## 4 Discussion
Poor weather conditions impeded a clear view of the summit crater area from the camera monitoring network, which only detected the mid-lower portion of the slope along which the lava flowed and PDCs were spreading. However, we could obtain the average spreading velocity of several lava flows and PDCs descending down the SdF slope (Table 4) and on the sea surface (Table 3). These data show an increase of the speed of the PDCs spreading on the sea surface from 6.9 m s\({}^{-1}\) to 23.3 m s\({}^{-1}\) between 01:50 and 02:51 on 31 March, which later decreased to 5.9 m s\({}^{-1}\) until 03:41. This is consistent with the greater speed of PDCs and lava flows along the SdF slope that was detected during the same lapse of time (Table 4), which led us to assume that the greater PDC velocity on the sea surface was probably caused by an initial greater thermal efficiency of the PDC starting from a higher elevation and flowing along the slope above the active, well-fed and hot lava flow. This might have increased PDC mobility, allowing for it to reach the coast and spread on the sea surface at high speed. By comparison, the velocity of some PDCs emplaced in 1997 at Montserrat by dome collapses whose velocity was between 8 and 21.9 m s\({}^{-1}\)[93; 94], whereas much greater values of \(\sim\)100 m s\({}^{-1}\) were obtained for the PDC that was emplaced during the 1888 phreatic eruption at Bandai Volcano [95]. Several PDCs on 30 and 31 March at Stromboli spread on the sea surface up to a maximum distance of \(\sim\)165 m from the coast, with an estimated speed up to \(\sim\)23 m s\({}^{-1}\) (Table 3). This velocity compares pretty well to the measurement of more than 27.8 m s\({}^{-1}\) (more than 100 km h\({}^{-1}\); [PERSON], INGV unpublished report, 29 January 2003) obtained on a video of the PDC spreading on the sea surface for more than 100 m during the landslide and flank collapse of Stromboli on 30 December 2002. Similar PDCs spread on the sea surface at Stromboli in 2014 during the initial stages of the flank eruption, propagating for several tens of meters from the coast at speeds of 5.9 and 9.8 m s\({}^{-1}\)[33]. The velocity to which the PDC expanded on the sea surface decreased along with the supply to the lava flow that was descending down the slope; at the same time, the PDC was travelling a shorter distance along the SdF slope (Table 4), thus causing a slower expansion of the PDC along the slope and reaching the coast at a lower speed. The measured distance that was travelled by the PDC on the sea surface from the coast varied between 108 and 165 m, with speeds between 4.4 and 23.3 m s\({}^{-1}\) (Table 3). Hence, we understand that also small volume PDCs, like those produced during 28 March-1 April 2020 at Stromboli, can spread on the sea surface for hundred meters distance from the coast, causing a potential hazard for bathers, fishermen, or touristic boats sailing along the North coast of the island. Moreover, it must be considered that the ash cloud spreading backwards from the sea to the craters might produce ash fallout that could possibly reach tourists trekking along the SdF margins, even at lower heights. PDCs spreading on the sea surface are not uncommon on Stromboli, and they surely represent an underestimated threat. They occurred during the subaerial and submarine landslide that caused a tsunami in December 2002 [16; 17; 96]; during the initial phases of flank effusive eruptions in 2002-2003, 2007, and 2014 [32; 36; 39]; after paroxysmal explosive eruptions triggered by column collapse [39; 97; 98]; and every time there is a small failure (with volumes of the order of 10\({}^{4}\) or 10\({}^{5}\) m\({}^{3}\)) of the summit craters due to overloading or instability [33; 34]. The two most impressive episodes occurred in 2019 as a result of the 4-5 km high eruptive column collapse after the paroxysmal explosions of 3 July and 28 August.