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The Christiana-Santorini-Kolumbo volcanic field (CSKVF) in the Aegean Sea is one of the most active volcano-tectonic lineaments in Europe. Santorini has been an iconic site in volcanology and archaeology since the 19th century, and the onshore volcanic products of Santorini are one of the best-studied volcanic sequences worldwide. However, little is known about the chronology of volcanic activity of the adjacent submarine Kolumbo volcano, and even less is known about the Christiana volcanic island. In this study, we exploit a dense array of high-resolution marine seismic reflection profiles to link the marine stratigraphy to onshore volcanic sequences and present the first consistent chronological framework for the CSKVF, enabling a detailed reconstruction of the evolution of the volcanic rift system in time and space. We identify four main phases of volcanic activity, which initiated in the Pliocene with the formation of the Christiana volcano (phase 1). The formation of the current southwest-northeast-trending rift system (phase 2) was associated with the evolution of two distinct volcanic centers, the newly discovered Poseidon center and the early Kolumbo volcano. Phase 3 saw a period of widespread volcanic activity throughout the entire rift. The ongoing phase 4 is confined to the Santorini caldera and Kolumbo volcano. Our study highlights the fundamental tectonic control on magma emplacement and shows that the CSKVF evolved from a volcanic field with local centers that matured only recently to form the vast Santorini edifice.

We use the tephrostratigraphic framework along the Aegean Volcanic Arc established in Part 1 of this contribution to determine hemipelagic sedimentation rates, calculate new tephra ages, and constrain the minimum magnitudes of (sub)plinian eruptions of the last 200 kyrs. Hemipelagic sedimentation rates range from similar to 0.5 cm/kyr up to similar to 40 cm/kyr and vary laterally as well as over time. Interpolation between dated tephras yields an eruption age of similar to 37 ka for the Firiplaka tephra, showing that explosive volcanism on Milos is similar to 24 kyrs younger than previously thought. The four marine Nisyros tephras (N1 to N4) identified in Part 1 (including the Upper (N1) and Lower (N4) Pumice) have ages of similar to 57 ka, similar to 63 ka, similar to 69 ka, and similar to 76 ka, respectively. Eruption ages for the Yali-1 and Yali-2 tephras are similar to 55 ka and similar to 34 ka, respectively. The Yali-2 tephra comprises two geochemically and laterally distinct marine facies. The southern facies is identical to the Yali-2 fall deposit on land but the western facies has slightly less evolved glass compositions. Overall, erupted plinian and co-ignimbrite fall tephra volumes range from

The Milos, Christiana-Santorini-Kolumbo (CSK) and Kos-Yali-Nisyros (KYN) volcanic complexes of the Aegean Volcanic Arc have repeatedly produced highly explosive eruptions from at least similar to 360 ka into historic times and still show recent unrest. We present the marine tephra record from an array of 50, up to 7.4 m long, sediment cores along the arc collected in 2017 during RV Poseidon cruise POS513, which complements earlier work on distal to ultra-distal eastern Mediterranean sediment cores. A unique set of glass-shard trace element (LA-ICPMS) compositions complements our major element (EMP) data on 220 primary ash layers and 40 terrestrial samples to support geochemical fingerprinting for correlations with 19 known tephras from all three volcanic complexes and with the 39 ka Campanian Ignimbrite from the Campi Flegrei, Italy. The correlations include 11 eruptions from CSK (Kameni, Kolumbo 1650, Minoan, Cape Riva, Cape Tripiti, Upper Scoriae 1 and 2, Middle Pumice, Cape Thera, Lower Pumice, Cape Therma 3). We identify a previously unknown widespread tephra from a plinian eruption on Milos (Firiplaka Tephra). Near the KYN we correlate marine tephras with the Kos Plateau Tuff, the Yali 1 and Yali 2 tephras, and the Upper and Lower Pumice on Nisyros. Between these two major tephras, we found two tephras from Nisyros not yet observed on land. The four Nisyros tephras form a systematic trend toward more evolved magma compositions. In the companion paper we use the tephrostratigraphic framework established here to constrain new eruption ages and magnitudes as a contribution to volcanic hazard assessment.

