Explosive Volcanism in the Hellenic Arc: a Summary and Review

Differences and similarities between individual events have been elucidated and help in constraining our models and reconstructions of the major volcanic eruptions.

The main centres of explosive eruptions are located on Milos, Santorini, Kos, Nisyros and Yali, and it is now well established that explosive volcanism was particularly intense in the Upper Quaternary of Santorini, Kos and Nisyros.

The Minoan eruption of Santorini is the most recent and one of the most powerful manifestations in Aegean explosive volcanism. The volcanic history of Santorini shows 12 major explosive cycles during about the last 200,000 years. A similar intensity of explosive volcanism in approximately the same time span characterizes the volcanic history of the eastern sector of the Hellenic arc, where Kos, Nisyros and Yali are prominent centres of important eruptions.

A major explosive unit, the Kos Plateau Tuff of ignimbritic nature covers several Aegean islands and western Turkey. The total volume is estimated to be > 100 km³. A caldera related to the Kos ignimbrite is located in the sea between Kos and Nisyros. The Kos ignimbrite starts with a phreatoplinian phase which reflects the position of the centre in the sea. The ignimbrite flows have crossed wide stretches of the open waters.

The Kos eruption is dated to 145,000 ± 5,000 years BP and represents an important volcanostratigraphic time-marker in the eastern Mediterranean.

Nisyros is a caldera volcano with many similarities to Santorini, although smaller in dimensions. The present caldera is 4 km across and has a volume of 6-8 km³. The caldera-phase pumice sequence is a dominantly plinian deposit, grading into phreatomagmatic and flow-type formations. The age of the caldera phase is not yet well defined, although a young age, possibly less than about 10,000 years. seems possible from the geological context.

The construction of the multi-centre pre-caldera andesite-dacite complex of Nisyros spans the last 100 ka. Radiometric ages for units of the pre-caldera evolution are 66, 37 and 24 ka. Of about 12 explosive eruption cycles of the pre-caldera Kyra sequence, six major plinian eruptions have been identified whose deposits cover several eastern Aegean islands and the western Anatolian coast. They post-date the Kos ignimbrite eruption. The activity of Yali is among the youngest events in eastern Aegean volcanism.

INTRODUCTION

Twenty years ago the First Congress on Thera and the Aegean World was held. One will be tempted looking back in many fields to ask what progress in our understanding of Thera and the Aegean World has been achieved by concentrated research efforts over these years. Physical volcanology, especially the quantitative understanding of explosive dynamics and the modelling of eruptive events through the analysis of pyroclastic deposits has experienced a revolutionary development just over these past twenty years.

An example of these developments is furnished by the concepts of magma-water interaction during explosive eruptions, which has led to our present understanding of phreatomagmatism and surge deposits. Following the now classical eruption of the Taal volcano in the Philippines in 1965 (Moore 1967), the first descriptions of phreatomagmatic phenomena, base surges and their deposits appeared in the literature about the time of the first Congress. In fact, the standard profile of the Minoan sequence could still be discussed in terms of Reck's work of 1936, in which the cross-stratified base-surge unit Minoan B, now mostly referred to as Minoan Phase 2, was taken as evidence for the interruption of volcanic activity and hence for a time gap with reworking and redeposition of the primary products. This is just one example, but we could extend such considerations to all other members of the profile. Certainly, the Rose Pumice layer of the initial Minoan A phase (Minoan 1) was recognized as an airfall deposit. But the concepts and quantitative modelling of plinian eruptions, with steady eruption columns several tens of kilometres high, grain-size sorting processes in these eruption columns, the duration of plinian eruptions, discharge rates from the vent, the deduction of vent evolution from the deposits, and the effects of the sudden collapse of the plinian columns - all these concepts and models had still to be developed.

This session on Explosive Volcanism is focused on the understanding of the Minoan eruption of Santorini. However, this understanding must be set within the framework of the broader Santorini evolution, and this in turn within the development of explosive volcanism in the Aegean (or Hellenic) volcanic arc.

