Geothermal Research on Santorini

In the first phase of the research, more than 100 temperature measurements and water samplings were made in Thera's springs, wells and bore holes. From the temperature gradient, the thermically anomalous areas were identified. Chemical analysis classified the waters in the alkaline-chloride type, indicating the mixing of 'deep' hot, meteoric and sea water.

The geothermometers of SiO₂, Na-K, Na-K-Ca were used to calculate the probable reservoir temperature of the deep-origin fluids, which seems to be between 140° and 180° C.

Eleven bore holes executed in the first phase of the drilling research at depths up to 200 m allow the drawing of the isotherms at depths from 50 to 250 m and the definition of the most interesting geothermal area. The resulting gradient was also high, reaching 16° C/10 m.

On the basis of the data stated above, the geochemically determined temperatures of 130-160° C should be located at depths between 800 and 1000 m.

In a second phase, three deeper bore holes were drilled in south Thera, to study the geothermal gradient in impermeable formations, the nature of the deep circulated water, and the tectonic structure of the basement.

A series of geo-electric soundings were carried out in the same area and yielded some interesting geothermal and tectonic information.

INTRODUCTION

The Santorini volcano is the main active volcanic centre in recent and historical times of the south Aegean volcanic arc. Volcanic activity is registered at Thera island at least 1.6 Ma BP (Ferrara et al. 1979) and continues up to the present day (last eruption in 1950) with some superficial thermal manifestations (fumaroles and hot springs) in the internal part of the caldera as well as at the external side of the island.

The tectonic-magmatic conditions which predominate in the area of an active volcanic centre like Santorini are characterized by the capacity to create thermal traps, in relation with the permanent important magmatic fluids at relatively shallow depths. These magmatic bodies are capable of creating a thermal anomaly in the surrounding rocks and a regional 'high' heat flow that is also due to the geotectonic position on the volcanic arc.

Also the creation of a series of calderic depressions require shallow magmatic chambers able to create an important thermal anomaly and active hydrothermal systems.

GEOLOGIC OUTLINES

Volcanic activity was developed on a metamorphic basement of the 'Cycladic massif (or Pelagonian zone, after other authors) that outcrops at the south-eastern part of the island of Thera. This basement consists of a phyllitic epimetamorphic series of Mesozoic age (Upper Jurassic-Lower Cretaceous; Tataris 1964). Microcrystalline thick-layered limestones with a total thickness exceeding 600 m overthrusted the phyllites. The age of this formation is Upper Triassic (Papastamatiou 1958), reaching Upper Cretaceous (our observations).

In the early Quaternary the first volcanic activity occurred, depositing pyroclastics and lavas which cover the entire range of the typical calc-alkaline suite.

The oldest volcanic deposits outcrop in the south-west part of Thera, produced by volcanic centres located in the actual southern submarine area. The products of these centres are pyroclastics (tuffs, tuffites, hyaloclastites, scoriae, pumices and ashes), domes, lava flows and pillow lavas of andesitic-dacitic composition. A great part of these series is formed by dacitic submarine tuffs and tuffites with a thickness greater than 200 m. At the upper part of the pyroclastic series, up to 200 m above sea level, there are sedimentary levels with marine fossils which testify to the intense tectonic activity of the area in the last 2 Ma. The entire series is slightly hydrothermally altered - mainly caolinized.

Volcanic activity in Santorini continued with Megalo Vouno, Thera, Mikros Profitis Ilias, Therasia and Skaros volcanic centres alternating effusive, explosive and extrusive activity. The last paroxysmal eruption was the Minoan phreatoplinian explosion, which formed the pumice fall, and flow and surge deposits that cover the entire Santorini area. The post-Minoan volcanic activity was mainly restricted to the area of Palaea and Nea Kameni islands.

