Dynamics of the 1873 CE “Breccia De Fiore” phreatic eruption at Vulcano (Aeolian Islands, Italy) through historical chronicles, physical volcanology, and numerical modelling

doi:10.21203/rs.3.rs-4545492/v1

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Dynamics of the 1873 CE “Breccia De Fiore” phreatic eruption at Vulcano (Aeolian Islands, Italy) through historical chronicles, physical volcanology, and numerical modelling

https://doi.org/10.21203/rs.3.rs-4545492/v1

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Phreatic events may represent precursors of magmatic eruptions, but they can also occur as single or multiple episodes punctuating the activity of volcanoes characterised by active hydrothermal systems. The Breccia De Fiore deposit represents evidence of the phreatic activity preceding by fifteen years the onset of the 1888–1890 magmatic eruption at La Fossa di Vulcano. This study integrates historical chronicles, sedimentological and physical descriptions of the deposit, and 3D numerical simulations to reconstruct eruption dynamics. Results indicate that this deposit was the product of multiple, shallow explosions of low magnitude (< 5×10⁴ m³ cumulative volume), possibly occurred during a short time interval at the beginning of the phreatic sequence in 1873. The explosions produced pyroclastic density currents and ballistic fallout, affecting the slopes of the La Fossa cone. The asymmetry of the deposit, outcropping on the north-western flank of the cone, is well reproduced by 3D simulations as the product of explosions from an inclined vent driven by shallow (i.e., < 150 m deep) pressure build-up (up to 5 MPa) from accumulating magmatic fluids. Simulations of explosion dynamics and the distribution of eruptive products allows us to put some constraints on the two main controlling parameters of phreatic scenarios: erupted total mass and specific energy. This study also highlights the potential impact of such relatively small phreatic explosions on the Vulcano island and the need for quantifying their hazards, especially given the proximity to populated sites and the growing tourism in this active volcanic area.

Phreatic eruption dynamics

Vulcano Island

Pyroclastic density currents

Ballistic fallout

Numerical modelling

Hazard

Vulcano Island (38°24’N, 14°58’E, 22 km²) is an active volcanic centre in the southernmost sector of the Quaternary Aeolian arc (Tyrrhenian Sea, Italy; Fig. 1). Renowned for its distinctive “Vulcanian” eruptions (Clarke et al. 2015), it also hosts an active hydrothermal system, which manifests through a high temperature fumarolic field, extensive CO₂ soil degassing, and rock alteration (Fulignati and Sbrana 1998; Di Giuseppe et al. 2023; Federico et al. 2023).

Since the last eruptive cycle in 1888–1890, the activity has been characterised by continuous fumarolic outgassing, concentrated at the summit of the La Fossa cone and Porto di Levante area (Fulignati et al. 2006; Paonita et al. 2013; Federico et al. 2023). Several unrest episodes occurred over the past century have been attributed to changes in heat and fluid supply to the hydrothermal system, and permeability variations of host rocks (e.g., Badalamenti et al. 1988, 1991; Barberi et al. 1991; Chiodini et al. 1995; Diliberto et al. 2002; Harris et al. 2012; Paonita et al. 2013; Federico et al. 2023). The first recorded episode in 1916 saw increased gas flux and temperatures (up to 416°C) in the fumaroles of the northern rim of the La Fossa cone (De Fiore 1922). Subsequent episodes of increased fumarolic gas discharge, temperatures, and exhaling surface areas occurred in 1978, 1987–1993 (when the fumarolic area reached temperatures up to 690°C), 1996–1998, 2004/2005, and 2021–2023 (Barberi et al. 1991; Chiodini et al. 1995; Harris et al. 2012; Capasso et al. 2014; Federico et al. 2023).

The recent unrest (2021–2023), attributed to increased heat and gas fluxes from a deeper magmatic source (Aiuppa et al. 2022; Inguaggiato et al. 2022; Federico et al. 2023), heightened the scientific interest in potential phreatic eruptions on the island, especially with the growing popularity of Vulcano as a tourist destination. The population varies from 800 during winter to peaks of 22,000–28,000 visitors in July and August (Bonadonna et al. 2021), raising risks even in the case of small-scale events.

Historical and stratigraphic records at Vulcano have provided evidence of at least three significant phreatic events (> 10⁴ m³) in the past 1000 years (Frazzetta et al. 1983, Gurioli et al. 2012, Rosi et al. 2018; Selva et al. 2020). Between 1873 and 1888, the La Fossa volcano experienced small-scale explosive activity, including low-magnitude phreatic events, according to historical descriptions of explosion dynamics and ejected products (Mercalli 1886; Mercalli and Silvestri 1891), as well as stratigraphic evidence (this work; Di Traglia et al. 2024).

We present a compilation of historical information on the 1873–1888 explosive activity and describe the 1873 phreatic deposit based on field survey and laboratory data. We then use a physical model and three-dimensional multiphase flow simulations to explore explosion dynamics. Comparing historical accounts, volcanological data, and simulation results, we discuss the eruption dynamics and associated hazard.