This introductory review is intended to summarize our current understanding of explosive volcanism, the timing, products and processes, in the volcanic arc which extends from the Saronic Gulf in the west to the Anatolian coast in the east (Fig. 1).

EXPLOSIVE VOLCANISM IN THE HELLENIC ARC

The most important centres of explosive eruptions in the Aegean are Santorini, Milos, Kos, Nisyros and Yali. In the context of this conference our interest is focused on the youngest of the major explosive events. The evolution of the explosive volcanism of Santorini is discussed in detail in the recent paper by Druitt et al. (1989). The Minoan eruption (Bond and Sparks 1976) is the subject of the papers by Heiken and McCoy, Sparks and Wilson, Sigurdsson and Carey, and Pyle (this volume), Therefore, the special emphasis of the present paper is placed on volcanic centres outside Santorini. Their volcanostratigraphic evolution and the pyroclastic products of their major explosive eruptions are compared with the evolution of Santorini.

VOLCANIC STRATIGRAPHY 

The age of Aegean volcanism has been reviewed by Ferrara et al. (1980), Innocenti et al. (1981), Fytikas et al. (1984) and Keller (1982). Some additional data have become available since (Fytikas et al. 1986; Rehren 1988; Keller et al. 1989). Volcanic activity of the Hellenic arc started about 3-4 million years ago. The oldest radiometric ages obtained from Hellenic arc volcanics are summarized in Table 1.

For Santorini the oldest ages are found in the Akrotiri volcanoes from which the K/Ar dates given in Table 1 have been obtained (Ferrara et al. 1980). Seward et al. (1980) reported fission track ages on zircons from the oldest pyroclastic units of Santorini with ages close to 1 million years. Although there is still some uncertainty and also some inconsistency with palaeomagnetic results (Huijsmans 1985), one can state in a general way that volcanic activity in Santorini started about 1.6 - 1 million years ago.

More important in the context of this conference is that the major evolution of Santorini is recent, which means most of the volume of the exposed volcanics has been erupted during the last 200,000 years. Druitt et al. (1989) have documented and defined at least twelve major explosive eruptions in this time span. Their volcanostratigraphic summary of the Thera Pyroclastic Formation is reported in Table 2.

The dating of the Lower Pumice (Unterer Bimssteinhorizont of Reck 1936) is essential for establishing the time span of major activity on Santorini. The Lower Pumice consists in fact of two independent, fully developed eruption cycles, Lower Pumice 1 and 2. In eruptive style and sequence both units have striking similarities with the Minoan eruption.

The dating of the Lower Pumice units has still to be constrained more precisely. At the Second Thera Congress, Pichler and co-worker presented fission track ages of 70-130 ka (Seward et al. 1980), and from this stems a generally used average age of 100 ka. This age seems supported by preliminary U-Th disequilibrium data of D.M. Pyle (unpublished, reported in Druitt et al. 1989).

TABLE 1. Radiometric ages for the oldest dated rocks in islands of the Hellenic arc (for references see text)

However, the deep-sea sediments from the eastern Mediterranean show an amazing gap in their record of major explosive events just in this age range (Federman and Carey 1980; McCoy 1981; Keller 1981).

According to chemical composition, a correlation of the Lower Pumice with the widespread V-1 layer in deep-sea sediments is possible (Keller 1981, Fig. 6). However, the V-1 tephra has an interpolated age of 160 ka, which is still in conflict with the reported information from Santorini

The deep-sea sediment stratigraphy shows the presumed Lower Pumice from Santorini below the most conspicuous time-stratigraphic marker-horizon in the eastern Aegean. This is the co-ignimbrite ash of the Kos Plateau Tuff (KPT). K/Ar dating and profile position give an age of 145,000 years (Keller 1981; Stadlbauer et al. 1986). This ignimbritic unit is described in detail in the following section on explosive volcanism in the Kos-Yali-Nisyros sector of the Aegean arc.