TECTONICS

The tectonics of Santorini is very intense and complicated, due also to the various subsequent calderic collapses. This is also proved by the fault system around the metamorphic basement and the dykes, which outcrops in the northern part of Thera island. The presence of an extension, active till now, in the central Aegean, with a NW-SE direction, is confirmed both by the neotectonic data of this region and the study of the source mechanism of the recent earthquakes (McKenzie 1978; Mercier 1979; Jarrige 1978, Mougiaris et al. 1989).

The above data are confirmed by the gravimetric measurements, and also by the development of the post-Minoan volcanic centres in a N 50° - 60° E direction (Fytikas et al. 1990). The direction of the Bouguer gravity anomaly (Fig. 1) shows the same trend (Budetta et al. 1984). The interpretative section based on the gravity data attested to the presence of a Graben with a maximum depth of the basement (about 1500 m) in the central part of the area.

GEOPHYSICAL DATA 

After the preliminary results of the exploration that localized the geothermal anomaly mainly in the southern part of Thera, a geophysical research programme was established, comprising gravimetric, magnetic and geo-electric measurements (Budetta et al. 1984), to achieve a better evaluation of the geothermal possibilities of Santorini and the construction of a relative model.

The results of the geo-electric research are not so satisfactory and do not permit the drawing of a credible geothermal model. This is because of the limited area covered, and also the relatively short development of the AB line. The electric section of BB' (Fig. 2) hows a continuous decrease of the resistance with depth, which is probably due to the presence of saline water in the deep hydrothermally altered formations, in addition to the increase of the geothermal gradient. The correlation of the geo-electric results with the drilling data (drill hole S-2) leads to the conclusion that it is not possible to distinguish, with such geophysics, the basement that is situated at a depth of 450 m. In the interior of the metamorphic basement the value of the resistance decreases continuously and, given the lithologic homogeneity, the decrease is probably due to the general increasing of the temperature with depth (about 13° C/100 m). At site 17 of the cross-section BB', the geo-electric resistance reaches very low values (1.5 Ohm.m at 700 m depth) in the basement. These values are probably due to the circulation of hot and salty fluids.

From the evaluation of the gravimetric measurements and the filtered map it is possible to distinguish a gravimetric 'high' of the basement at the eastern and western parts of the volcanic complex and a structural low in the central area. The same structure is confirmed by the magnetic measurements (Budetta et al. 1984).

In the area of the geothermal anomaly there is an abrupt fall of the gravity values, which is interpreted by the existence of a north-south Graben between Emborio and Faros (southern edge of Thera); the higher temperature values are encountered in the eastern edges of this Graben.

The existence of the above-cited depression is confirmed also by the seismic profile JI (Fig. 3; Heiken and McCoy 1984) as well as by the field data of the Akrotiri area, where a dense framework of faults and fractures with associated hydrothermal alteration of the volcanic formations is present.

TEMPERATURE DISTRIBUTION

For the evaluation of the temperature distribution and the stratigraphic control of the volcanic sequences and the metamorphic formations a series of slim bore holes was carried out on Thera to a depth of 200 m. The first two bore holes up to 120 m deep were carried out in northern Thera (Cape Kolombos area) at a site interesting for its intense faulting. No significant temperature gradient was detected at these depths due to the massive intrusion of sea water into these non-altered pyroclastic formations. A significant temperature gradient, which is controlled by the stratigraphy and lithology of the existing formations, was registered in the southern part of the island, especially in the bore holes S-6, S-10 and S-11 (Fig. 4).