We present the results of laboratory experiments on the behaviour of sustained, dense granular flows in a horizontal flume, in which high-gas pore pressure was maintained throughout the flow duration by continuous injection of gas through the flume base. The flows were fed by a sustained (0.5-30 s) supply of fine (75 +/- 15 mu m) particles from a hopper; the falling particles impacted an impingement surface at concentrations of similar to 3 to 45 %, where they densified rapidly to generate horizontally moving, dense granular flows. When the gas supplied through the flume base was below the minimum fluidization velocity of the particles (i.e. aerated flow conditions), three flow phases were identified: (i) an initial dilute spray of particles travelling at 1-2 ms-1, followed by (ii) a dense granular flow travelling at 0.5-1 m s-1, then by (iii) sustained aggradation of the deposit by a prolonged succession of thin flow pulses. The maximum runout of the phase 2 flow was linearly dependent on the initial mass flux, and the frontal velocity had a square-root dependence on mass flux. The frontal propagation speed during phase 3 had a linear relationship with mass flux. The total mass of particles released had no significant control on either flow velocity or runout in any of the phases. High-frequency flow unsteadiness during phase 3 generated deposit architectures with progradational and retrogradational packages and multiple internal erosive contacts. When the gas supplied through the flume base was equal to the minimum fluidization velocity of the particles (i.e. fluidized flow conditions), the flows remained within phase 2 for their entire runout, no deposit formed and the particles ran off the end of the flume. Sustained granular flows differ significantly from instantaneous flows generated by lock-exchange mechanisms, in that the sustained flows generate (by prolonged progressive aggradation) deposits that are much thicker than the flowing layer of particles at any given moment. The experiments offer a first attempt to investigate the physics of the sustained pyroclastic flows that generate thick, voluminous ignimbrites.

The fluidal behavior of pyroclastic flows is commonly attributed to high gas pore pressures and associated fluidization effects. We carried out experiments on flows of fluidized volcanic ash at 170 degrees C, which is hot enough to reduce cohesive effects of moisture. The flows were generated in a 3- m- long, horizontal lock- exchange flume. The ash was fluidized and expanded uniformly in the flume reservoir by up to 43% above loose packing and was then released. Each flow defluidized progressively down the flume until motion ceased. Initial expansion E and initial height h(0) were varied independently of one another. The flows traveled in a laminar manner. Flow fronts exhibited three main phases of transport: ( 1) a brief initial phase of gravitational slumping, ( 2) a dominant, approximately constant velocity phase, and (3) a brief stopping phase. Phase 2 frontal velocities scaled with root gh(0), like other types of dam- break flow. Deposition from initially expanded flows took place by progressive sediment aggradation at a rate that was independent of distance and varied only with E. Despite rates of shear up to 80 s(-1), aggradation rates were identical to those determined independently, at the same value of E, in quasi-static collapse tests. Sedimentation caused the flows to thin progressively during transit until they ran out of volume. The dynamics were governed to a first order by two dimensionless parameters: (1) the initial aspect ratio h(0)/x(0) in the lock reservoir and (2) the ratio t(sett)/t(grav) of two timescales: a particle settling time t(sett) and a gravitational acceleration time t(grav).

International audience; The 7500 BP Socompa sector collapse emplaced 25 km 3 of fragmented rock as a thin, but widespread (500 km 2) avalanche deposit, followed by late-stage sliding of 11 km 3 as Toreva blocks. Most of the avalanche mass was emplaced dry, although saturation of a basal shear layer cannot be excluded. Modelling was carried out using the depth-averaged granular flow equations in order to provide information on the flow behaviour of this well-preserved, long-runout avalanche. Results were constrained using structures preserved on the surface of the deposit, as well as by deposit outline and run-up (a proxy for velocity). Models assuming constant dynamic friction fail to produce realistic results because the low basal friction angles (1 to 3.5°) necessary to generate observed runout permit neither adequate deposition on slopes nor preservation of significant morphology on the deposit surface. A reasonable fit is obtained, however, if the avalanche is assumed simply to experience a constant retarding stress of 50-100 kPa during flow. This permits long runout as well as deposition on slopes and preservation of realistic depositional morphology. In particular the model explains a prominent topographic escarpment on the deposit surface as the frozen front of a huge wave of debris reflected off surrounding hills. The result that Socompa avalanche experienced a small, approximately constant retarding stress during emplacement is consistent with a previously published analysis of avalanche data.