TABLE 2. Summary of the major volcanic events in the history of Santorini (after Druitt et al. 1989)

THE ISLAND OF KOS

Unlike the other islands of the arc, Kos is dominantly non-volcanic. However, the metamorphic and sedimentary basement is covered by volcanic formations of a great petrological and volcanological variety, ranging in age from Miocene to the Recent Quaternary (Table 3 and Fig. 2).

The Miocene rocks on Kos belong to a potassic series and are dominantly K-trachytic ignimbrites related to a pre-Hellenic arc subduction zone (Keller 1982). Contemporaneous and petrologically related volcanic rocks occur in the neighbouring Turkish peninsula of Bodrum (Ercan et al. 1984; Robert 1985).

     The Kefalos Series:      On Kos, the volcanic evolution related to the recent Aegean arc starts with dacitic domes in the Kefalos peninsula. Hornblende dacites yielded K/Ar mineral ages of 3.4-2.7 ma (Altherr, Keller and Kreuzer, unpublished). The dacites are followed by the rhyolitic domes of the Kefalos peninsula with ages ranging from 2.7 to 1.6 ma (Bellon et al. 1979). Both rock types show typical calc-alkaline compositions.

A highly explosive volcanic sequence, the Kefalos Pumice Tuff Series, was formed during the Middle Quaternary in the Kefalos peninsula of western Kos. Dalabakis (1987) has studied this area in detail and explained the Kefalos Pumice Tuff Series as a tuff ring consisting of rhyolitic base surges and flow deposits with a total thickness of about 50 metres. The Kefalos Pumice Tuff Series covers an area of about 6 km². The centre of this remarkable hydromagmatic activity is located in the bay of Kamari, which is interpreted as a related caldera, about 4 km in diameter (Fig. 2). Within the Kamari caldera the rhyolitic Zini-dome was emplaced. K/Ar ages for this extrusion range from 1 ma (Bellon et al. 1979) to 0.55 ma (Pasteels et al. 1986; Boven et al. 1987).

TABLE 3: Volcanic evolution of Kos Island

     The Kos Plateau Tuff:      The most widespread volcanic deposit on Kos is the Kos Plateau Tuff, KPT (Keller 1971), resulting from the most powerful explosive event in the Recent Quaternary Hellenic arc. The KPT has an estimated total volume of at least 100 km³ (Stadlbauer 1988). A detailed volcanological interpretation of this major rhyolitic deposit is given by Stadlbauer et al. (1986), Dalabakis (1986, 1987) and Stadlbauer (1988). The dominant volcanological characteristics are summarized in Table 5. The most important feature of the Kos Plateau Tuff is the widespread distribution of the related ash-fall and the obvious ability of ignimbritic pumice flows to cross high mountain ridges and large stretches of the open sea (Stadlbauer et al. 1986; Stadlbauer 1988).

KPT is up to 30 m thick and its multiple pumice-flow facies is strictly unwelded with no indication of secondary erosion or reworking within the sequence.

Two main stratigraphic units are recognized: 1) a basal fine-grained and cross-stratified unit representing a powerful phreatoplinian initial phase, which is overlain by 2) unwelded ignimbritic pumice-flow deposits, consisting of several flow units with a co-ignimbrite ash layer on top of the sequence (Table 4).

Important deposits belonging to the KPT eruption occur on seven Dodecanese islands (Kos, Kalymnos, Pserimos, Tilos, Pachia, Perigusa and Chalki) and on the Turkish peninsulas of Bodrum and Datça (Fig. 2, 3). On Kos, KPT covers nearly the whole central part of the island. This is a plateau-like landscape with maximum elevations of about 200 m. On the other islands primary KPT is accumulated in the actual valleys and is eroded from all higher slopes.

A K/Ar mineral age of 145,000 years was obtained by P. Gillot on sanidine from the KPT pumice (Keller et al. 1989). This is in perfect agreement with the tephrostratigraphic correlation of Keller (1981) and Stadlbauer et al. (1986) giving 140,000-150,000 years for the W-3 ash layer in the deep-sea sediments of the eastern Mediterranean. The correlation of W-3 ash with KPT is based on identical glass and phenocryst compositions. A somewhat older K/Ar age of 240 ± 20 ka is given by Pasteels et al. (1986) and Boven et al. (1987) for the KPT pumice on Kos (whole rock and feldspar concentrates). These data are obviously not in agreement with the deep-sea record.

