hydrothermal eruptions of nisyros (dodecanese, greece). past events and present hazard

Journa l o[ Volcanology a n d G e o t h e r m a l Research, 56 ( 1993 ) 71-94 71 Elsevier Science Publishers B.V., Amste rdam

Hydrothermal eruptions of Nisyros (Dodecanese, Greece). Past events and present hazard

L. Marini ~, C. Principe b'c, G. Chiodini d, R. Cioni b, M. Fytikas" and G, Marinelli' "Geotermiea l lahana ~rl, k,:mgarno Mediceo 1(~, 56127 t'isa, l ta/v

bls'tilulo di Geocronologia e Geochimica lsolopJca, ( N R . 17a ( 'ardinale Ptetro .lla[]i 3(~. 5(~ 12" l'tsa, ]ta/v CGruppo Nazionale per la I ulcanologia, CNR, Via Nizza 12& 001%' Ron~a, huh"

dDipartimenlo di Scienze della 7i,rra, UpHve~w~la di Perugia. Piaz=a dell'UHiver~it~t, 061 O0 t'eru~Va, h a l l %'chool ~/Gcology, Facuhy o/);cien, 'e, ,.lrisloI/e ('nivep:rily, 540 06 Sa/omka, (;re~'~ e

~Diparltmenlo di &' ienze della 7~,rra. [ :niversith di t'i.~a. I "m Santa Maria 53, 56 l 2 r l'lwl, l:~¢h

( Received January 24, 1992 revised version accepted No~ember 30, 1902 l

~BSTRACT

The detailed analysis of the craters of hydrothermal eruptions and related products present on Nisyros Island demon- strafes the ephemerality of these morphological forms. In other words, the mere recognizable existence of the craters anti associated deposits implies recency of hydrothermal activity. The minimum temperature required to cause the explosive phenomenon and, possibly, the depth of the reservoir (which can be evaluated on the basis of the correlation between the diameter of the crater and the depth of explosion as proposed by Fytikas and Marinelli, 1976 ) arc therefore closely representative of the current hydrothermal circulation.

Both field evidence and historical records indicate that all the deposits of hydrothermal eruption recognized on N isyros Island were emplaced as debris flows. Almost all the ballistic ejecta were entrained in these debris flows and either rede- posited far from their landing sites or involved in later crater collapse and erosion, This emplacing mechanism implies that the original products were characterized by a water content higher than about 5% by weight.

Steam-driven hydrothermal eruptions, one of which took place in 1871, originated deposits ot limited dispersion, as nn sign of these erodible products can be found in the field today.

Surface geology and fluid geochemistry, together with subsurface information (e.g., primar> and hydrolhcrmal lilholo- gies, distribution of temperature with depth, physical-chemical characteristics of deep water-bearing zones ) indicate that two distinct hydrothermal aquifers are present underneath the southeastern part of the caldera floor. Both aquifers were probably involved in the most important historically documented hydrothermal eruptions, which occurred in 1873.

At that time, violent earthquakes fractured the brittle aquiclude separating the two aquifers and caused a sudden transf~:r ol" fluids from the deep to the shallow aquifer, thus triggering the hydrothermal eruptions.

Hydrothermal eruptions will probably occur m future, and this hazard must be laken into serious consideration. The southern half of Lakki plain, where all past eruptions took place and active fumaroles ark concentrated is the zone ~t highest risk.

At present, gas geochemistry represents an effective tool to detect changes in the 1',7 conditions of the shallow aquifcT. and particularly the phenomena of pressure build-up that may lead to a hydrothermal eruplion.

Introduction

The term "hydrothermal" was introduced by Lloyd ( 1959 ) to denote eruptions driven by the expansion of hydrothermal fluids against atmospheric pressure, without the involvement

( ( ) r t e spondence lo. R, Cioni.

of fresh magma. Related deposits are made up of fragments of pre-existing rocks, mostly affected by hydrothermal alteration prior to eruption. Long before, these eruptions were called "phreatic" by Suess ( 1885 ), to convey the crucial role of groundwaters in this phenomenon, and many authors (e.g., Marinelli, 1969; Fytikas and Marinelli, 1976; Goguel,

0377-0273 /93 /$06 .00 © 1993 Elsevier Science Publishers B.V. All rights reserved.

https://www.researchgate.net/publication/233227283_The_hot_springs_and_hydrothermal_eruptions_of_Waiotapu?el=1_x_8&enrichId=rgreq-0cc06410-7095-4c28-9300-83f8b85c14a5&enrichSource=Y292ZXJQYWdlOzIyMzgzODM0NztBUzoxNzg5NDgwMTMwNDM3MTJAMTQxOTY3NTkzOTE1Nw==

72 L. MARINI ETAL.

1977; Le Guern et al., 1982) have also used this term. Sometimes the term "phreatic" has been used incorrectly by other authors (e.g., Fairbridge, 1969; Hedenquist and Henley, 1985) as a synonym of "phreatomagmatic", whereas the latter denotes eruptions in which "the material ejected is partly or wholly mag- matic" (Macdonald, 1972, p.23). In order to avoid ambiguity the term "hydrothermal" instead of "phreatic" will be used in this work.

Nisyros Island has been the site of several hydrothermal eruptions in recent times, as clearly testified by the presence of craters and deposits, first recognized as such by Fytikas and Marinelli (1976), and erroneously as- cribed to phreatomagmatic activity by St. Sey- mour and Vlassopoulos ( 1989 ).

The available information on the island includes: ( 1 ) historical descriptions of the last events (Gorceix, 1873a-e, 1874; Martelli, 1917 ), summarized in Appendix 1; (2) the re- constrnction of the geological, volcanological and structural frameworks (Di Paola, 1974; Vougioukalakis, 1984); and (3) a considera- ble amount of data (such as subsurface geology, distribution of temperature with depth, physical-chemical characteristics of deep water-bearing zones) provided by deep geothermal drilling, carried out by PPC, the Greek Energy Authority, in 1982-1983 (Marinelli et al., 1983; Koutroupis, 1983; Vrouzi, 1985; Barberi et al., 1988 ). Nisyros Island has therefore been chosen as the test site for a geological-geochemical study aimed at reconstructing past hydrothermal eruptions and evaluating the hazard of such events re-occurring. The available information has been integrated by a detailed geological survey of the area occupied by hydrothermal craters and products (Principe, 1989) and a geochemical survey of the fumarolic manifestations (see companion paper, Chiodini et al., 1993 ).

Geological framework

Nisyros lies at the eastern end of the Aegean active volcanic island arc, which extends across

the Aegean Sea and includes Crommyonia, Methana, Poros, Aegina, Milos, Santorini, Kos and numerous minor eruptive centres, four of which are close to Nisyros (Fytikas et al, 1975: Keller, 1982 ).

