Structure of the NW and SW straits

The new multi-beam bathymetric data show that the NW strait consists of a 3 km long, 1 km wide, U-shaped, submarine channel with an arcuate headwall (Fig. 1a). The erosional morphology of the headwall, 2 km across and with slopes of 5–10°, is scarred by landslides and multiple narrow submarine drainages that converge towards the caldera (Figs 1b–d and 2a,b). In the middle course of the strait, two distinct drainages merge downstream to form a single V-shaped drainage (section p2 in Fig. 2c). A seismic reflection profile along the axis of the strait (Fig. 2a; Supplementary Fig. 1) reveals that the headwall is carved into NW-dipping, coherent lithologies representing the lava succession that comprises much of northern Santorini12. Prominent, more or less continuous, reflections are probably lavas and acoustically transparent layers are either tuffs11 or hemipelagic sediments13. Phase-reversed bright spots may indicate bedding-parallel fluid flow11. The NW-dipping reflectors underlie much of the strait, covered only by thin sediment layers of units 1 and 2 (ref. 10. To the SE, a seismic profile into the caldera shows all three sediment units (Fig. 3; ref. 11). A seismic reflection profile perpendicular to the strait axis (Fig. 2a,d) reveals superficial landslides on the margins and thin layers of units 1 and 2 on the floor (Fig. 2d). A prominent landslide deposit at the foot of the headwall can be distinguished (Fig. 1a,d). Deep faulting is not observed, beneath either the breach headwall or beneath the margins (Figs 2d and 3).

Figure 2: More detailed topography and seismic profiling of the NW strait of Santorini caldera. (a) Shaded slope gradient with the location of profiles shown in 2c (p1, p2 and p3), 2d (seismic profile A-A′) and Fig. 3 (seismic profile B-B′) marked. The dotted orange line marks the location of the erosional headwall, the dashed orange line the channel sides, the dotted pink line the rim of the caldera, and the dotted black line the small-scale drainages superimposed on the large-scale channel. (b) Perspective view of the NW strait. (c) Bathymetric cross sections perpendicular to the NW strait. (d) Seismic profile perpendicular to the strait axis, showing caldera-fill stratigraphic units 1 and 2 of ref. 10. Full size image

Figure 3: Interpreted multi-channel reflection seismic profile along the axis of the NW strait. Transect B-B′ in Fig. 2. Stratigraphic units 1–3 of ref. 10 are shown numbered. The light blue lines indicate stratigraphic horizons within Peristeria lavas, and the dotted dark blue line indicates the presence of a deep, but poorly defined reflector. What appears as an edifice in the upper part of the slope is a ledge of the channel rim. A TWT (two-way travel time) interval of 0.1 s corresponds approximately to a water depth interval of 75 m or a sediment interval of 100 m. Please refer to Supplementary Fig. 1 for the uninterpreted seismic profile. Full size image

The southwest straits are morphologically fresh, and have landslide scars with well-preserved headwalls and intervening septa (Fig. 1a). The headwalls are steeper than that of the NW strait, and are less scarred by secondary slumping and drainage channels. Seismic reflection profiles of the SW straits reveal the fill of the western basin (Figs 4 and 5, Supplementary Figs 2 and 3). High-resolution bathymetry of the SW straits (Supplementary Fig. 4) is presented for comparison with that for the NW strait.

Figure 4: Interpreted W-NE striking multi-channel reflection seismic profile across the SW strait. This profile cuts across the outer edge of the western basin of Santorini caldera, as shown in the inset. Stratigraphic units 1–3 of ref. 10 are shown numbered; The dotted dark blue line indicates a deep, but poorly defined reflector; faults are shown in red. A TWT (two-way travel time) interval of 0.1 s corresponds approximately to a water depth interval of 75 m or a sediment interval of 100 m. Please refer to Supplementary Fig. 2 for the uninterpreted seismic profile. Full size image

