater and the magma discharge rate and intensity of the eruption varied intermittently (Fig. 9b). During this phase, large volumes of unconsolidated rhyolitic pumice accumulated around a submarine vent that may have been as deep as 500 m. Growth of the pyroclastic deposits led to a shallowing of the edifice and eventually a transition to subaerial discharges of highly fragmented magma that generated convective eruption columns that rose to [5 km in the atmosphere (Fig. 9c). As the vent approached the sea surface, the hydrostatic pressure was reduced. This may have significantly increased the role of phreatomagmatic fragmentation. The increase in phreatomagmatic activity is reflected in the stratigraphy as the upper sequences of the crater wall are finer grained and thinner bedded than those at depth. The presence of large lithic clasts throughout the pumice lapilli units indicates that conduit erosion and vent expansion occurred periodically throughout the eruption. Different facies of the submarine pyroclastic deposits indicate that deposition occurred by both pumice fallout and sediment gravity flows, some of which were likely to at high temperature. Historical reports of fatalities by asphyxiation and burns in the vicinity of Kolumbo and on the shores of Santorini strongly suggest that pyroclastic flows and/or surges were generated from eruption columns that broke the sea surface (Fig. 9d).
5 Tsunami generation mechanisms of the 1650 AD Kolumbo eruption Tsunamis are often associated with explosive eruptions that occur within or in close proximity to the marine environment (Latter 1981). Research on syneruptive mechanisms of tsunami generation has focused on submarine explosions (Stehn 1929), pyroclastic flow discharge into the sea (Watts and Waythomas 2003; Waythomas and Watts 2003; Carey et al. 2000), island flank collapse (Ward and Day 2001; Day et al. 2005; McQuire 2006; Abadie et al. 2009), and submarine caldera formation (Cooke et al. 1976). We consider each of these mechanisms within the context of the historical records and interpretive eruption sequence based on the pyroclastic stratigraphy. Shallow water explosive volcanism has generated small-scale tsunamis during several historical eruptions. For example, submarine explosions during the growth of a dacitic dome at Myojinsho Japan during the period 1952–1953 generated tsunamis with a maximum height of about 1 meter and the explosive emergence of Anak Krakatau in Indonesia in 1927 produced waves up to 4 meters high at the site of generation (Stehn 1929). Neither of these events produced sustained explosive eruption columns and instead was characterized by short-lived phreatomagmatic explosions. We suggest that small waves are also likely to have been generated by the initial emergence of Kolumbo in 1650 AD when explosions breached the surface but may have been relatively small. The generation of the most energetic tsunamis at Kolumbo appears to coincide with the climax of the eruption (September 27–30) when the historical observations and stratigraphy of the submarine deposits suggest that pyroclastic gravity currents and sustained
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Fig. 9 Model of Kolumbo 1650 AD eruption processes from Cantner et al. (2010). a Sustained submarine eruption column—ingestion of seawater at the plume edge causes pumice quenching and fallout of negatively buoyant clasts; b Simultaneous deposition from submarine gravity flows and eruption column fallout; c Sustained subaerial plume—fallout accumulates in water column and forms vertical density currents; d Phreatomagmatic activity increases fragmentation
eruption columns were generated. Establishment of convective eruption columns capable of transporting tephra to Turkey raises the possibility that column collapse occurred. Entrance of collapse material into the sea would provide a potentially important mechanism for the generation of the main tsunamis that impacted the coast of Santorini and neighboring islands. Watts and Waythomas (2003) presented a detailed theoretical analysis of tsunami generation during pyroclastic flow discharge into the sea. They identified five wave-generating mechanisms, likely to operate during flow/water interactions. These include steam explosions, submarine transport of a dense underflow component of pyroclastic flows, pressure from a less dense component of the flows that travels on the sea surface, shear from a laterally moving flow component on the sea surface, and pressure impulse from dense clasts striking the sea surface. Of these mechanisms, they determined that the most significant involves entrance of the dense lower part of the pyroclastic flow into water, in a manner analogous to a decelerating solid body moving down a submarine slope.