As opposed to other major islands of the Ae- gean arc, the pre-volcanic basement is not exposed on Nisyros. Early submarine volcanics, which have been extensively eroded, form the base of the exposed stratigraphic sequence. The island (Fig. 1 ) represents the emerged portion of an andesitic composite volcano (Di Paola, 1974), that wasbuilt up during the last 100,000 years (Keller et al., 1990). This is truncated by a 3.8-km diameter summit caldera that formed as a consequence of intense explosive activi- ties, less than 24,000 y.B.P. (Keller et al., 1990). Most of the caldera depression was later filled by a series of dacitic-rhyodacitic domes, the highest of which, Profitis Ilias, rises about 600 m above the caldera floor, whose altitude is 110-120 m asl. The southern half of the caldera depression not occupied by the domes, known as Lakki plain, and the southeastern- most small dome, Lofos, have been devastated by a series of very recent hydrothermal eruptions.

The Lofos dome and the opposite side of Lakki plain appear strongly affected by fracturing along the NE-trending tectonic system, which has been active until recent times. Frac- tures and fissures of Lofos dome are commonly filled with quartz. The presence of ep- ithermal gold in this small dome (82 ppb), as well as in the adjacent Profitis Ilias (2,500 ppb), indicates the occurrence of important hydrothermal circulation phenomena. Later argillic alteration, caused by reaction with fumarolic fluids, has further altered the lavas of Lofos dome.

Subsurface information

Two deep geothermal wells have been drilled in Lakki plain (Fig. 1 ).

Well Nisyros-1 crossed the carbonate base-

HYDROTHERMAL ERUPTIONS OF NISYROS (DODE( "ANF SE, GREECE ) 73

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Fig. l. Simplified geological map of Nisyros Island (modified from Di Paola, 1974 ). Sampled springs and shallow bore- holes arc indicated by filled circles and squares, respectively (their physico-chemical features arc given b\ ('hiodini et al.. 1992 ). H'I and H'2 are the deep geothermal wells Nisyros-1 and Nisyros-2, respectively,

ment underlying the volcanic sequence from a depth of 691 m down to 1350 m, whereas well Nisyros-2 did not encounter this carbonate sequence, indicating that the central parts of the caldera underwent a larger collapse than the peripheral parts (Fig. 2). Both wells ended within a complex made up of an alternance of quartz-dioritic subintrusive rocks and thermometamorphic rocks. The original lithology of the thermometamorphic rocks has been completely obliterated by thermometamorphic phenomena, probably related to the composi te volcano magma chamber, and by hydrothermal activity. The subintrusive rocks

have been interpreted, given their petro- graphic affinity, as either the feeding conduits of the recent dacitic-rhyodacitic domes or batches of magma that did not succeed in reaching the surface.

The distribution of permeability is similar in both wells, which crossed a shallow permeable sequence and a deeper permeable section. Both measured temperatures and the hydrothermal parageneses of the permeable zones (Marinelli et al., 1983; Geotermica Italiana, 1983, 1984) provide further details on the deep fluid circulation.

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HYDROTHERMAL ERUPTIONS OF NISYROS (DODECANESE, GREECE) 75

in well Nisyros-1 (from 400 to 700 m depth ) than in well Nisyros-2 (from 250 to 350 m depth) . Chlorite, in well Nisyros-1, and sericite, in well Nisyros-2, are the most abundant minerals of this zone. They are associated with quartz, zeolites, pyrite, anhydrite and carbon- ates. This phyllitic-zeolitic hydrothermal par- agenesis indicates formation temperatures of 120-180 ° C. The abundance of chlorite in well Nisyros-1, probably caused by rock-sea water interaction, suggests that this zone of the well has been affected by sea water inflow.

The deep permeable zone occurs from 1400 m to well-bottom ( 1816 m) in well Nisyros- 1 and from 1000 to 1350 m in well Nisyros-2, which reached a total depth of 1547 m. Tem- perature values higher than 290°C have been measured in the deep permeable zone, characterized by propylitic hydrothermal minerals. Abundant subeuhedral epidote, adularia and albite have been found in this zone of both wells, associated with quartz, pyrite, anhydrite, stilpnomelane, wairakite, garnet, tremo- lite and pyroxene. Scapolite, brucite, pyrrho- rite, and arsenopyrite occur in well Nisyros.-1 only, while chlorite and sericite-muscovite are present in well Nisyros-2 only. Near the bottom of well Nisyros-1 the appearance of bio- tite, typical of potassic-propylitic assemblages, and the disappearance of epidote and tremo- lite indicate formation temperatures above 350°C.

ment of the craters and distribution of both active and extinct fumarolic vents.

The relative chronology of the hydrothermal eruptions has been determined from the stratigraphic relationships between the different products and the degree of conservation of lhe morphological forms (Principe, 1989).

The oldest hydrothermal craters

The oldest hydrothermal craters are located at Kaminakia and north of Lofos dome.

The two craters of Kaminakia, linked to a NE-trending fault, lean against the caldera wall. Their position has favoured the erosion of morphological forms and products. They are also partly filled by the talus of the caldera wall and by the deposits of Stephanos. The average diameter of both craters is about 150 m.

Three old craters are located north of Lofos dome and east of Profiti Ilias dome. The largest is sited along a major NE-trending fault. Al- though these craters have been partly filled by the talus of the domes, the distribution of hydrothermal products, consisting of blocks of lava dome, is still recognizable. Anhydrite veins are observed in the largest blocks, which can reach 4 m in diameter. Blocks are affected by exfoliation, which indicates the relatively old age of these deposits.

Stephanos area

Field evidence: hydrothermal craters and deposits

Only the hydrothermal craters that are clearly recognizable in the field, based on their morphological forms and products, are considered here. They are concentrated in two distinct areas of Lakki plain (Figs. 3, 4 and 5 ): at the foot of the southeastern caldera escarpment and near Lofos dome. Both areas are dis- sected by active NE-trending faults, which are marked by fault scarps, displacement of the corresponding layers, lengthening and align-

The Stephanos crater (Fig. 6) is almost circular in shape. Its floor measures 240 m along the major NE-trending axis and 180 m along the minor NW-trending axis and reaches a depth of 27 m.

The old talus of the caldera escarpment, mainly made up of fragments of andesitic lavas, crops out at the bottom of the stratigraphic sequence exposed along the steep inner crater walls. The original upper surface of the old talus gently slopes towards the north- west, that is, towards the center of the caldera. Moving in this direction, the size of the lava

76 L, MARINI El" AL.

fragments decreases, their roundness in- creases, and they appear to be organized in alluvial beds. The talus material is completely grey in color due to argillic alteration that preceded the hydrothermal eruptions.