After the 11 slim bore holes, three deeper wells were carried out in the central and southern part of Thera at the eastern flank of the above-cited tectonic Graben:

1. The bore hole S-1 (near Megalo Vouno) perforated: (a) young pyroclastics from the surface to 200 m, consisting of pumice, ash, scoriae etc; (b) metamorphic formation from 210 to 255 m with a contact metamorphism near 255 m; (c) from 255 to 270 m a granitic intrusion was perforated which is responsible for the skarn-type mineralization. The temperature gradient in the S-1 is constantly high, (16° C/100 m, five times the mean terrestrial value). The maximum temperature registered was 64.7° C at a depth of 240 m.
2. The bore hole S-2 (south-south-east of the previous one) perforated: (a) pyroclastics of Thera volcano from 0 to 240 m, consisting of pumice, ash, scoriae etc; (b) Akrotiri series consisting of an upper level of andesitic-dacitic lavas and a lower one of tuffs and tuffites; (c) metamorphic rocks from 455 to 460 m. The temperature gradient here presents sharp changes: there is a first maximum of 39° C at 120 m, then an inverted gradient with a minimum of 34.5° C at 220 m. Entraining into the impermeable Akrotiri tuffs the geothermal gradient turns to normal, with a high mean value of 13° C/100 m. The maximum registered temperature is 52.3° C at 440 m.
3. The bore hole S-3 (near Megalochori) perforates: (a) pyroclastic products (0-145 m); (b) metamorphic formations (145-260 m). The maximum registered temperature was 51.2° C at the bottom (260 m), while the gradient decreases in the calc-schistic members of the series.

The drawn iso-temperature maps at the depths of 50 and 250 m (Fig. 5) show that the temperature distribution pattern is high along the eastern flanks of the Graben as well as inside it.

GEOCHEMICAL RESEARCH

From the chemical analysis of the waters collected on Thera from springs, wells and drill holes, it emerged that the samples having geothermal interest are located mainly in the southern part of the island.

For the estimation of the probably temperatures at depth, there are further indications from the hot waters of the springs in the areas of Thermia and Vlychada. The contribution of the remaining samples, which help us to understand better the mechanism of the circulation of the geothermal fluids, is not ignored.

The geothermal waters of Thera belong to the sodium-chloride type, while the salt content is relatively high (two of them present a higher content than the sea water) reaching the value of TDS = 54.223 meq/1 (Fig. 6). As deduced from the geochemistry of the water samples, there exists a constant relation in the hot springs between superficial and sea water that influences the springs at a different ratio, changing their content into dissolved salts. This is mainly due to the changes of the concentration in Na and Cl ions in these waters (Fig. 7). The Cl/Na ratio in the superficial cold waters is nearly 1, while in the hot water samples it was observed that when the temperature increases, the solubility of the salt also increases and the ratio changes in values > 1 (Fig. 8). From the T/Cl diagram it is clear that a change in the Cl concentration is connected with a corresponding change of the temperature at the surface (Fig. 9).

The hot water ascends to the surface and is mixed with sea and meteoric water. Representative samples of deep water are those from the thermal springs at Thermia (No 1, 2, 3, 4, 103) and Vlychada (No 23, 24, 104). The outlet of the hot water at the surface is obtained either through different fault systems in each area (Thermia - Vlychada) or through the same system but by different conductors. Inside these systems of faults, the water is moving at different velocities; so given the different time of circulation, the higher temperatures and content in salts are registered where important fractures permit a faster outlet (Thermia springs). Lower temperature and salt contents are registered where the water follows longer channels, remaining a longer time below the surface (Vlychada springs).

Applying the geothermometers based on the content of the dissolved SiO₂ and the relation between the Na-K-Ca elements, we calculated the probable temperature of the fluids at depth (Fig. 10). With the application of the Na-K-Ca geothermometer, we deduced as indicative values of the original temperature of the fluids 160°-190° C, and with the SiO₂ geothermometers values of 130° C. The Na-K geothermometer gives almost the same results as those of the SiO₂ geothermometers (140° C).

Taking into account the main geothermal gradient of the area (~16°C/100 m) we can suppose that the most probable temperatures of deep origin (130°-160° C, e.g. the reservoir) are expected at a depth of 800-1,000 m. That is true if the gradient is constant until this depth.

