Precursory Activity to the Minoan Eruption, Thera, Greece

The thin ashfall deposits from these eruptions are easily overlooked and may or may not contain juvenile tephra. Phreatic eruptions are small but impressive, causing little damage beyond the immediate vicinity of the vent, and provide a warning of events to come. Phreatomagmatic eruptions occur following the explosive mixing of rising magma with ground or surface water.

A deposit of phreatic and phreatomagmatic tephra overlies the Minoan palaeosol and underlies the plinian pumice fall of the Minoan eruption. This deposit has a maximum thickness of 9.5 cm and forms a 3.8-km-wide, SSE-trending deposit across southern Thera, from Balos to Cape Exomiti. On land this deposit has an approximate volume of 0.9 x 10⁶ m³. Within the Akrotiri excavations, this deposit has a maximum undisturbed thickness of 2.2 cm; in some places thicknesses of as much as 8 cm were possibly the result of clean-up efforts by Minoan inhabitants. Along the centre-line of the deposit there are four sub-units: (1) yellow-brown ash containing accretionary lapilli, abraded and porphyritic pumices with bubble-wall textures, and dacitic or andesitic lava clasts; (2) partly-graded, white to light-grey ash with pyroclast types similar to those of the first sub-unit; (3) pale yellowish-brown fine ash; and (4) greyish-brown to brownish-red coarse ash and lapilli (clast types are the same as in the first sub-unit, but lithic clasts make up 70%).

Lava lithic clasts in the phreatic and phreatomagmatic tephra are similar to those lavas that make up the youngest lavas of the Skaros volcano. In an earlier study (Heiken and McCoy 1984), we presented data supporting a hypothesis that the Minoan eruption began along vents of the Skaros volcano; this theory is also supported by the petrologic studies of Huijsmans (1985).

This phreatic and phreatomagmatic ash deposit provides evidence both that the Minoan eruption was preceded by phreatic and phreatomagmatic activity from vents on pre-caldera Skaros volcano, and that this activity may have prompted the evacuation of the island's residents.

INTRODUCTION

In the archaeological excavations at Akrotiri no human skeletons have been found (Doumas 1983). Cultural data from the site can be interpreted to indicate that the village had been damaged by an earthquake, was partly repaired, then was evacuated before the Minoan eruption. What phenomena warned the residents of the imminent eruption? One interpretation is that they were warned by numerous earthquake swarms that precede most eruptions; this may be so, but these phenomena may leave no tangible evidence of their occurrence. The other major precursor of volcanic eruptions is phreatic explosive activity, which does leave minor but distinctive pyroclastic deposits.

Phreatic eruptions consist of accelerated hydrothermal activity, during which fumaroles and hot springs erupt explosively. These blasts can be triggered either by tectonic activity that opens fracture permeability within hydrothermal systems or by the rise of magma into the volcanic edifice, where ground water is superheated. If the shallow magma body stops rising, this precursor activity ceases after a period of several months - as was the case for La Soufrière de Guadeloupe in 1976-1977 (Heiken et al. 1980). Phreatic activity preceded the explosive eruption of Mount St. Helens for several months, as ground water within the volcano was turned to steam (Sarna-Wojcicki et al., 1981). In that case, magma continued to rise and eventually erupted explosively, following pressure release caused by catastrophic collapse of the volcano summit. Volcanic ash deposits from phreatic activity consist of fragments from vent walls and may or may not contain juvenile ash from the magma body. Phreatic deposits can grade upward into or alternate with phreatomagmatic deposits, in which the predominant pyroclasts consist of fine-grained glassy particles formed by interaction of water and magma.

The question of what prompted the residents of ancient Akrotiri to evacuate stimulated our search for phreatic tephra deposits at the base of the Minoan Tuff; preliminary results were presented in Heiken and McCoy (1984).

PHREATIC / PHREATOMAGMATIC TEPHRA AT THE BASE OF THE MINOAN TUFF

     Description and distribution:      The phreatic deposit overlies the Middle Tuff Sequence, the older dacitic domes and flows, and an andesitic scoria cone of the Akrotiri Peninsula, and metamorphic rocks of the Gavrilos Ridge. A sandy loam (Minoan palaeosol) had developed on rocks of the Middle Tuff Sequence. At every measured section, the phreatic ash is overlain by the pumice fall deposits of the first phase of the Minoan eruption. A total of 65 stratigraphic sections of this deposit were measured, including 43 within the environs of the Akrotiri excavations (combined as H on Fig. 1).

The ash deposit has a maximum thickness of 15.3 cm and forms a 3.8 km wide south-south-east trending deposit across southern Thera, from Balos to Cape Exomiti (Fig. 1). On land, the deposit has a volume of 0.9 x 10⁶ m³ (calculated by integrating the volume within the area of the ashfall); however, much of the original volume may be hidden at sea inside and outside the caldera. Within the Akrotiri excavations, this deposit has a maximum undisturbed thickness of 2.2 cm; in places there are ash thicknesses of as much as 8 cm, which may be the result of clean-up attempts by the villagers or wind swirling around the buildings.