This deposit is covered by a red fine-grained layer of variable thickness, up to a maximum of 30 cm.

At the top of this red layer there is a deposit, up to 5-6 m thick, that crops out in the southeastern wall of the pit only. It is made up of lava blocks, some of which are affected by argillic alteration, supported by a pale yellow matrix rich in small gypsum crystals. This deposit is probably related to the Kaminakia craters.

The deposit linked to the Stephanos crater occurs at the top of the sequence. Of 3-4 m in thickness, it is made up of a clayey to sandy matrix, pale yellow in color, containing up to 20-30% of lava blocks. The matrix is rich in small gypsum crystals, while anhydrite veins are frequently observed in the largest lava blocks.

The small Andreas crater cuts the Stephanos crater rim and its products unconformably overlie the Stephanos deposits. Abundant re- worked fragments of Stephanos products are found in the small deposit produced by the eruption of Andreas.

Lofos area

The complexity of this area (Figs. 3 and 7a) stems from the concentration of several hydro-

thermal craters and related products within a morphologically uneven zone.

The oldest crater of this area is Logothetis, which is sited on the southwestern flank of Lo- fos dome and is demolished towards the south. As it is filled in part by the products of Poly- bores Megalos eruption, Logothetis can be assumed to precede the latter event.

The Polybotes Megalos hydrothermal eruption was the most important event in this sector of Lakki plain. It left a large elliptical crater that has been the site of subsequent events. The northeastward lengthening of Polybotes Me- galos crater reflects the influence of the NE- trending tectonic system. At present this crater occupies a surface area of about 180 X 350 m, but collapses and erosive processes have probably enlarged the original pit.

The deposit of Polybotes Megalos hydrothermal eruption consists of altered lava fragments and blocks of the dacitic-rhyodacitic domes, supported by a clayey to sandy matrix. The lava blocks are characterized by brown- reddish oxide coatings and anhydrite veins.

Polybotes Megalos deposit is covered in part by Phlegethon products and seems to overlie Stephanos deposit. In the northwestern sector, Polybotes Megalos deposit occurs on the morphological steps cut by the NE-trending faults in the slope of the dacitic-rhyodacitic domes and in their old talus, affected by argillic alteration. Here Polybotes Megalos deposit ex- hibits a thickness of about 3 m, whereas about

Fig. 3. Geological map of Lakki plain, with location of the deep geothermal wells Nisyros-1 ( WI ) and Nisyros-2 (W2). Legend: 14 = recent alluvial deposits; 13 = recent talus; 12 = recent (post- 1873 ) lacustrine sediments deposited inside Po- lybotes Megalos depression and at the southern end of Lakki plain; / /=debr i s flow deposit emitted from Po/ybotes Micros crater in 1887; 10= debris flow deposits emitted from Phlegethon and Polybotes craters in 1873; 9 = old lacustrine deposits cropping out inside Polybotes Megalos depression; 8=debris flow deposits emitted from Polybotes Megalos crater; 7=debris flow deposit emitted from Andreas crater; 6=debris flow deposits emitted from Stephanos crater: 5 = debris flow deposits emitted from Kaminakia craters; 4 = deposits of hydrothermal eruption linked to the old craters located north of Lofos dome; 3a = old talus of the post-caldera dacitic-rhyodacitic domes, affected by argillic alteration; 3b = old talus of the caldera escarpment, affected by argillic alteration; 2 = post-caldera dacitic-rhyodacitic domes; 2a = zones of Lofos dome affected by fracturing and argillic alteration; 2b=zones of Lofos dome affected by anthropogenic activity. mainly sulphur quarrying; 1 = lava flows of the composite volcano exposed along the caldera walls.

H Y D R ( ) T H E R M A L E R r J P F I O N S O F N [ S Y R O S ( D O I ) E ( " A N F SE, ( I I~ .EE( 'E ) "7 7

5 m of this deposit can be observed along the northeastern crater rim.

A fine-grained red layer crops out near the Polybotes Megalos crater rim. This layer, similar to that exposed in the Stephanos crater

walls, underlies the Polybotes Megalos products and covers the old topographical surface of Lofos dome.

A 15-20-m-thick sequence of lacustrine sediments, pale yellow in color, partly fills Poly-

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Fig. 4. Hydrothermal explosion craters recognized on Nisyros Island. Also shown are craters of practically no morphological prominence and no identifiable products. Hydrothermal craters have the following names (see Appendix 1): a = Polybotes Micros; b= Phlegethon; c= Polybotes i d= Achelous; e= Polybotes Megalos; f = Logothetis; g= Andreas: h = Stephanos; i j= Kaminakia. Other craters are unnamed.

HYDROTHERMAL ERUPTIONS OF NISYROS ( DODE( 'ANESE. GREECE ) 79

Fig. 5. The hydro thermal erupt ion craters viewed from the southeast.

Fig. 6. Nor thern inner wall of Stephanos crater. N u m b e r as in Fig. 3. A thin red level separates lhc talus of the caldcra wall (3t~), strongly affected by argillic alteration, f lom the overlying debris flow deposits of Kaminakia 15 I and Stcphanos (~) craters.

80 L. MARIN1ET A,L,

J

Fig. 7. (a) General view of Polybotes Megalos depression. Numbers as in Fig. 3. In the foreground, the diatomaceous clays (12) deposited in the small lake formed after the 1873 eruptions. Polybotes crater and deposit (I0), related to such eruptions, appear strongly affected by erosive processes. Further away are the small Polybotes Micros crater and deposit ( 11 ) and the summit of Lofos dome. In the background the caldera wall.(b) A view of the same area from the summit of Lofos dome. This picture (reproduced from Martelli, 1917 ) was probably taken in 1912. Polybotes crater still appears as an almost intact hole.

botes Megalos crater and forms the base of the stratigraphic sequence recognized inside it (Figs. 7a and 8 ). Some darker levels, probably rich in organic substances, are observed in the upper part of this lacustrine sequence.

This is overlapped by a vanilla coloured deposit, 8-9 m thick, made up of altered lava fragments and blocks of the dacitic-rhyodacitic domes, supported by a clayey to sandy matrix enriched in sulphur and small gypsum

HYDROTHERMAL ERUPTIONS OF NISYROS ( DOI)E('ANESE, GREECE ) ~ I

Fig. 8. Southwestern part of Polybotes Megalos depression. Numbers as in Fig. 3. The deposits of the 1873 erupt ions (10) overlie an older lacustrine sequence (9) and are covered by the dia tomaceous clays ( 12 ) deposited in the small lake lhat formed after these eruptions.

crystals. These products were emitted from Polybotes crater during the 1873 eruptions (Gorceix, 1873a-e, 1874; see Appendix 1 ).