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Evidence for the emplacement of pyroclastic gravity flows around Kolumbo volcano can be found in seismic records collected on the flanks of the edifice and in surrounding basins. Seismic profiles 6–7 (Fig. 10) and 8–9 (Fig. 11) run parallel to the northwestern and southeastern slopes of Kolumbo volcano, respectively. In Fig. 10, the upper sequence is acoustically transparent or mostly chaotic and can be interpreted as pyroclastic flows emplaced during the second phase of 1650 AD volcanic activity (KPT). In the same profile, the middle one is thicker and layered with sub-continuous, slightly undulating internal reflectors (b) which onlap the basal contact and the lower one has sub-parallel reflectors (c) with good continuity, deformed by synsedimentary faults. The thickness of the upper unit (KPT) decreases as the distance from Kolumbo volcanic center increases. Maximum thickness may reach 100 ms twtt (about 80 m) on the western slopes (Fig. 10), up to 40 m in the eastern slopes (e.g., Fig. 11), and diminishes gradually. The seismic profile 8–9 (Fig. 11) shows a step morphology created on top of a listric plane at the northeastern slopes of Santorini. This morphologic feature does not affect the top unit, indicating that the emplacement of the latter took place during the 1650 AD eruption. Another pyroclastic flow (PF) can be distinguished in the lower unit and may be associated with Santorini eruptions during the Quaternary. It is worthy to note that the gravity flows produced from the 1650 AD eruption spread around the volcanic center of Kolumbo, especially toward Santorini. The eastern part of the profile 97–98 (Fig. 12) exhibits a segment of Anydhros fault zone (AFZ) bordering the SE margin of Anydhros Basin. The structure of Kolumbo submarine volcano is also shown on this profile. The interpretation of this profile suggests that at least one older volcanic cone exists and is located underneath the present one, indicating that Kolumbo has been active at least once, prior to the 1650 eruption (Huebscher et al. 2006; Sakellariou et al. 2010). Another possible tsunami-generating mechanism during the 1650 AD Kolumbo eruption was the formation of a submarine caldera due to collapse of the structure as magma was evacuated from the crustal magma chamber. The rate and geometry of caldera collapse
Fig. 10 Seismic reflection profile NE of Santorini. KPF Kolumbo pyroclastic flow deposits
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Fig. 11 Seismic reflection profile NE of Santorini. KPF Kolumbo pyroclastic flow deposits
is poorly understood, because there have been no direct observations of the process, but only of the resulting post-eruption morphology. The size of the Kolumbo crater is rather small compared with other known caldera collapse structures, and the current configuration may reflect a crater that formed around the active vent site. However, seismic profiles that cross Kolumbo (Fig. 12) reveal a possible pre-existing structure that has been built upon by deposition of the thick 1650 AD pyroclastic deposits. Thus, it is possible that some partial collapse of the structure may have taken place as a result of the eruption. Previous
Fig. 12 Seismic reflection profile NE of Santorini and crossing Kolumbo volcano. KPF Kolumbo pyroclastic flow deposits
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estimates of the total erupted volume based on the caldera volume suggested a total of 2 km3 (Vougioukalakis et al. 1996). New correlation of all the seismic profiles around Kolumbo volcano shows the distribution and thickness of the pyroclastic deposits of the 1650 AD eruption (Fig. 13). The maximum thickness of these deposits is approximately 180 m at the western part of the crater, and they cover an area of about 97.4 km2. If an average thickness of 50 meter is assumed for the area, then a conservative estimate of the deposits is of the order of 5 km3 bulk volume or about 2-km3 dense rock equivalent. Given the great thickness of inferred Kolumbo pyroclastic flow deposits in the basin surrounding the edifice, the large volume discharge increases the likelihood of partial edifice collapse.