     Appearance in the field:      Within the phreatic/phreatomagmatic ash deposit there are four sub-units (1-4 on the stratigraphic sections shown in Fig. 2); in the field, they have the following characteristics:

1. Sub-unit 1 is a yellow-brown (10YR5/4 to 6/4) to pinkish-grey (5YR8/1) lithic ash containing subangular to subrounded pumice clasts. Within the thickest sections and in the Akrotiri excavations, there are 1- to 3-mm-diameter accretionary lapilli.
2. Sub-unit 2 consists of poorly graded white to light grey ash with thicknesses as great as 5 cm. This sub-unit contains rounded pumice lapilli.
3. Sub-unit 3 is a pale yellowish-brown (10YR6/2) fine ash, 0.2 to 0.5 cm thick. It was too thin to be sampled with confidence for later analysis.
4. Sub-unit 4 is a greyish-brown to brownish-red bed of coarse ash to lapilli. In the field it was estimated that 40% of the clasts were lithic and are mostly lavas of intermediate composition. The colour variations, from 5YR3/2 to N6, are related to the types of lithic clasts.

All of the sub-units are distinct and easily distinguished from the underlying tuff units of the Middle Tuff Sequence.

     Petrographic characteristics:     

     Sub-unit 1:      This is a coarse-grained lithic ash consisting of (a) subangular clasts of dark brown, fine-grained tuff, (b) slightly metamorphosed arkose, (c) glomeroporphyritic clots of zoned plagioclase, orthopyroxene, and Fe-Ti oxides, (d) subrounded clasts of flow-banded, hyalocrystalline andesite or dacitic lavas, (e) hornfelsed lava (andesite), (f) basaltic lava with a trachytic texture, (g) 1- to 2-mm-long plagioclase phenocrysts exhibiting oscillatory zoning, (h) orthopyroxene with inclusions of Fe-Ti oxides, (i) rare clinopyroxene, and (j) pumice pyroclasts (Fig. 3a and b, 4a and b, Table 1). The coarse-ash-size pumice pyroclasts contain about 5% of (mostly) plagioclase phenocrysts in colourless glass; vesicles are usually spherical or ovoid, and there is some radial vesicle growth around phenocrysts (Fig. 3b). Many of the phenocrysts have adhering glass and a bubble-wall texture. All of the pyroclasts have an adhering rim of fine-grained ash or clay (removed for the scanning electron micrographs).

     Sub-unit 2:      The variety of clast types is the same as in sub-unit 1, although the ratios are considerably different (Fig. 4). The 1- to 4-mm-diameter glomeroporphyritic clots of plagioclase, Fe-Ti oxides, and orthopyroxene often have thin, vesicular glass coatings and appear to have been broken out of the pumices common to this ash (Fig. 3c and 3d). The aphyric pumice clasts are bits of pumiceous melt separated from large phenocrysts and glomeroporphyritic clots during explosive fragmentation.

     Sub-unit 3:      This fine ash layer was too thin to be sampled without contamination from overlying ash beds.

     Sub-unit 4:      This very coarse ash consists of mostly weathered pumice clasts and lithic clasts (some showing effects of hydrothermal alteration); all show considerable rounding. Most of the pumice clasts in this ash appear to be lithic clasts from an older deposit; most were not considered juvenile (Fig. 4a and b). In contrast with the juvenile pumice pyroclasts of lower sub-units, sub-unit 4 pumices have high vesicularity and abundant elongate vesicles (Fig. 3e). The abundant 'andesitic' lava clasts are both glassy and holocrystalline; at this scale, they could have been broken from either andesitic or dacitic lavas.

For comparison, plagioclase compositions (cores) were determined with an electron microprobe (10-μm-diameter spot) of phenocrysts in lava clasts, in glomeroporphyritic clots, and in pumice pyroclasts. They ranged from An₃₁ to An₉₇. Also for comparison, plagioclase phenocrysts were measured in lava samples from the Skaros volcano, including the youngest dacite flow at Merovigli. The plagioclase compositions all overlap, and there is more variation within anyone sample than between samples. It is possible that many of the lava clasts within the Minoan phreatic/phreatomagmatic ash were derived from the Skaros shield volcano. Huijsmans (1985) and Den Tex et al. (1984) present petrologic data supporting initiation of the Minoan eruption from vents on the ancestral Skaros volcano.

DISCUSSION

Bond and Sparks (1976) and Heiken and McCoy (1984) presented evidence that the Minoan eruption began at vent(s) located between what is now Merovigli (Skaros) and the Kameni islands and what was then the ancestral Skaros volcano. Based upon the isopachs of the underlying Minoan phreatic/phreatomagmatic tephra (Fig. 1), it appears that the eruptions also came from the general area of the vent of the Minoan pumice fall-out deposit. The similarities between lavas of the Skaros volcano and lava clasts in the phreatic/phreatomagmatic ash deposit can be used to hypothesize that the vent for these phreatic (and phreatomagmatic) ashes was Skaros. The ash deposit overlies soils developed on tuffs of the Middle Tuff Sequence (Heiken and McCoy 1984)and underlies, with no major erosional break, the massive pumice fall-out deppsits of the Minoan Tuff.