At the end of the 1871-1873 eruptions the western sector of the Polybotes Megalos depression, constituting a relative morphological low, was occupied by a small lake, which was sampled and analyzed by Gorceix. Today, a 1.5-m-thick sequence of varved-like diatomaceous clays, alternatively yellow and violet in color, testifies to this lacustrine episode. The thickness of each bed varies from some cm up to 10 cm. Laminae rich in heather leaves (probably Erica carnea) can be observed. Small gypsum crystals, whose major axis is ori- ented vertically, can be recognized at the top of the violet beds. These features suggest that the lake water cyclically experienced dilution and concentration (up to gypsum saturation ) episodes, whose main controlling factors were meteoric recharge and inflow of fumarolic fluids.

The lacustrine sequence is covered by a 31)- cm-thick alluvial conglomerate, capped by a :5- cm-thick white laminated diatomaceous level

containing minor barite. This is overlain by very recent talus produced by rock-fall from the steep walls of Polybotes Megalos crater.

Achelous crater, located north of Polybotes Megaios crater rim, has a very elliptical shape and a relatively small size. In fact its major axis and its depth measure only a few tens of meters. The crater is carved into the old altered talus of the dacitic-rhyodacitic domes and into the lava of Lofos dome. At least 3 m of Poly- botes Megalos deposit crop out on the slopes of the domes adjacent to Achelous crater, but no deposit ofhydrothermal eruption is present inside Achelous depression. Achelous eruption, for which no historical records are available, is therefore younger than Polybotes Me- galos eruption but probably older than the Polybotes-Phlegethon event.

The large elliptical Phlegethon crater is located on the southern flank of Lofos dome. It is separated from the morphological rim of Po- lybotes Megalos by an exiguous wall made up of altered lavas of Lofos dome. Phlegethon crater is demolished towards the south and partly filled with its deposits. The extension

82 L. MAR1NI ET AL.

Fig. 9. View of Phlegethon crater from the southwest. The crater is carved into the southern flank of Lolbs dome. An exiguous crest separates it from the Polybotes Megalos depression behind it.

Fig. 10. View of Phlegethon crater from the south. Person (arrow) for scale. The crater is very elliptical in shape and its major axis follows the NE-trending tectonic system.

towards the SW of its northern rim corre- sponds to a fracture belonging to the main active tectonic system. Phlegethon crater resem- bles a large tub (Figs. 9 and 10) from which the debris flows of 1873 poured outwards into the lowest part of Lakki plain. The deposit of

this eruption is very similar to the coeval products emitted from Polybotes crater, It is made up of fragments of the lava domes supported by fine-grained materials, rich in small gypsum crystals. It directly covers the products of Stephanos and Polybotes Megalos. The distal

HYI)R()THERMAL ERUPTIONS OF NISYROS (DODECANESE, GREECE) 83

portion of the deposit is covered by a more than 2 m thick sequence of varved-like lacustrine clays, alternatively yellow and violet in colour, with laminae rich in heather leaves.

Polybotes Micros has a diameter of only 30 m and is the youngest hydrothermal crater on Nisyros. This crater is carved into the eastern morphological rim of Polybotes Megalos and is surrounded by a small ring made up of products (mud and altered rocks, according to Martelli, 1917; see Appendix 1 ) emitted during the 1887 eruption.

Fig. 11. Debris flow deposits emitted in 1873 from Phle- gethon crater. Pre-existing alluvia are present below. The two debris flow units of June and September are separated by a decimetric level (indicated by a), which is probably representative of the mudflows emitted at the end of the June eruption.

Fig. 12. Lava block of the Polybotes Megalos deposit showing a fracture that formed and was filled with anhydrite prior to the hydrothermal eruption. ~nhydri te was later gypsified upon exposure to atmospheric air.

Characteristics of the deposits of hydrotherrnal eruptions

All the deposits of hydrothermai eruptions recognized in the southern half of Lakki plain exhibit similar characteristics. It is therefore likely that they were originated through the same mechanisms of emplacement.

These deposits contain poorly sorted matrix-supported fragments of pre-existing rocks (Fig. 1 1 ). The fragments vary in shape from angular to subrounded and exhibit their maximum size, up to a few meters, close to the crater rims. They are generally affected by fumarolic alteration processes preceding ~he

84 L. MARINI ET AL,

Fig. 13. Detail of the matrix of the debris flow deposits emitted in 1873 from Phlegethon crater.

eruption. Some lava blocks have also been fractured and recemented prior to eruption. The fractures are commonly up to a few centi- meters in width and completely filled with anhydrite (Fig. 12 ).

After the emplacement of the deposits of hydrothermal eruption, brown-reddish coatings of hydrous iron oxides formed on the surface of most of the lava blocks (Fig. 12). These coatings and the yellow color of the matrix (at least where sulphur is absent) are produced by the leaching of iron from the rocks by sul- phuric acid (through oxidation of hydrogen sulfide on reaction with atmospheric air). After oxidation to iron-Ill, aqueous iron is precipi- tated by the neutralization and evaporation of the aqueous solution.

The blocks typically make up less than 20- 30% of the deposits linked to the most recent eruptions, such as Phlegethon-Polybotes and Stephanos. Higher blocks/matrix ratios are observed in the oldest deposits, as fine materials were removed by the action of atmospheric agents. The blocks within the oldest deposits are also characterized by exfoliation, a process that presumably begins after the de-

posit has been substantially depleted of its matrix.

The matrix is clayey to sandy in granulome- try and yellow in colour. It is made up of fragments of comminuted pre-existing rocks and contains variable amounts of secondary sulphur and euhedral gypsum crystals (Fig. 13 ). These crystals originated through evaporation of the aqueous fraction after the emplacement of the deposit and are more abundant in recent than in old deposits.

The characteristics of these deposits of hydrothermal eruption (i.e. presence of gypsum crystals, low blocks/matrix ratio, poor sorting, absence of impact sags and lack of any matrix- poor basal breccia made up of ballistic ejecta) suggest that they were all emplaced as laterally transported debris flows characterized by an interstitial mud-water fluid content higher than 5% by weight. This is the minimum amount required to lubricate the movement of the granular component, thus producing a debris flow (Cas and Wright, 1987, p. 325). Ballistic ejecta (described by historical chronicles) were either entrained in these debris flows and re- deposited far from their original landing sites

HYDROTHERMAL ERUPTIONS OFNISYROS (DODECANISE, (3REECE) g5

or involved in the subsequent enlargement of the crater by collapse and erosion. Conse- quently the observed distances of the blocks from the vent are not the ballistic distances and the diameter/dis tance ratio of the blocks inside all these deposits cannot be used to evaluate the ballistic velocity of the ejecta.

