Synchronous Pumice Mantle Found on Santorini Volcano

It is generally accepted that the vent of the 3.6 ka Minoan eruption was situated in the water-filled Santorini caldera prior to the Minoan eruption. One should therefore expect to find huge quantities of pumice and ash on the inner side of the caldera walls, but there is only a relatively small amount preserved. An unexpected discovery of remnants of a synchronous pumice mantle of the Minoan eruption appears to solve this enigma. A lengthy period of erosion and the intensive quarrying of pumice for the construction of the Suez Canal (1859 to 1869) led to the removal of an enormous amount of material and information for generations of geologists. The synchronous pumice mantle covered the whole caldera wall from rim to sea level. Archaeological finds under the pumice mantle show that the caldera wall was accessible and inhabited in the Bronze Age. Furthermore, this discovery documents that only one Minoan eruption took place and that the so-called “Lower pumice” does not exist on Santorini.

. The photo shows the three main phases of the Minoan eruption in Fira quarry (ca. 200 m asl). The eruption column of the first phase (yellow arrow) reached a height of several kilometres and gave rise to air fall deposits of pumice. The entry of sea water into the magma chamber triggered ring-shaped explosions that produced base surge deposits during the second phase (green arrow). The crater widened in the third eruption phase and created pyroclastic flow (red arrow). Material on the walls of the chamber was also ejected and created bomb sacs (above the persons) in the unconsolidated deposits (Photo Friedrich).
Marinatos [3]. Here a Bronze Age town with houses with up to three stories has been excavated [4]. The houses were decorated with colourful wall paintings [5] and contained decorated pottery and other artwork. A detailed description of the Minoan eruption and its consequences for Santorini and the surrounding area is given elsewhere [6].
Here we focus on the first eruption phase, the Plinian air fall pumice that produced a huge fan that covered the Eastern Mediterranean area, creating a valuable marker horizon for both geological and archaeological correlations. The synchronous ash and pumice mantle provides a valuable tool for the chronology since all objects that are in direct and undisturbed contact with this volcanic tephra fan have the same maximum age. This pumice mantle covered the entire volcanic edifice on Santorini. One should therefore expect to find a huge amount of pumice and ash on the inner side of the Santorini caldera wall. The absence of this material remained an enigma until recent discoveries.
A new series of eruptions on Nea Kameni (1866-1870) attracted numerous scientists to Santorini to study the spectacular events in the caldera and the he met a volcanic island that had experienced recent major changes. Pumice that had been deposited by the Minoan eruption on the inner side of the Santorini caldera was the first to be removed by this quarrying. This pumice removal led to the loss of an enormous amount of geological and archaeological information-a loss that led to wrong conclusions being made. In the 1870s Fouqué [7] considered that the volcano had produced two distinct pumice units; the Upper Pumice Series (ponce superieur) representing Minoan products) on the top of the island and the Lower Pumice Series (ponce inferieur) that represented older eruption products that were sandwiched between older and younger volcanic layers

Abandoning the Two Eruptions Concept
Fouqué's two eruptions concept was supported by a detailed illustration of the caldera walls by Neumann van Padang [9]. Since that time, the two eruptions concept was implemented in geological maps by Pichler and Kussmaul [10] and Druitt et al. [11]. Generations of geologists [12]- [18], including the senior author The still active Karageorghis quarry is visible in the right centre. Pumice is clearly present at two levels. Here Ferdinand Fouqué showed in 1879 the two pumice units (inserted drawing with arrows pointing to the lower and upper pumice series) thus giving the impression of two different eruptions. We now claim that both represent Minoan eruption products that were deposited on two terraces on the caldera wall (Photo Friedrich).  [20]. Today we claim that the so-called Lower Pumice Series does not exist ( Figure 3). It is in fact part of a synchronous pumice

Pumice Quarrying on Santorini and Its Consequences
Construction of the Suez Canal (1859 to 1869) led to enormous improvements in world trade since the sea-route from Europe to SE Asia became much shorter.
There were also some unexpected consequences. On the Greek island group of Santorini, prehistoric ruins were discovered when pumice of the Minoan eruption (also called the Late Bronze Age eruption) was quarried on Therasia (Figure International Journal of Geosciences 3(d)) and shipped to Egypt. Several prehistoric ruins were found under the pumice. On Thera-the main island-ruins were discovered at Akrotiri and above Balos harbour. The latter locality was excavated by the French scholars Gorceix and Mamet [27] where they found a prehistoric (later classified as Cycladic) house (  Santorini's medieval history. In medieval times [28] a large bell at Skaros castle was rung when pirates were seen in the caldera, and the inhabitants could hide or escape through tunnels that were excavated in the Minoan pumice. This accident also solved a geological enigma. It provided important evidence in the long-standing discussion concerning the two similar-looking pumice series on the caldera wall [29]. Up to 2 km long tunnels (Table 1,