The ash is, in part, made up of hydrothermally altered and weathered clasts derived from older rocks consisting of arkose, fine-grained (phreatomagmatic) tuffs, and andesitic to dacitic lavas. Many of the clasts are coated with clays or silt. They can be interpreted as deposited by a phreatic eruption. However, there is a substantial component of what appear to be juvenile pumice and associated glomeroporphyritic clots with vesicular glass coatings. The fresh pumices imply that these early, small-volume eruptions were both phreatic and magmatic and were early leaks from the Minoan magma chamber. Den Tex et al. (1984) have proposed that the Minoan eruption was the culmination of activity responsible for construction of the Skaros volcano; this conclusion is compatible with our interpretation that the Minoan phreatic/phreatomagmatic precursor ash was erupted from that volcano and left a fall-out deposit across the Akrotiri peninsula.

Earthquakes usually do not force total evacuation of a town unless it is completely destroyed. As summarized in Doumas (1983), it appears that the residents of ancient Akrotiri had initiated repairs to buildings damaged by an earthquake. However, most of these people appear to have left before the eruption of the Minoan pumice fall-out deposit. Why did the residents finally evacuate? We believe that the early phreatic and minor magmatic eruptions that produced this small ash deposit were sufficiently impressive to force evacuation of the island. Maximum tephra thicknesses of 8 to 9 cm are sufficient to cause collapse of some flat roofs (Blong 1984). Even if roofs had not collapsed, this much ash is enough to have made breathing difficult and the explosive eruptions would have been sufficient to cause fear of yet more energetic volcanic activity. If this eruption followed the course of more recent eruptions with premonitory phreatic activity, a period of several weeks to several months could have passed before the first large-scale eruption of pumice began.

ADDENDUM

Criteria not emphasized in the preceding manuscript, in addition to new criteria from two new exposures of the precursor ashlayers (newly slumped face in the Cape Therma quarry, new construction site on the caldera edge near Akrotiri), lead to the following addendum.

The yellow ash mixed with subrounded, white lapilli pumice along the northern boundary of the deposit (e.g. Phira, Alonaki and Cape Therma quarries) is equivalent to sub-units 2 and 3. In the quarries, the two sub-units appear mixed; at the construction site, the two sub-units are distinctly different.

The entire deposit mantles the Minoan topography, indicating ash fall as the primary depositional mechanism. There is evidence for secondary lateral re-deposition of sub-unit 2.

All four sub-units indicate a vent approximately near the Bo1 (Minoan A of Druitt et al. 1988) vent inferred by Heiken and McCoy (1984). Water had access to this vent, in adequate quantities to allow phreatomagmatic and phreatic eruptions. Dispersal of tephra was to the south-east, presumably by atmospheric winds.

Sharp contacts between the lower two sub-units, 1 and 2, indicate distinct pauses between these eruption episodes. These contacts are not erosional; thus, no significant hiatus between the deposition of the two sub-units is indicated. Gradational contacts between the upper two units, sub-units 3 and 4, imply continuity between these eruption episodes; the gradational contact with the overlying plinian airfall pumice (Bo1) also implies no significant break in eruption activity leading into the major part of the eruption.

We envisage the following sequence of events for the eruption and deposition of the precursor ash:

1. Eruption of ash (sub-unit 1).
2. A pause in the eruption. This hiatus could be minutes, hours or days, but certainly not as long as through a rainy season, so as to allow erosion of the thin deposit.
3. Eruption of pumice (sub-unit 2). Some near-ground turbulence is indicated by the variable thickness of the deposit and the rounding of the pumice; inferences from a few outcrops suggest south-east directed lateral transport similar to tephra dispersal winds. This turbulence was adequate to cause only local erosion and re-deposition and inadequate to erode the thin underlying ash of sub-unit 1.
4. Another hiatus in volcanic activity may have occurred, as implied by the sharp boundary between sub-units 2 and 3. However, mixing of these two sub-units in distal parts of the deposit, particularly to the north-east, could indicate approximately contemporaneous eruptive episodes.
5. Eruption of yellow ash (sub-unit 3) with dispersal more to the east-south-east, suggesting a small change in wind direction.
6. Continuation of eruption activity, without a hiatus, with ash and lapilli (sub-unit 4) dispersed to the south-east. This activity continued into the first phase of the major eruption.

Finally, we note that the combination of precursor seismic activity and of the eruption of the precursor tephra likely warned the Minoan inhabitants to leave. Seismic activity was known to these Minoans; eruptions of volcanic ash and pumice on their island was unknown, however, and the combination of the tephra blanket and persistent seismicity must have been threatening.

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