Consistently with the description of the 1873 eruptions from Phlegethon and Polybotes craters (Gorceix, 1873a-e, 1874), the characteristics of these deposits indicate that they were emplaced during the open-conduit stage of the hydrothermal eruption, after the initial explosive event, which is largely responsible for both cratering and the opening of the conduit.

Age of Nisyros hydrothermal eruptions and relevance of hydrothermal craters in geothermal exploration

Numerous papers on the relationships between hydrothermal eruptions and geothermal phenomena have been published during the last 10-15 years. Fytikas and Marinelli (1976) proposed a classification of hydrothermal eruptions based on their genetic mechanisms and an empirical method to establish the minimum temperature required to cause the explosive phenomenon and, possibly, the depth of the reservoir. This method is based on the correlation between the diameter of the crater and the depth of explosion. The latter corre- sponds with the depth of the reservoir, unless the eruption is triggered by an uprise of hydrothermal fluids along fractures opened by a tectonic event. In this case the depth of the reservoir cannot be evaluated, but the min imum temperature required to cause the explosion can be estimated anyhow.

The application of this relatively simple and low-cost method could give conclusive indica- tions on the existence of an active high-temperature fluid circulation, that is, of an eco- nomically exploitable geothermal field. The main problem is to determine the age of the craters caused by the initial explosion. If the

morphology of these craters were persistent for a long time (e.g., one or more m.y. ), a fossile anomaly could be erroneously considered an active one. In the opinion of the authors, this problem can often be solved with a satisfactory approximation.

Details on the mechanisms of hydrothermal eruptions are provided both by field evidence and historical descriptions of events, particularly for recent eruptions such as those of Ni- syros (Gorceix, 1873a-e, 1874: Martelli, 1917), La Soufribre, Guadeloupe (Tazieff, 1978), Dieng Plateau (Stehn, 1939; Le Guern et al., 1982) and the Afar triangle (Marinelli, 1971 ). These eruptions took place in areas of active volcanism. Recent hydrothermal eruptions, however, have also been known to occur in zones where only sedimentary formations crop out, such as at Larderello, Tuscany (Mar- inelli, 1969). Analysis of historical descriptions and field investigations are important as they give information on the type of materials emitted, which is a basic characteristic controlling changes in crater morphology with time. Knowledge of emitted materials allows one to judge, at least approximately, the dura- tion of the morphology of the craters of hydrothermal explosion. A study of this type was re- cently carried out for the Larderello area, in which about fifty craters of hydrothermal explosion were identified ( Mignani, 1989 ). Sim- ilarly, the characteristics of the twenty craters recognized in lhe flat part of Nisyros caldera have been analyzed in order to assess the ephemerality of these morphological forms.

It is known that a hydrothermal explosion can take place only where an impervious cover prevents, or at least drastically reduces, the upflow of hot fluids towards the surface. This cover can be made up of either rocks characterized by p r ima~ impermeability, owing to an abundance of clay minerals, or rocks that have acquired secondary impermeability through shallow low-temperature t 100-150 ~ C or so ) self-sealing processes (under these conditions

8 6 L. MARINI E T A L

the cover material will mainly be made up of clay minerals also in the second case). The explosive phenomenon will, therefore, eject these comminuted impermeable materials, together with a mixture of vapor and hot water. The products of this process are, as a result, largely made up of an extremely fluid and mobile mud, provided that a sufficient amount of liquid water is available. Part of this mud will form one or more debris flows, which will move away from the crater rim; another part will obviously fall back inside the crater immediately after the end of the explosive phenomenon. Phlegethon crater on Nisyros is a perfect example of this eruptive mechanism, as testified both by historical chronicles and field evidence. Because of the immediate removal of mud, the emitted materials accumulate around the crater in smaller quantities than in phreatomagmatic eruptions, whose craters are, however, morphologically similar to the craters of hydrothermal eruptions (Lorenz et al., 1970; Lorenz, 1973, 1986).

In addition to immediate removal of mud during the eruptive phenomenon, progressive erosion will slowly take fine-grained materials away from the prominent ring of deposits dis- tributed around the crater. If the products of the explosion are wholly made up of fine- grained materials, the prominent ring of deposits will attenuate and disappear in a relatively short time, perhaps less than a thousand years; meanwhile, the crateric depression will be progressively filled by part of the materials washed away from the ring of deposits. If the products of the explosion are partly made up of coarse materials, these will be more persistent than fine particles. Nevertheless, the coarse component will also be gradually involved in the common erosive phenomena produced by atmospheric agents.

The Larderello area is a typical example of this phenomenon (Mignani, 1989). The impermeable cover of the geothermal field is mostly made up of clay-rich flyschoid formations, while the underlying Mesozoic lime-

stones represent the first geothermal reservoir. It is not surprising, therefore, that remnants of crater rims and relatively large areas nearby are often occupied by scarce angular fragments of limestones and sandstones of the flyschoid units, as well as by silicified breccias of the Me- sozoic limestones. However, these materials have not accumulated to form a prominent ring around the crater, indicating that these fragments were either comparatively rare in the erupted products or, as is more likely, were laterally transported away from the crater as debris flows.

Let us now consider the morphologic persistence of the twenty or so craters of hydrothermal explosion recognized on Nisyros Is- land (Fig. 4). Craters a, b and c, little more than a century in age as documented by historical sources, and craters d, e, f, g, h, i and j are all well preserved. Yet these young craters also exhibit a progressive filling of the depressions and a certain attenuation of the rings of emitted products.

The morphology of Phlegethon (b in Fig. 4 ) is characteristic in this respect. The northeastern part of this elliptical crater is carved into a portion of the small Lofos dome, which has been compacted by silicification. Not surpris- ingly, this part of the crater is still well preserved, although the underlying depression is partly filled with collapsed materials. The southwestern part of the crater, which is cut into the talus of the large post-caldera domes, is considerably attenuated, almost obliterated. Although the emitted materials contain numerous large fragments, the outer slope of the ring shows evident shoestring rills, indicating that this crater too will hardly be recognizable in a relatively short time.

Further evidence of the ephemerality of these craters is provided by Polybotes (c in Fig. 4). It was reported as a 40 m in diameter circular depression in the 1884 map at scale 1:5,000 (Martelli, 1917 ) and an almost intact circular hole can be seen in a photograph (Fig. 7b) probably taken in 1912 (Martelli, 1917 ). To-

HYDROTHERMAL ERUPTIONS OF NISYROS (DODECANESE. GREECE) b{7

day Polybotes looks entirely different, as its original morphology has been greatly modified by the collapse and erosion of the walls (Fig. 7a), carved within unconsolidated lacustrine deposits.

Craters i and j (Fig. 4) now show a barely prominent ring, although the crateric depressions are still clearly recognizable.