A Cycladic House on a Terrace at Balos
Balos harbour is situated north east of the village Akrotiri. Two excavation sites Minoan pumice is still visible at several levels on the caldera wall at Balos harbour ( Figures 5(a)-(c)). Neuman van Padang [9] was not able to locate the prehistoric house above Balos harbour. When he mapped the "Lower Pumice  Series" there he considered that all remnants of this house had already been removed or eroded away. He was therefore not able to appreciate that this house provided a maximum age for the pumice that buried it. Today, however, we are aware of the site where the house stood (Friedrich, 2009) and are certain that it was the Minoan eruption that buried it and not the "Lower Pumice Series". Furthermore, Cycladic objects found in the house are depicted in Fouqué's book [7] International Journal of Geosciences and are kept in the collections of the French school in Athens.
According to Simmonds et al. [32] LP2 has an absolute age of 154 ka. It is clearly impossible that pumice of this age could have buried a Bronze Age house.
Even though the pumice here has been removed by quarrying it is reasonable to claim that it was pumice from the Minoan eruption that buried the house and not that from a much older eruption. Study of the geological setting around the platform on top of the lava where the house stood [33] also indicates that the pumice here had been deposited on a terrace and was not sandwiched between older and younger layers.
The terrace where the house stood can be followed for several hundred metres to the south. At some places it is still covered by the remains of Minoan pumice.
A total of 10 caves have been excavated and a monastery has been built in this pumice. All the caves are only 4.50 metres deep since older deposits forming the caldera wall in the Bronze Age were encountered at this depth. This strongly supports the concept that this unit is not the product of an older pumice eruption that covered the entire island.

Terraces and Pumice Mantle at Cape Plaka
Every geologist working on Santorini knows the Plaka profile ( Figure 6). It The unconformity on Santorini between phyllites and volcanizes that is exposed at Cape Plaka and at the nearby Cape Thermia, where the Christos church is located, might have been in use before the Minoan eruption. This can be deduced from the finding of pigments and lead minerals in the Akrotiri excavation [34] which were probably collected from this area [35]. When quarrying removed the pumice mantle the terrace became accessible again. The unconformity at Plaka is still inhabited; the Panaghia church and a summerhouse are located on this terrace. At the nearby Thermia locality the Christos church is directly built on the unconformity (Figure 7). The largest of three caves here is occupied by the church and houses have been built in the other two. The church is still in use but the two houses were abandoned a few decades ago.

Pumice Mantle and Terraces in Old Profiles
Scientific study of the Minoan eruption started around 1830 when the French  to the harbour at Katofira (Figure 8(a)). Virlet's drawing contains some important details that are of interest for present-day researchers. His number 1 layer represents a pumice layer which he recognized at several levels on the caldera wall. These pumice layers are emphasized in Figure 8

Pyroclastic Relicts on Analfi Island
Anafi is a non-volcanic island situated about 30 km east of Santorini. Relicts of Minoan pumice were reported on Anafi [15] [37] by Keller (1981). Furthermore, Doumas & Papazoglou [38] reported vestiges of Minoan pumice from both Anafi and Rhodes. It is important to establish the age of these deposits and whether they come from Santorini. Gertisser et al. [39] reported that pumice on Anafi could represent "Lower Pumice 2" (LP2 with a supposed age of 154 ka). Keller et al. [40] reported, based on chemical analyses, that the LP2 pumice is present on Anafi. However, an OSL study by Theodorakopoulou et al. [41] showed that no traces of pumice older than 21 k years were present on Anafi. This result supports the non-existence of the "Lower Pumice Series", at least on Anafi. However, more investigations on both Santorini and Anafi are necessary to shed further light on this issue.

Historic Eruptions
The discovery of the pumice mantle on the inner side of the caldera solves some stratigraphic problems. However, it is important to realise that there were some International Journal of Geosciences

Conclusions
The discovery of a synchronous pumice mantle on the caldera wall shows that 172,000 years) and LP2 (ca. 154,000 years) are no longer relevant because they provide ages of the sampling sites on the caldera wall upon which the pumice mantle was deposited but not of the pumice itself.
Many researchers have noted that the "Lower Pumice" is remarkably similar to the "Upper Pumice", but have nevertheless considered that they were produced by two eruptions separated by up to 172 k years. We claim that these two pumice units are identical. Both represent products of the synchronous pumice mantle produced by the Minoan eruption. The three classic phases (Figure 1) of the Minoan eruption are visible in both the "Upper" and "Lower Pumice Series".
The chemical compositions of the two pumice units are also remarkably similar [29], including the rare earth diagrams [42]. Furthermore, archaeological findings below the "Lower Pumice" at two localities (Karageorghis quarry and Balos) show that these sites were buried by the synchronous pumice mantle of the 3.6 ka Minoan eruption and not by pumice with an age of some 170,000 years. For these reasons, the two eruptions concept should be discarded. International Journal of Geosciences To summarize, our main conclusions are: 1) After the Minoan eruption, pumice mantled the entire inner side of the Santorini caldera; 2) There was only one major eruption that took place in the Bronze age (about 3.6 ka ago) and not two separated by about 150,000 years; 3) The so-called "Lower Pumice Series" does not exist on Santorini. Like the "upper Pumice Series" it is part of the Minoan eruption; 4) The inner side of the Santorini caldera had several natural terraces prior to the Minoan eruption. The pumice came to rest on these terraces. Much of it has been removed by quarrying and erosion; 5) Archaeological finds of Cycladic ruins and pottery prove that the inner side of the Santorini caldera was inhabited during the Bronze age.