6 Discussion and conclusions Recent oceanographic explorations of the Kolumbo submarine volcano have shed new light on the morphological nature of the center, the composition of its eruptive products, and the sequence of events that took place during the catastrophic eruption in 1650 AD. We suggest that tsunamis from the 1650 AD were likely generated by a variety of mechanisms including shallow submarine explosions, discharge of pyroclastic flows into the sea, and edifice collapse as a result of magma withdrawal from a shallow crustal magma chamber. Of these mechanisms, discharge of pyroclastic flows into the sea may have played a dominant role, but our data precludes us from making a quantitative assessment of the relative role of each mechanism. It is worthy to note, however, that the eruptive behavior of Kolumbo clearly presents numerous ways for generating tsunamis in close proximity to Santorini. There are several lines of evidence which suggest that Kolumbo could pose significant risk to inhabitants of Santorini and neighboring islands from future eruptive activity. First, recent seismic studies indicate that the most frequently occurring zone of earthquakes in the Santorini volcano field is located beneath Kolumbo (Bohnhoff et al. 2006). Furthermore, tomographic studies have suggested the existence of an active crustal magma chamber at depth of about 5 km beneath the volcano (Dimitriadis et al. 2010). This is the same depth as the pre-eruption pressure/temperature conditions inferred for the 1650 AD rhyolite magma (Cantner et al. 2010). In addition, a widespread hydrothermal vent field was discovered on the northern part of the Kolumbo crater floor during ROV dives in 2006 (Sigurdsson et al. 2006). A Kuroko-style massive sulfide deposit is currently forming there with fluid venting temperatures in excess of 200 °C. It is difficult to predict the exact nature of a future eruption of Kolumbo volcano given that a detailed history of the volcano has yet to be determined. ROV explorations of the crater indicate that dacite and rhyolite are the most common products of the volcano with little evidence of more mafic compositions. In addition, the inner crater wall consists dominantly of bedded pyroclastic and dike injections. There has yet to be any evidence of extensive dome-building episodes in the evolution of the volcano. If renewed activity taps a volatile-rich rhyolite magma from the current chamber at 5 km depth then eruptive scenarios similar to the 1650 AD sequence could be anticipated. In this case, the east of coast of Santorini and neighboring island could experience tsunamis of similar magnitude to those generated during the last eruption. According to historical accounts, it has been possible to assess that the 1650 AD tsunamis had a maximum intensity of VI degrees in the six grade modified Ambraseys–Sieberg tsunami intensity scale (Ambraseys 1962; Papadopoulos and Chalkis 1984). This is in contrast to more recent studies (Dominey-Howes et al. 2000) which suggest that the magnitude of the 1650 AD tsunami has probably been
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Fig. 13 Map of the Kolumbo area showing the distribution of pyroclastic deposits from the 1650 AD eruption in the Anydhros basin. The thickness of the upper sequence is shown with 30-meter-spaced contours
overestimated since palaeontological and lithological studies along the eastern coast of Thera have shown no bio- or litho-stratigraphic evidence for the deposition of (tsunami) marine-deposited sediments. In light of the recognition of the potential tsunami-generating mechanisms at Kolumbo volcano, a useful next step would be to carry out computer simulations of possible tsunami magnitudes using geologically constrained parameters determined from new knowledge about the scale of the 1650 AD eruption. This modeling could include wave generation from pyroclastic flow discharge into the sea and small-scale caldera collapse to investigate the plausible ranges of coastal impacts in the Santorini area. Such modeling could help the local authorities in their efforts toward hazard evaluation, emergency planning, and disaster management.
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Nat Hazards Acknowledgments This work was supported by the collaborative projects ‘‘Thera Exploration 2006’’ and ‘‘New Frontiers in the Ocean Exploration 2010’’ between the Graduate School of Oceanography at the University of Rhode Island (URI-USA), the Department of Geology & Geoenvironment of University of Athens (NKUA-GREECE), the Hellenic Centre for Marine Research (HCMR-GREECE) and the Institute of Geology and Mineral Exploration (IGME-GREECE). The officers and the crew of the R/V AEGAEO and E/V NAUTILUS are gratefully acknowledged for their important and effective contribution to the field work.
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