TABLE 4: Kos-Plateau-Tuff: Synoptic presentation of important sedimentary features, medium grain-size data and an interpretation of eruptive phases

The volcanological description of the KPT sequence follows the stratigraphic subdivision of Stadlbauer (1988) into six sub-units (Fig. 4, Table 4):

1. KPT-A is a stratified widespread layer of fine ash with accretionary lapilli. It has the characteristics of a typical phreatoplinian deposit (Self and Sparks 1978; Cas and Wright 1987). Isopach maps localize the eruptive centre to the south of Kos (Stadlbauer 1986; Dalabakis 1987). Total thicknesses on Kos range from 40 to 5 cm.
2. KPT-B is characterized by its distinct cross-stratification and a high content of rounded pumice lapilli, mostly coated with fine ash. This specific cross-stratified phreatomagmatic facies is restricted to central Kos, where. it is up to 4 m thick and grades continuously into KPT-C. The cumulative deposit of KPT A+B has been found as an independent deposit without flows with a thickness of 0.5 m in the central part of the Datça peninsula in Turkey, about 50 km from the presumed centre (see Fig. 3 for thickness of phreatomagmatic deposits KPT A+B on the other islands).
3. KPT-C is dominantly massive with minor lamination and a thickness of 1-2 m. The boundary between KPT-B and KPT-C is in most cases characterized by an oxidation zone showing a pinkish coloured ash matrix. It is explained as a transition zone between wet and dry sedimentation due to increasing temperature.
4. KPT-D consists on Kos of up to four pyroclastic flow units (sensu Sparks et al. 1973; Freundt and Schmincke 1985) with topographically controlled basal lithic concentration zones (LCZ) and lithic breccias (LB). Only one or two flows of sub-unit D have reached the other islands. Maximum pumice diameters (MP₅ = average of the five largest components) of single flow units show nearly no variation, except for the typical fine-grained basal layers (see Table 4 for MP₅ values of flow units).
5. KPT-E is formed of up to three flow units on Kos and one or two on the other islands. The boundary between KPT-D and KPT-E is always marked by a sudden increase in MP₅ values, with single pumice block diameters reaching more than 50 cm in the KPT-E flows. The boundary is further characterized by a changing spectrum of lithic types. Andesites and other volcanics are dominant in KPT-D, whereas granitic and metamorphic lithics appear in addition in KPT-E. In some places on Kos spectacular lithic breccias with single lithic diameters reaching up to 5 m occur at the base of KPT-E. On Tilos and the Datça peninsula, i.e. about 30-50 km from the presumed vent area, single lithic diameters are still close to 0.5 m. As in KPT-D, the MP₅ values within single flow units do not vary significantly but decrease step-wise from one flow unit to another. On Kos the uppermost layer of KPT-E is in some places characterized by a gradual transition from the massive flows to a cross-stratified surge facies.
6. KPT-F is the fine-grained top layer above KPT-E. It is characterized by its high content of fine ash and a dominantly massive appearance. Accretionary lapilli up to 5 cm in diameter are found in KPT-F Kos and Tilos; on Chalki, about 80 km from Kos, accretionary lapilli are still 2 cm in diameter. The deposition of KPT-F and the formation of large accretionary lapilli are obviously characterized by the presence of condensing water and a temperature below 100^o C. In terms of the flow-unit concept, KPT-F is interpreted as a cumulative dust layer on top of the whole sequence.