Figure 5: Interpreted NW-SE sparker reflection seismic profile. This profile crosses the two SW straits, as shown by the orange line in the bathymetric map inset. Stratigraphic units 1–3 of ref. 10 are shown numbered; The dotted dark blue line indicates the presence of a deep, but poorly defined reflector. A TWT (two-way travel time) interval of 0.1 s corresponds approximately to a water depth interval of 75 m or a sediment interval of 100 m. Please refer to Supplementary Fig. 3 for the uninterpreted seismic profile. Full size image

Origin of the straits

We interpret the NW strait as a flood-modified landslide scar, formed by northward propagating regressive erosion and landsliding, and the headwall as a huge fossilized water chute (Figs 1a and 2a,b). It is clearly erosional in origin, since no deep faulting is observed. The arcuate headwall scar is concave towards the caldera, thus implying sliding and water flow into the caldera. The large-scale sea floor morphology, with its wide, rounded headwall scar and narrow, steep-sided passage, shows marked similarities to features commonly observed when a manmade dam fails with a sudden outflow of water (as shown on Figs 1, 5 and 6 from ref. 14, on front cover and Plates 11.1, 11.5, 11.6 from ref. 15, in figures 3 and 5 from ref. 16). This strongly suggests a sudden breach of the caldera wall analogous to a dam failure. The breach must have required significant force, given the structural integrity of the channel walls. A strong inward-directed flow of water suggests that the caldera was either dry, or that the level of water was much lower than that of the surrounding sea, when the breach took place. The onset of inflow by the sea was probably accompanied by large-scale landsliding into the caldera, followed by erosion due to the rapidly flowing water. The much smaller-scale drainage pattern superimposed on the strait sea floor morphology (Fig. 1c,d) either formed at a late stage of the flooding event, or was cut by the present-day flow (8–19 cm s−1) of cold Mediterranean bottom waters into the warmer waters of the caldera17.

Figure 6: Summary of the development of Santorini caldera before and during and following the LBA eruption. (a–c) Pre-eruptive morphology of the volcano18,19,20,21 and the main phases of the eruption1,2,3,4,5. Before the eruption there existed an ancient caldera in the northern half of the volcanic field18,19. This caldera was lagoonal, as shown by the presence of fragments of ancient travertine, stromatolites and brackish to marine fauna in the LBA ejecta20,21. There was also an andesitic edifice within this caldera5. In eruptive phase 1 a Plinian eruption took place, which in phase 2 was joined by the production of syn-plinian pyroclastic surges. In phase 3, eruption of ‘cold’ phreatomagmatic pyroclastic flows constructed a large tuff cone that filled the old caldera, cutting it off from the sea. In phase 4, eruption of hot pyroclastic flows took place from multiple subaerial vents, forming at least three ignimbrite fans (NW, E and S), and associated caldera collapse enlarged and deepened the ancient caldera. The main eruptive vents are shown in these figures as red stars (locations well constrained for phases 1 to 3, but speculative for phase 4). Black arrows show schematic emplacement vectors for the pyroclastic flows of phases 3 and 4. (d,e) Post-eruptive opening of the NW and SW straits (based on the present research). At the end of the eruption the caldera was dry and isolated from the sea, probably due to thick accumulations of LBA tuff. The sea first broke through to the NW, where a combination of water erosion and landslip carved out the NW strait and flooded the caldera (blue arrows in d) in less than a couple of days. Submarine landslides (black arrows in e) then opened up the SW straits once the caldera was largely flooded. (f) The present-day volcanic field. Full size image

In contrast, the morphological freshness of the two SW straits (Supplementary Fig. 4) suggests that they formed by slumping once the caldera was already flooded (Fig. 1a). We envisage that, during formation of the NW strait, there was little water inside the LBA caldera (high pressure difference with outside the caldera), whereas during formation of the SW straits the caldera was already flooded (small pressure difference with outside the caldera).