The other eleven craters are unnamed, which is not surprising considering that their morphological prominence is almost nil. Related products are not identifiable. These craters can only be recognized on close examination of aerial photos, on the basis of subtle differences in ground colour, vegetation and other details that mark their circular shape.

It is difficult to determine how long it takes for the morphology of these hydrothermal explosion craters to attenuate and disappear. The very young age of Nisyros caldera (probably less than 24,000 y.B.P., according to Keller et al., 1990, despite some inconsistencies with other age determinations, see discussion in Limburg and Varekamp, 1991 ), indicates that explosion craters generated by hydrothermal eruptions have a very short morphological life. If it takes only a few thousand years to obliter- ate them, it is clear that their presence is a val- uable indicator of the existence of high-temperature fluid circulation, as the cooling time of a high-enthalpy geothermal system, though difficult to evaluate, is probably in the order of 0.1 m.y.

Possible exceptions, such as craters carved into very resistant rocks, which may have a comparatively long persistence, are easily discernible.

It can therefore be concluded that the study of craters produced by hydrothermal eruptions is an important tool in geothermal exploration. For instance, in Milos Island, the method by Fytikas and Marinelli (1976) provided an estimated temperature of 310°C, which was later confirmed by physical measurements carried out in production wells Zephyria 1,309 ° C, and Adamas 1,308 ° C.

Conceptual geochemical model

As discussed in the companion paper de- voted to fluid geochemistry (Chiodini et al., 1993 ), two distinct hydrothermal aquifers are present underneath Lakki plain:

( 1 ) A deep aquifer located at more than 900 m below sea level. It is characterized by temperatures higher than 290 °C and chloride-rich fluids (C1 content over 50 g/ l) .

(2) A shallow aquifer sited between sea level and 500 m below. It is characterized by a chloride content probably close to 21 g/1 and temperature values, under Lofos dome and Ste- phanos crater, in the 225-255 ° C range. Boiling of this aquifer produces a vapor phase discharged at fumarolic vents.

The subintrusive apophyses of a still-hot in- trusive body have been reached by deep geothermal drillings. This body supplies heat to the overlying hydrothermal system and, through thermometamorphic reactions, is responsible for the formation and flow of acid gases (CO> H2S) towards the surface.

Mechanism of hydrothermal eruptions

Pre-eruption and eruptive trig,~,er

Pressure distribution in boiling hydrothermal aquifers brings them close to becoming mechanically unstable with respect to their confining burden (White, 1955). A decrease of external pressure or an increase of internal (fluid) pressure can then trigger a hydrothermal eruption. For instance, the sudden decrease of external pressure consequent to draining an ice-dammed lake overlying the system has been proposed as the triggering mechanism for the hydrothermal craters in Yellowstone (Muffler et al., 1971 ). An increase of fluid pressure above the lithostatic value leading to hydrothermal eruption may be caused by one of the following mechanisms:

( 1 ) impermeabilization of the cover due to self-sealing (Fytikas and Marinelli, 1976): the

8 8 L. MARINI ET AL

local sealing mechanism proposed by Heden- quist and Henley ( 1985 ) is an example of this phenomenon;

(2) a sudden uprise of deep hot fluids along a fracture system opened by tectonic movements (Lloyd, 1959; Cross, 1963; Fytikas and Marinelli, 1976 );

(3) a gradual increase in the upward flow of deep hot fluids into the hydrotherrnal aquifer (Barberi et al., 1991 ).

In the case of Nisyros Island, Gorceix's letters relative to the 1871-1873 hydrothermal eruptions document that each eruptive episode was accompanied, and probably preceded, by important seismic shocks. These shocks were felt by local people but not by the inhabitants of nearby islands, at least as re- gards the 1873 events. It is obviously impossi- ble to establish with any certainty the nature of these earthquakes, whether fault-controlled, fluid-driven or of other type, because of the purely descriptive nature of Gorceixs letters. However, the heavy damages inflicted on buildings and the opening of fractures in Lofos area and elsewhere on the island as well as out- side it, up to a distance of about 5 km, suggest that at least the 1873 seismic events were of tectonic nature. It can be concluded therefore that seismic shocks played a fundamental role in triggering the 1873 hydrothermal eruptions.

In the framework of the conceptual geochemical model described above, a sudden transfer of deep, hot water from the deep aquifer to the shallow one could occur along the fractures opened by the seismic shocks in the brittle aquiclude between the two aquifers, thus triggering the hydrothermal eruption.

The cause-effect relationship between the earthquake and the 1871 hydrothermal eruption is not well documented, because Gorceix was not on Nisyros at the time. Although an earthquake could also have triggered the 1871 hydrothermal eruption, it is alternatively possible that the aquifer reached a pressure value sufficient to trigger the explosive event, thus causing the seismic shock. The increase in fu-

marolic activity recorded in Lofos area during the months preceding the eruption could re- flect a gradual pressure build-up in the shallow aquifer.

Eruption

Hydrothermal eruptions develop in two stages, each with a very distinct transfer of energy: (1) an initial explosive event, which is largely responsible for both cratering and opening of the conduit; and (2) an open-conduit eruptive stage afterwards.

The energy relationship proposed by Muf- fler et al. ( 1971 ) considers that water partly flashes to steam owing to the sudden depressurization of the system and that, meanwhile, further heat is transferred from the comminuted rock to the water, causing additional flashing to steam. This process is described by the following equation:

mwcw( Tw- T) + mrcr( Tw- T)= XmwL ( l )

where mw and mr are the masses of water and rock, Cw and Cr are their specific heats, Tw is the aquifer temperature, T is the boiling temperature of water at atmospheric pressure, L is the latent heat of evaporation of water at T, and X the fraction of steam produced. The following hypotheses are considered in this model:

(a) a single liquid phase is considered to be present in the aquifer;

(b) water and rocks are assumed in thermal equilibrium under initial conditions;

(c) only aquifer rocks are considered to take part in the process;

(d) total rock/water heat transfer is hypothesized;

(e) the role of carbon dioxide is negligible (which is not always true, as pointed out by Nelson and Giles, 1985 ). Although hypothesis (c) is obviously never fulfilled in natural systems, it is probably a condition that is approached only in the initial explosive stage. On the contrary, during the subsequent open-conduit eruption, there is

H Y D R O T H E R M A L ERI JPTIONS OF NISYROS ( D(-)DECANtiSE. GREECE ) $9

only minor involvement of rock fragments in the system, particularly those coming from the aquifer zone. In other words, during this second eruptive stage the system is mainly made up of water that expands close to adiabatically, a theoretical limiting condition described by the following equation:

m,, Cw ( T,,. - T) = X m w L (2 )

If hypothesis (a) is correct and there is no physical flow restriction (e.g., poor aquifer permeability, partial obstruction of the conduit due to collapse of walls,...), the products erupted during the open-conduit stage should contain a large fraction of liquid water.