According to Freundt and Schmincke (1985) and Wilson (1985), ignimbritic dust layers are formed as a consequence of strong fluidization and elutriation of fine ash out of hot pyroclastic flows during their high-velocity transport. Minimum estimates for the velocity of KPT flows are based on their ability to cross the open sea and to surpass mountain ridges of at least 300 m on Kos, Kalymnos and the Datça peninsula, yielding flow velocities of 50-100 m/s (Stadlbauer 1988). Surprisingly, the grain-size characteristics of KPT flows are basically similar to published data on unwelded ignimbrites from dry environments (Fig. 5). Moreover, the grain-size of the flow matrix does not vary significantly between the different islands.

Another interesting feature of KPT is the amount and size of the lithics in flow deposits, especially on Kos. These may occur either as single components 'swimming' in the matrix of flows, or in basal concentration zones with a pronounced fines-depleted matrix (lithic concentration zone = LCZ and lithic breccia = LB in Fig. 5).

According to thickness and grain-size variations, combined with the evidence for transpon directions, the eruptive centre of this gigantic explosive event must be located south of Kos in the sea. As a consequence of the estimated KPT volume of more than 100 km³ a caldera with a diameter of at least 5-10 km should be present in the source area. Dalabakis (1987) and Stadlbauer (1988) place this caldera near the recent volcanoes of Nisyros and Yali (Fig. 3). However, there is no bathymetric indication of a caldera depression around these islands. New stratigraphic results (Bohla 1986; Rehren 1988; Keller et al. 1989) show that these islands were mainly formed by post-KPT activity and it is concluded, therefore, that the KPT caldera has been obliterated by the volcanic accumulations of these islands. Yali and, in part, Nisyros can be considered as post-caldera formations with respect to the KPT caldera.

TABLE 5. Summary of volcanological characteristics of the Kos ignimbrite (KPT)

INITIAL PHREATOPLINIAN PHASE

COMPOUND NON-WELDED IGNIMBRITE SEQUENCE

SPECTACULAR LITHIC BRECCIAS AS GROUND LAYERS AND INTRA-FLOW SEGREGATIONS

IGNIMBRITE PUMICE FLOWS TRAVELLED 30-50 km ACROSS THE OPEN SEA AND OVER 300 m HIGH MOUNTAIN RIDGES

HIGH MOMENTUM AND FLOW VELOCITIES OF 50 - 100 m/SEC

CO-IGNIMBRITE ASH LAYERS ?500 cm DISTANCE

CO-IGNIMBRITE ACCRETIONARY LAPILLI 5 cm Ø

DISTINCT FINES DEPLETION DUE TO HIGH DEGREE OF FLUIDISATION

VENT AREA IN THE SEA. HIDDEN CALDERA NEAR YALI-NISYROS

THE VOLCANIC EVOLUTION OF THE ISLAND OF NISYROS

Nisyros is a Quaternary stratovolcano marking the eastern edge of the Hellenic arc (Keller 1982). Nisyros has many similarities in its evolution with Santorini. Like Santorini, it is a caldera volcano. A complex stratocone was built up during the last 100 ka. Caldera formation and post-caldera domes are of Recent Quaternary age. The whole compositional range varies from basaltic andesites to rhyodacites and rhyolites and is of typical calc-alkaline characteristics, as at the other centres of the active arc.

Three principal evolutionary stages for Nisyros are recognized (Keller 1971; Di Paola 1974; Limburg 1986; Rehren 1988) and summarized in Table 6. These are: (1) the caldera and post-caldera phases, (2) the pre-caldera stratocone complex, and (3) the older submarine volcanic basement (Table 6).

Important sequences of explosive activity are the young 'Caldera Sequence' and the older 'Kyra Sequence', which belongs to the pre-caldera stratocone (Rehren 1988).

TABLE 6. Volcanic evolution of Nisyros

     The caldera and post-caldera phases:     The caldera of Nisyros has a diameter of 3-4 km. The two rhyodacitic to rhyolitic caldera-forming eruptions, Upper Caldera Pumice and Lower Caldera Pumice, are separated by a distinct palaeosol and the Nikia rhyolites, an unusually extensive lava effusion which covers the entire south-east sector of Nisyros. Caldera formation was followed by the extrusion of dacitic-rhyodacitic post-caldera domes which form the highest summits of the island and completely fill the western part of the caldera. In spite of the quite recent appearance of the domes, their exact age is still to be established, as is the age of the caldera formation. Nisyros is still active, with historical phreatic eruptions inside the caldera (Gorceix 1873).