Constraints on the onset of caldera flooding

Constraints on the timing of the caldera flooding event are provided by previous studies of the LBA eruption products, and of studies of the changes in morphology of the volcano resulting from the eruption and associated caldera collapse. The eruption took place in four main phases. It began with a Plinian phase (phase 1) from a subaerial vent, then became phreatomagmatic (phases 2 and 3) (refs 1, 2, 3, 4, 5). The main phase of the eruption (phase 4) involved outpouring of hot, fluidized pyroclastic flows, forming multiple ignimbrite (deposit from a pyroclastic flow) fans1,2,3,5.

Before the eruption, Santorini already had a shallow caldera that had formed during an eruption 18 ky previously5,18,19 (Fig. 6a). This ancient caldera was lagoonal, as inferred from fragments of travertine, stromatolites and brackish to marine fauna in the LBA ejecta20,21. The eruptive vent was situated outside the caldera lagoon during phase 1 of the LBA eruption, then migrated into it during phases 2 and 3, causing phreatomagmatic explosions3,5 (Fig. 6b). Phase 3, the most violent phreatomagmatic phase, used up most of the water in the lagoon, and built up a huge tuff cone that probably cut off any connection to the sea4. The subsequent eruption of hot, fluidized pyroclastic flows during phase 4 shows that, by the end of phase 3, the caldera was dry and the vents were subaerial3,5. Caldera collapse triggered by the LBA eruption deepened and widened the old caldera3,4,5. The pyroclastic flow deposits of phase 4 are rich in rock debris of diverse lithologies, suggesting that collapse took place mostly during phase 4 (refs 1, 3, 5) (Fig. 6c). Indeed it is known that caldera collapse takes place syn-eruptively at many calderas worldwide, although final settling may continue after the eruption22,23,24.

The newly collapsed LBA caldera must have been essentially dry by the end of the eruption, and isolated from the sea by thick accumulations of LBA tuff, because there was no return to phreatomagmatic activity following the hot pyroclastic flows of phase 4 (ref. 3). Moreover, phase 4 ignimbrite bordering each side of the NW strait is known from lithic thermal remnant magnetism analysis to have been emplaced hot (150–350°C) (ref. 25), and it contains none of the phreatomagmatic ashes typically produced when hot pyroclastic flows enter the sea. Hence, by the end of phase 3, the region between present-day Thera and Therasia islands must have been above sea level, probably due to a thick accumulation of phase-3 tuffs, with this barrier remaining intact throughout phase 4 until the end of the eruption (Fig. 6c). Then, once the caldera had largely collapsed, this wall of easily eroded tuffs failed, allowing the sea to rush in, accelerated by retrogressive landsliding into the caldera (Fig. 6d). That this occurred once the eruption was over would explain the apparent lack of LBA products on the floor of the NW strait (Figs 2d and 3). Finally, the two remaining SW straits collapsed once the caldera was already largely flooded, accounting for their fresh landslide morphologies (Fig. 6e).

Debris from these breaching events has not been recognized on seismic profiles inside the caldera (Figs 2d and 3). However, the high energy of water flow associated with the NW breach may have fragmented the debris and scattered it across a large area inside the caldera. We have estimated the total volume of rock removed to create the NW strait as 2.0–2.5 km3 (Supplementary Figs 5 and 6). The joint volume of seismic units 1, 2 and 3, in the northern basin, is only 0.94–1.1 km3 (as shown in figures 9–11 from ref. 10). The volume of material removed during the breach therefore cannot be accommodated in these units. Although unit 3 has been interpreted previously as the top of the LBA tuff succession10, we speculate that it might be composed of sediment deposited during the waning phase of the flood event, such as mass flows and suspension fallout of sediment from the highly turbid water column. The main layer of landslide and flood debris from the NW breach would then lie, unresolved, beneath unit 3, and the LBA intracaldera tuffs below that.