The amount of liquid water of the products emitted during the initial explosion deserves further discussion. Thick breccia deposils, whose distribution around the crater is essen- tially controlled by the ballistic trajectories of the ejecta, have been reported as the products of hydrothermal explosions by many authors (e.g., Muffler et al., 1971; Nairn and Wiradi-

radja, 1980; Hedenquist and Henley, 1985 ). As little as 5% of interstitial mud-water fluid is apparently enough to lubricate the movement of the granular component, thus producing a debris flow. It is evident, therefore, that the water content of the deposit is a critical factor that largely controls its emplacing mechanism. Although only approximations are possible, equation ( 1 ) may be used to evaluate the pairs of water/rock weight ratio and aquifer temperature values for which the water content of the deposit is 5% (Fig. 14 ). Liquid water contents above this threshold (or a somewhat lower threshold, corresponding to 5% of interstitial mud-water fluid) cause the formation of debris flows. These events may therefore be called water-rich hydrothermal explosions. Hydro- thermal explosion breccias are generated instead for pairs of water/rock weight ratio and aquifer temperature values for which the water content of the deposit is lower than roughly 5%, that is, by water-poor hydrothermal explosions. Another limiting condition is given by

, , , / , , /

, , , / / j S T0c

0 ~ ) 5 I ~ i[ ;' ' { ( : * [ } I i

Fig. 14. Aquifer temperature vs water/rock weight ratio plot. The theoretical conditions leading to either water-rich or water-poor hydrotherrna} explosions arc shown. The e×plosions driven b~ dr], steam represent an extreme case of the latter type.

90 L. MARINI ET AL.

the pairs of water/rock weight ratio and aquifer temperature values for which X = 1, separating the explosions driven by bi-phase fluids from the explosions driven by dry steam. Bal- listic ejecta and ash fallout are typically generated by the latter type of explosion, as observed, for instance, at Wairakei geothermal field (Allis, 1984) and at Guagua Piehincha volcano (Barberi et al., 1991 ).

For the hydrothermal eruptions triggered by the sudden uprise of fluids along fractures as a result of tectonic movements, such as at Nisy- ros in 1873, hypothesis (b) is not satisfactory. In this case there is an obvious separation of fluids from the original aquifer rocks. The shallower rocks involved in the explosion are therefore colder than the fluids, although they too supply some heat. Moreover, the water/ rock ratio controlling the phenomenon is likely to be higher than under aquifer conditions. These factors favor the occurrence of water- rich hydrothermal explosions. It is not surprising, therefore, that related debris flow deposits were already emplaced during the initial explosive stage of the eruptions that took place on Nisyros in 1873, as documented by field observations and historical documents.

Other details of the 1873 Nisyros eruptions are given in the following discussion.

The provenance of the liquid phase from the Cl-rich deep aquifer rather than from the shallow system (C1 content 21 g/ l ) is indicated by the deposition of NaC1 in stalactites hanging from tree branches. This implies that the flashed brine was already saturated with respect to halite when it collected in the pond at the southernmost end of Lakki plain, or that it reached this condition upon minor evaporation, when the tree branches were still submerged.

Let us consider the unlikely case in which only the shallow system was involved in the eruption. It is assumed that heat was totally transferred from debris to water, both origi- nally in thermal equilibrium at 230 ° C, and that the debris flows emitted were rich in water, in

accordance with Gorceix's observations. Water content is taken to be 20% by weight, which is the upper limit for debris flows (Fisher and Schmincke, 1984). Based on these assump- tions and figures, the calculated chloride content of the aqueous fraction is 33 g/1 only. This aqueous system should lose a large fraction of steam (0.87 by weight) through evaporation to become saturated with respect to halite, which means a 87% decrease of the water level of the pond. The involvement of the deep CI- rich aquifer is therefore considered more likely.

At the beginning of the open-conduit eruptive stage of the June 1873 episode (the event described in greatest detail by Gorceix), the fluids were easily drained from the deep hydrothermal aquifer, so that the liquid phase reached the surface continuously. Later, due either to the collapse of the wall rocks and consequent obstruction of the vents and underlying fracture system, or to the depressurization of the deep aquifer, the liquid phase reached the surface intermittently, until only steam was discharged. The pressure along the fracture system continued to decrease with time. It is possible that when the pressure of the shallow hydrothermal aquifer at the inflow point in the fracture system was higher than the pressure of the fluids coming from below, the shallow aquifer started to discharge instead of the deep aquifer.

Present hazard and surveillance

Hydrothermal eruptions wilt probably occur in future, and this hazard must be taken into serious consideration. The southern half of Lakki plain, where all past eruptions took place and present fumarolic activity is concentrated, is the zone at highest risk.

All the deposits of hydrothermal eruption recognized on Nisyros Island were emplaced as debris flows, consistent with the historical description of the hydrothermal eruptions that took place in 1873 from Phlegethon and Poly- bores craters. Steam-driven hydrothermal

HYDROTHERMAL ERUPTIONS OF N1SYROS (DODECANt!SE, GREECE) 9 1

eruptions, one of which occurred in 1871 according to historical sources, originated ballistic deposits of limited importance, as no trace of these erodible products can be found in the field nowadays. Therefore, debris flows rather than ballistic ejecta are expected to be produced in future hydrothermal eruptions.

Geochemical evidence points out that the boiling parts of the shallow hydrothermal aquifer underneath fumarolic vents are, at present, anomalously heated through either conduction or steam injection from below. The latter heating mechanism is considered more likely because fumaroles are located along active NE-trending faults and along the caldera fault. Given this framework, only steam would ascend along these faults both from the deep aquifer to the shallow aquifer and from the latter to the surface fumaroles. If the shallow aquifer is able to dissipate, through boiling, the heat entering into it without exaggerated pressure increase with respect to the hydrostatic value, the risk of hydrothermal eruption re- mains low. However if self-sealing processes hinder heat transfer toward the surface dis- charges, pressure in the shallow aquifer may reach the threshold for the occurrence of a hly- drothermal eruption.

Furthermore, a sudden upflow of deep, hot water may occur along the NE-trending faults or along the caldera fault re-opened by an earthquake, thus leading to a quick evolution of the system toward a hydrothermal eruption. These events took place on Nisyros in 1873, as discussed above.

Therefore, two kinds of triggering mechanisms are expected in future:

( 1 ) overpressurization of the shallow aquifer consequent to increasing heat- and mass- transfer from below;

(2) sudden uprise of deep, hot water from the deep aquifer to the shallow one along faults re-opened by seismic shocks.