The Upper Caldera Pumice (UCP) is an eruptive unit of fall, flow and surge deposits, with a strictly north to north-east directed distribution pattern. Limburg et al. (1986) distingwish three sub-units in the Upper Caldera Pumice, which they call the WYP series; (1) a plinian pumice-fall layer; (2) a second topography-mantling unit associated with cross-stratified dune-like surge deposits; (3) a final flow sequence.

Both airfall and flow deposits exhibit a north-directed dispersion axis. The maximum thickness of plinian fall deposits is about 15 m on the north coast, while the thickness of the flow units reaches its highest value of 40 m in two palaeovalleys trending north and north-east.

The whole depositional sequence of UCP is the result of an eruption column with an estimated height of 12-15 km (Limburg et al. 1986), and with an increasing degree of column collapse due to a rupturing vent and water interaction.

Volume estimates of this eruption are difficult and ill constrained due to the lack of distal outcrops outside the island. The caldera volume lies in the range of 6-8 km³, while the approximate volume of the UCP pyroclastics is estimated at 3 km³ DRE (Limburg et al. 1986).

The UCP is rhyodacitic; its mineralogy includes plagioclase, orthopyroxene, two different clinopyroxenes, amphibole, magnetite and ilmenite. Banded and mixed pumices and the wide range in composition of plagioclase, pyroxenes and opaques indicate magma mixing of differently evolved melts before or during the eruption.

The Lower Caldera Pumice (LCP) is separated from the UCP by a distinct palaeosol and has two different dispersion lobes. In the northern part of Nisyros, LCP is an unstratified, well-sorted plinian pumice deposit up to 10 m thick. In the southern part of the island its thickness is reduced by erosion to about 1 m. However, there is no definite evidence yet that the northern and southern lobes belong strictly to the same eruption phase. The LCP pumice is more silica-rich than the UCP and shows plagioclase, orthopyroxene, clinopyroxene, magnetite and ilmenite as phenocrysts.

The occurrence of this LCP plinian phase also suggests an earlier LCP related caldera. However, no direct geological evidence of this caldera remains.

     The pre-caldera stratocone complex:      The pre-caldera complex is a composite andesitic-dacitic stratocone with lava series and pyroclastic deposits from several centres (Rehren 1988). Three new radiometric age determinations have been presented recently for the pre-caldera cone (Rehren 1988; Keller et al. 1989), which range from 66,000 to 24,000 years BP. Accordingly, the caldera phase is younger than 24,000 years. The K/Ar age of 66,000 a was obtained on the Avlaki rhyodacite lava, formerly dated to 0.2 ma (Di Paola 1974). The whole pre-caldera cone post-dates the eruption of the Kos plateau ignimbrite (145,000 a) (Bohla 1986; Rehren 1988).

Most of the eruption centres of the pre-caldera evolution were effusive, producing dominantly basalt-andesitic to dacitic lava and forming the complex, multi-centre pre-caldera stratocone (Di Paola 1974; Rehren 1988). An essentially pyroclastic sequence is exposed in the coastal section in the eastern sector of Nisyros, near the Kyra monastery. This is the pyroclastic pre-caldera Kyra Sequence (KyS) of Rehren (1988).

The Kyra Sequence is characterized by intense explosive activity. It is a sequence of andesitie to dacitic scoria, pumice and tephra layers. It spans from very local strombolian activity to plinian eruptions with pumice distribution over > 500 km².

Twelve pyroclastic units are distinguished in this sequence. Six major eruptions of the Kyra Sequence from Nisyros reached the island of Tilos and deposited plinian pumice and lapilli layers up to 1 metre thick at a distance of 18-26 km from Nisyros (Bohla 1986; Bohla and Keller 1987).