Dynamics and duration of flooding

Initially, the flooding event through the NW strait would have been analogous to those associated with dam failure, with rapid erosion of the retaining wall leading to inflow. However, lake- or dam-breach floods cease once the upper reservoir is empty. In the LBA case the upper reservoir (the sea) was effectively infinite, and the flood stopped when the lower reservoir (the caldera) filled up to sea level. Thus the late-stage dynamics was different: at the end of a dam failure, the water still flows down a similar topographic drop, giving it similar potential energy drop per unit volume of water, whereas in our case, the potential energy drop decreases as the caldera fills. The erosive power of the influx will also decrease as it flows into increasingly deep water. A close, if smaller-scale, analogue was the flooding of a Malaysian open cast tin mine, when the wall separating the mine from the sea collapsed26. A much larger-scale analogue was the flooding of the Black Sea 8,400 years BP27.

The time that it took to fill the caldera was constrained by numerical modelling of the water flow through the NW strait. The model used the depth-averaged equations for water flow and is described in the ‘Methods’ section; it neglects shoreline wave breaking, wave energy dispersion and the Coriolis force, but captures the first-order behaviour of water flow in a deep environment28. Similar models have been widely used in tsunami modelling29. The initial conditions for modelling were created as follows: the caldera floor bathymetry was modified by removing the post-caldera Kameni edifice, which would have been absent immediately following the LBA eruption. An artificial wall was placed across the two SW straits in order to prevent entry of the sea from this direction. The bathymetry of the northern strait was modified by reconstructing the original NW-dipping flank of the volcano using a conical surface with an outward-dipping 2°-slope, then setting the water depth in the breach (equal to the entry depth of the subsequent inundation flow) to a specified value by cutting the cone by a horizontal surface of that depth. In this way we were able to simulate caldera inundation through a series of entry channels of five different specified depths, from 20 to 300 m (Fig. 7). Finally an artificial wall was placed across the NW strait at its entry point into the caldera. This wall was then removed instantaneously in order to allow the sea to flow into the caldera through the NW strait.

Figure 7: Numerical modeling of flooding. Model set up and results for the flooding of the caldera resulting from the breach of the NW caldera wall. (a) Calculation domain for the numerical modelling, showing the initial (pre-flooding) conditions for the 20 m entry model presented in the Supplementary Movie 1. The black line represents the bathymetric profile shown in b. (b) Bathymetric profile along the traverse in a, showing the five different starting profiles with caldera-entry depths of 20, 50, 100, 200 and 300 m. The bathymetry was kept constant over the duration of each model. (c) Intracaldera sea water volume as a function of time for the five models. (d) Caldera filling time as a function of caldera-entry depth for final conditions of 5 and 1 m below sea level. Full size image

For each fixed entry depth, we modelled the inundation of the caldera, and measured the time for filling to −5 m of the final water level, then −1 m of the final water level (Fig. 7c,d). In reality, the entry depth, and indeed the bathymetry of the entire strait, would have evolved with time due to landslip into the caldera, followed by erosion by the rapidly flowing water; however, this time-variation is unknown and cannot be predicted by our model. By fixing the entry depth, and keeping the breach bathymetry constant in each model run, we place constraints on the possible range of caldera-filling times for entry depths >20 m (the lowest value we chose to simulate). The models simply predict caldera-filling times under a range of fixed conditions.

In the 20 m entry depth model, the inflow velocity reached 19 m s−1, the water flux reached 2.5 × 105 m3 s−1, and the filling time was about 50 h. In the 300 m model, the corresponding values were 45 m s−1, 92 × 105 m3 s−1 and 0.6 h. Hence, irrespective of the exact time-variation of breach bathymetry through landslip and water erosion, once the entry depth had reached 20 m, caldera filling would have proceeded to completion in <2 days, and possibly in as little as a few hours. Our model provides no constraints on how long it would have taken to initially erode the entry point to 20 m depth, but given that the NW strait was blocked by unconsolidated LBA tuffs, it seems very likely that, once breached, erosion to this depth would have proceeded very quickly.

Inflow of the sea through the NW strait could have generated large waves inside the caldera (with run-ups up to ∼200 m above the caldera floor on the eastern and southern cliffs), but no significant (amplitude <∼10 m) waves outside the caldera (Supplementary Movie 1).