At present, total pressure in the portions of the shallow aquifer directly feeding most fiJ- maroles is 25 to 45 bar, as estimated through

gas-geobarometry (Chiodini et al., 1993). As proven elsewhere (Barberi et al., 1991 ),

phenomena of pressure build-up in the shallow aquifer, consequent to gradual increase of heat- and mass-transfer from below, can be detected through systematic sampling of the fumarolic effluents and application of geobarometric techniques (Chiodini and Cioni, 1989; Chiod- ini et al., 1993). The evolution of the shallow system towards the critical conditions leading to a hydrothermal eruption can thus be as- sessed with sufficient prior warning.

A tectonically triggered hydrothermal eruption could be less predictable, on the basis of geochemical evidence only, although signifi- cant changes in the composition of the fumarolic effluents can also be expected in this case, resulting from an increase in vertical permeability produced by strain accumulation.

In order to be effective, surveillance should include monitoring of local seismicity, as v, ell as geochemical techniques (Chiodini et al., 1991).

Acknowledgements

Research carried out with the partial finan- cial support of the National Group for Volcan- ology, Italy. The authors are grateful to Prof. L. La Volpe, Dipart imento Geomineraloglco dell'Universit/~ di Bari, Italy and Dr. L. Vez- zoli, Dipart imento di Scienze della Terra dell'Universit/~ di Milano, Italy for the critical reading of the manuscript. The reviewers, Prof. R. I. Tilling, Branch of Igneous and Geother- mal Processes of U.S.G.S. at Menlo Park, (a l - ifornia and Prof. J. Keller, Mineralogisch-Pe- trographisches lnstitut der Universitat Freiburg, FRG are thanked for their construc- tive remarks to the first version of ihe manuscript.

Appendix 1. Historical records

Names o/'hydrothermal crater.s

Most hydrothermal craters (i.e. Stephanos, Kaminakia, Polybotes, Phlegethon, Logothe-

92 L. MARINI ET AL

tis, Achelous) were named during the sulphur mining periods, in the second half of the 19th century (Martelli, 1917 ). The names of other craters (i.e. Andreas, Polybotes Micros) are probably more recent (Ribacchi, 1967-68). The name Polybote Megalos is proposed here to denote the largest and oldest crater carved into Lofos dome.

Some crater names are taken from Greek mythology (e.g., Graves, 1982).

For instance, Polybotes is the name of the giant defeated by Poseidon during the revolt of the giants againsts the gods. It is said that Po- seidon cut a piece off Kos with his trident and threw it at Polybotes, thus creating Nisyros. Polybotes, who was imprisoned below the island, still tosses and turns in anger, shaking the island to its roots.

Achelous is the god of the eponymous river who lost the right horn of his bull's head in the fight against Hercules. The connection between this mythological character and this crater is unclear, but perhaps is linked to the emission of water during the hydrothermal eruption.

The names of other craters refer to obvious characteristics, such as Stephanos, which is Greek for crown, or to imaginary characteristics, such as Phlegethon and Kaminakia, which mean "flaming" and "furnace" respectively.

The hydrothermal eruptions of 18 71-18 73

During 1871, the activity of the fumaroles inside Stephanos crater decreased, and that of the fumaroles inside Polybotes Megalos depression increased.

In October 1871, or at the end of November 1871, a violent earthquake took place during the night (with no damage reported ), followed by a series of detonations. Red and yellow flames rose over the island. Fragments of rocks flew over the highest peaks and down into the sea. Two small eruptive vents opened that night: Polybotes, in the central part of the Po- lybotes Megalos depression, and another on the

southern flank of Lofos dome. The discharged steam smothered the island in a kind of fog. The fields within the caldera depression were covered in white ash. The deposits of this steam blast were, however, of limited importance, as no trace of them can be found in the field nowadays. Considering how easily they were eroded, it seems likely that these products were either entrained in the 1873 debris flows or dispersed by atmospheric agents.

In March-April 1873, during his first jour- ney to Nisyros, Gorceix noted a strong discharge of fumarolic fluids from the two vents that opened in 1871.

On 3rd or 8th of June 1873, numerous violent seismic shocks were felt by the inhabitants of Nisyros, but not by people on nearby islands. A 6-7-m-wide vent, Phlegethon, opened on the southern flank of Lofos dome, repre- senting the extremity of a 50-m-long fracture trending N22°E. This fracture links Phlege- thon crater and Polybotes crater. Seismic events also triggered the opening of a north- trending 110-m-long fracture. Its precise location is not known, because no description is available. For three hours a hot brine was discharged and blocks and fragments of rocks ejected. Dark fluid mud was emitted intermittently from both craters over the next three days. The brine and the mud discharged from Phlegethon moved southwards down the slope to accumulate in the most depressed sector of Lakki plain. Subsequent water evaporation caused the deposition of salts, mainly sodium chloride, in thick layers and in stalactites hanging from the branches of trees reached by the erupted fluids. The average thickness of the mud is about 3 m, and the mudflow is approximately 500 m long and 150 m wide. No cas- ualties were reported, although some were slightly injured.

From June to September 1873, weak seismic shocks were felt from time to time by the local people. A large amount of H2S-bearing steam was discharged from the newly formed vents, without ejection of liquid and solid materials.

HYDROTHERMAL ERUPTIONS OF NISYROS (DODECANESE, GREECE) 93

On l lth September 1873, violent seismic shocks preceded the opening of a submarine fracture sited a few meters from the coastline near Mandraki. The sea water turned milky (possible presence of colloidal sulphur) and a

jet of H2S-bearing steam was observed for a few seconds. The same phenomenon simultane- ously took place on the nearby island of Yali, about 5 km north of Nisyros. Both the monas- tery and the church of Mandraki suffered mi- nor damage. Many of the walls around the kitchen-gardens of the village collapsed.

On 26th September 1873, there were new emissions of hot brine and mud, less important than the previous ones, from both Phle- gethon and Polybotes. Blocks and fragments of rocks were ejected from both craters, whose size increased considerably as a consequence of this eruption. Polybotes crater spread over half the floor of the preexisting depression, Po- lybotes Megalos, probably because of collapses triggered by the emptying and depressurization of the underlying system.

Phlegethon continued to discharge steam fi)r a few months, while Polybotes from time to time behaved like a geyser, ejecting columns of liquid water and steam up to a height of 30 m. The seismic shocks became less violent.

The hydrotherrnal eruptions of 1887

The last eruption, which occurred in Sep- tember 1887 and originated Polybotes Micros crater, is also recorded in historical chronicles ( Martelli, 1917 ), although its description is far less detailed than that of 1871-1873, probably because it was less impressive. Martelli ( 1917 ) says only that "according to the islanders who worked at the sulphur mines, it consisted of an eruption of mud and altered rocks with a pow- erful emission of steam".

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hydrothermal eruptions of nisyros (dodecanese, greece). past events and present hazard

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