Several layers have also been found on the island of Chalki, 55 km south-east of Nisyros, and on the Turkish Datça (or Knidos) peninsula (Fig. 3). These plinian deposits demonstrate a hitherto unknown explosive history of Nisyros during the cone-building pre-caldera period.

The most instructive distal profiles are, however, on Tilos. The nomenclature of the distal units with wide regional distribution follows their sequence on Tilos, starting with the oldest Nisyros Fall 1 (NF 1) and ending with the youngest, NF 6 (Bohla 1986). According to distal thickness and areal distribution, the plinian fall layers NF 4, NF 5 and NF 6 represent the major eruptions.

All distal profiles show well-sorted, plinian-type deposits, whereas the corresponding Kyra Sequence profiles on Nisyros show a much more complex eruptive history for most of the units, which are often dominated by surge and flow deposits.

The Kyra Sequence has been subdivided by Rehren (1988) into several eruptive units, as shown in Fig 6. The correlation between the Kyra Sequence and the Tilos profile is based upon a detailed volcanological, geochemical and mineralogical comparison (Rehren 1988; Bohla 1986). The first pyroclastic unit of the Kyra profile is a lightly coloured dacitic pumice layer, the Pink Kyra Pumice (PKP). Its maximum thickness is 2 m; the correlated Tilos unit NF 2 reaches 15 cm. PKP on Nisyros and NF 2 on Tilos are well-sorted lapilli deposits covered by a palaeosol.

The following andesitic Lower Surge Series (LSS) consists of tuffs with dune-like structures and accretionary lapilli, and a characteristic layer of scoria bombs. The LSS is known only from Nisyros itself and has not been found in the distal profiles. The total thickness is 2 to 6 m.

The Zoned Kyra Fall (ZKF) is a well-sorted topography-mantling fall-out deposit with a total thickness of 2 m. ZKF is chemically zoned from dacitic to andesitic. The corresponding unit NF 3 on Tilos is a 5 cm thick andesitic lapilli layer.

The following major units of the Kyra profile, MBU, MGU and WKP (Fig. 6) are correlated with the major plinian fall units NF 4, NF 5 and NF 6 on the island of Tilos. NF 4 and NF 6 also occur on the Datça peninsula of western Anatolia, and NF 5 and NF 6 on Chalki.

TABLE 7. Volcanological characteristics of the zoned Plinian Fall Unit NF 4 of the Pre-caldera Kyra Sequence of Nisyros (measured values from Bohla 1986)

COMPOSITIONALLY ZONED FROM 66-59% SiO₂

AGE BRACKETS: > 24 ka / < 66 ka

Volcanological characteristics and grain-size data for these plinian fall units on Tilos are given in Bohla (1986). On average, maximum pumice diameters, MP, and maximum lithic diameters, ML (Walker 1971) vary around MP 3.5-5 cm and ML 2-3.5 cm. Parameters for NF 4 are summarized in Table 7. NF 4 is a distinctive, chemically zoned fall with a sharp colour change from light-coloured pumice in the lower two-thirds of its thickness to dark grey lapilli in its upper part, corresponding to a silica variation from 66% to 59% SiO₂. NF 5 reaches 60 cm on Tilos. It grades from light to dark grey, which reflects a gradual decrease in silica content of more than 3%. Its Nisyros equivalent is the Middle Grey Unit of the Kyra profile.

The next Nisyros unit, the dacitic White Kyra Pumice (WKP) is a cycle of intermittent plinian fall-out, surge and flow deposits. Cross-stratification, accretionary lapilli and bomb sags characterize the phreatomagmatic parts of the deposit, well-sorted pumice beds the plinian periods. The total thickness is > 10 m. On Tilos the equivalent unit NF 6 is a typical plinian lapilli deposit of 60 cm thickness and of dacitic composition,

The Kyra Sequence terminated with the Grey Kyra Pumice (GKP) and the several tens of metres thick Upper Surge Series (USS) of Rehren (1988). No equivalents are known outside Nisyros. The Upper Surge Series (USS) is followed by andesitic lava and scoriae of the Emborio centre, the youngest of the pre-caldera stratocone centres. Part of these andesites near Pali are younger than the reported C-14 age of 24,000 years (Rehren 1988; Keller et al. 1989).