DISCUSSION

An interesting active hydrothermal system exists on Santorini. This is evidenced by the following elements:

1. The existence of shallow magma chambers, taken for granted by the type of the recent volcanism (paroxysmal eruptions in the last 200 ka) (Druitt et al. 1989).
2. The existence of thermal springs and the extended geothermal anomaly in various areas of Thera island, which is confirmed by the high geothermal gradient measured in the bore holes.
3. The numerous hot springs and fumaroles that exist on Palaea and Nea Kameni. The heating of the water and its enrichment in salts is due mainly to the ascending magmatic hot fluids, principally to the gas phase.

The most important hot springs (from the geothermal point of view) appear mainly along faults, fractures and fissures of the non-volcanic basement at the internal part of the caldera as well as at the external side of Thera island. These hot springs are situated at sea level near to the shore line.

The 'geothermal' waters are of deep origin and uprise across faults, more or less isolated from the sea. If this did not happen the waters would be cooled completely and not partially, as is the case.

The geothermal anomaly is located almost inside or near the metamorphic basement. The hydrothermal system is developed in the basement. This formation contains impermeable rocks that partially isolate the hot water from the superficial cold meteoric and sea water.

The scarcity of vigorous thermal manifestations in such an active volcanic island is probably due to the hydrogeological conditions: the superficial very fresh volcanic products are generally permeable and permit in many areas the massive intrusion of sea water which acts as a 'buffer' for the uprising hot fluids. This is proved by the drill holes, the most characteristic of which are those of Cape Kolombos and south of Akrotiri village where no significant thermal gradient was measured.

The area having a particular geothermal interest is located on the west side of the Megalochori-Vlychada line, as is clear from the iso-temperature maps. From the geological and tectonic point of view, the following favourable conditions exist in this area:

     (a) A tectonic depression (Graben) of the oldest formations (basement) in an almost N-S direction, as emerges mainly from geophysical data. In the eastern margin of the 'depression' there is a big step and across it is registered a high geothermal gradient. This means that the marginal faults act as conductors of circulating-uprising fluids. This is confirmed also by the existence of zones of intense hydrothermal alteration in the oldest pyroclastic formations that are located at the edge and principally in the interior of the depression. Such hydrothermally altered formations (tuffs and tuffites of Akrotiri) create a sufficient 'cover' of the deeper geothermal system.

     (b) At the western margins of the 'depression' as well as the central part of it (area of Akrotiri) the pre-volcanic basement is relatively 'shallow' (not exceeding 500 m) and is composed of more or less impermeable formations (phyllites, greenschists, calc-schists etc.). The impermeability of the formation is also deduced from the geothermal gradient curves (temperature increases constantly with depth). This creates favourable conditions for the research of high temperature systems of primary or secondary permeability, below the impermeable formation where intercalations of crystalline limestones and conglomerates are expected.

     (c) In the same area should be found not only superficial thermal manifestations but also deep origin waters with a general geochemical anomaly, and it seems that these are partially mixed with shallow meteoric or sea water. Applying some geothermometers it appears that the most probable temperature of the deep fluids in the area under consideration seems to be of the order of 130°-160° C. If it is true, taking into account the geothermal gradient of the area, those waters should be prevented from a probable 'reservoir' 800-1000 m deep. Of course the existence of a deeper and 'hotter' reservoir is not excluded, but this cannot be sustained from the data at our disposal, neither is it possible to define its real characteristics (depth, temperature, etc.).

In conclusion it may be said that the central-southern part of Thera presents an increased geothermal interest and has a priority for future exploration. In this area there probably exist geothermal fluids at least of medium enthalpy, at economic depths. Of course, the area of the caldera also has an intense hydrothermal system masked more or less by the sea, at least at shallow depths. The area could probably have a geothermal interest at greater depths, but because of the general morphology and other conditions creating economic and technical problems, it is not possible to consider this area as a realistic target.

The results of the eventual complementary exploratory work and their evaluation along with the existing data should give more certainty to the research, with adequate deep drill holes for economically exploitable geothermal fluids. The first deep bore holes should also have an exploratory character, verifying the stratigraphy and the hydrothermal situation.

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