The Kyra Sequence is characterized by a phenocryst association of amphibole, plagioclase, pyroxene and opaques, in contrast to the other pre-caldera series, which are dominantly amphibole-free (Rehren 1988). The specific features of the Kyra Sequence can be explained by a high water-content of the related magma system (Rehren 1988).

     The submarine volcanic basement:      The oldest unit of Nisyros is the submarine volcanic basement of the north-west sector of the island (Keller 1971). This stratigraphically lowermost unit is separated from the overlying pre-caldera units by a major unconformity produced by marine abrasion. The submarine basement pre-dates the Kos ignimbrite (KPT), but has not given a more precise dating. We refer to it as a possibly Plio-Quaternary phase.

It can be concluded that the most active part of the history of Nisyros is younger than 145,000 years, with a complex pyroclastic activity of regional importance probably younger than 100,000 years. The rhyodacitic eruptions leading to the caldera collapse, followed by post-caldera domes, are supposed to belong to the Holocene.

THE ISLAND OF YALI

Yali has been described by Davis (1967), Keller (1971, 1980) and Di Paola (1974). It is essentially an Upper Quaternary rhyolitic centre with obsidian domes and pumice deposits. The only available dating is a fission track age of 24,000 years for the obsidians (Wagner et al. 1976). The main Yali pumice, unit Yali 1 of Keller (1980), is a stratified sequence of pumice tuffs with a thickness of about 160 metres. Yali pumices cover the youngest deposits on Nisyros (Keller 1971; Rehren 1988). A detailed volcanological interpretation for the Yali pyroclastic deposits is still lacking.

SUMMARY

According to the information now available about the major phases of explosive activity, both Santorini and Nisyros, i.e. the central and eastern Hellenic arc, experienced the period of most intense volcanic activity, within the last 100,000 years. The most recent event on Kos is the gigantic Kos ignimbrite (KPT) resulting from the most powerful explosive event in Quaternary Hellenic Arc volcanism, dated by deep-sea tephrochronology and K/Ar dating to 145,000 BP. A detailed volcanological interpretation of this major event is now available (Stadlbauer et al. 1986; Dalabakis 1986, 1987; Stadlbauer 1988). The KPT is found on Kos, on the adjacent islands Kalymnos, Pserimos, Pachia, Tilos and Chalki, and on the western Anatolian mainland.

It is a major silicic eruption, with phreatomagmatic features throughout its evolution. The vent area has been localized (isopachs, isopleths, flow directions) off the south coast of Kos in the sea near the islands of Yali, Stronghyle and Nisyros. A presumed large caldera in this area is completely obliterated in its bathymetric appearance by post-caldera activity. The formation of the volcanic islands of Yali, Stronghyle and large parts of Nisyros is post-ignimbritic and can be considered an intra-caldera activity in relation to the KPT eruption.

Nisyros is a caldera volcano, with a complex pre-caldera stratocone built up after the KPT evolution. Caldera formation and post-caldera domes are of Recent Quaternary age. Three principal evolution stages for Nisyros are (1) the caldera and post-caldera phases; (2) the pre-caldera stratocone complex and (3) the submarine volcanic basement. Two important sequences of explosive activity are the young 'Caldera Sequence' and the older 'Kyra Sequence' of the pre-caldera stratocone. Six major eruptions of the Kyra Sequence of Nisyros deposited plinian pumice and lapilli layers up to 1 metre thick on Tilos, at a distance of 18-26 km from Nisyros, on Chalki, 55 km south-east of Nisyros, and on the Turkish Datça peninsula, 30-40 km from the eruption centre. These plinian deposits demonstrate a highly explosive history of Nisyros during the cone-building pre-caldera period, approximately during the last 100,000 years.

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