Geological background and last 1000 years of activity at La Fossa

Vulcano Island formed from the progressive coalescence of diverse volcanic structures over approximately 120 ky (Keller 1980; Frazzetta et al. 1983; De Astis et al. 2013). The La Fossa composite cone (391 m asl), along with Vulcanello, is the most recent volcanic centre, rising in the middle of its namesake caldera. Over the past 5.5 ky, La Fossa has experienced multiple eruptive cycles and craters (Fig. 1), producing explosive (phreatomagmatic/phreatic, Vulcanian, Strombolian, Sub-Plinian) and effusive activity (Keller 1980; Frazzetta et al. 1983, 1984; De Astis et al. 2013; Di Traglia et al. 2013, 2024; Selva et al. 2020). To avoid ambiguity, we adopt the term phreatic to denote shallow explosive events driven by the expansion of steam and/or gas associated with an increase of heat and gas flux from magma stored in a relatively deep reservoir (Mastin 1995; Browne and Lawless 2001). Erupted products range from latites to rhyolites, with minor shoshonites (Keller 1980; Frazzetta et al. 1983; Gioncada et al. 2003; Boyce et al. 2007; De Astis et al. 2013; Costa et al. 2020).

La Fossa has been particularly active in the last 1000 years (Fig. 2a). Di Traglia et al. (2013) initially divided this period into two main Eruptive Clusters: the Palizzi-Commenda Eruptive Cluster (XII–XIII century) and the Gran Cratere Eruptive Cluster (XV century–1890 CE). Later, Di Traglia et al. (2024) refined this into the Palizzi, Commenda, Fossa-Forgia Vecchia, Pietre Cotte, and Gran Cratere eruptive units, separated by seven erosive unconformities.

The Palizzi sequence, starting in 1150 CE from summit vents, involved four explosive and two effusive phases, the latter producing the Commenda rhyolitic lava and the Punte Nere, Campo Sportivo, and Palizzi trachytic lava flows (Gurioli et al. 2012; Di Traglia et al. 2013, 2024; Rosi et al. 2018).

The Commenda unit comprises the lithic-rich Breccia di Commenda deposit (1230 ± 40 CE; Gurioli et al. 2012; Rosi et al. 2018) emplaced as pyroclastic density currents (PDCs) and minor fallout, followed by a long-lasting ash fall recorded as a sequence of thinly stratified reddish ash layers named “Tufi Varicolori” (Frazzetta et al. 1983; Capaccioni and Coniglio 1995; Dellino et al. 2011; Rosi et al. 2018) that concurred in building part of the La Fossa cone. The Breccia di Commenda event represents the largest (> 2×10⁶ m³) phreatic eruption recorded on Vulcano Island (Gurioli et al. 2012; Rosi et al. 2018).

The Fossa-Forgia Vecchia period (XV–XVI centuries) is characterised by alternating phreatic and magmatic events from La Fossa, including the phreatic deposits expelled from two lateral vents: Forgia 1 (1444 CE) and Forgia 2 (1444–1550 CE). This activity was followed by the Pietre Cotte pyroclastic units and the overlying Pietre Cotte rhyolitic lava flow (1631–1786 CE; Di Traglia et al. 2024).

The uppermost sequence of the Gran Cratere unit was emplaced after 1786 CE (Di Traglia et al. 2024). It includes phreatic deposits emplaced between 1873 and early August 1888 (Fig. 2a; GCa in Di Traglia et al. 2024) and the subsequent magmatic Vulcanian eruption of 1888–1890 (GCb in Di Traglia et al. 2024).

Since 1890, La Fossa has been characterised by continuous fumarole and soil degassing (Selva et al. 2020; Inguaggiato et al. 2022; Aiuppa et al. 2022; Federico et al. 2023).

The 1873–1888 activity from contemporary chronicles

The La Fossa crater has been industrially exploited since at least 1800 for the extraction of sulphur and alum and the production of boric acid. Possibly also for these reasons, crater changes or activity have been often described in detail by local observers.

A few years before 1873, La Fossa was characterised only by mild fumarolic degassing (Salino 1874; Mercalli 1883; Mercalli and Silvestri 1891; De Fiore 1922). Approximately two months before the onset of an explosive crisis, in July 1873 workers within the La Fossa crater witnessed intensified outgassing and uncommon hiss from fumarolic vents (Salino 1874). An enlargement of the exhaling area was also observed (De Fiore 1922). This escalated in a series of explosive events affecting the flanks of the cone from 1873 to 1886, including the opening phase (3–5 August) of the 1888–1890 eruption.

Explosive activity clustered into four main eruptive phases: 1873, 1878/1879, 1886 and 1888 (Fig. 2b). Mercalli (1886) and Mercalli and Silvestri (1891) described the material ejected in 1873, 1879, 1886, and the early August 1888 eruptions as consisting of non-juvenile, variably altered fragments.

The September-October 1873 activity was directly observed by Ambrogio Picone, mining director on the island between 1873 and 1876; his descriptions depicted the 1873 activity as impulsive, marked by repeated short events of varying intensity (Fig. 2c; Salino 1874; Mercalli and Silvestri 1891; De Fiore 1922). Noticeable fumarolic activity preceded each explosion and, a few minutes before an explosion, loud noises emanating from inside the crater were heard, accompanied by changes in fumarolic emissions. Explosions lasted for 10 to 15 minutes and occurred at intervals of 20 to 30 minutes. These events ranged from simple ash emission to explosions ejecting also blocks (De Fiore 1922). Larger ejecta were projected up to 800 m above the eruptive vent, with most landing near the La Fossa crater rim and on the northern flank, while only a few settled within the crater. Over the 44-day-long activity, at least 10 explosions ejecting blocks occurred at intervals ranging from one day to a few more than a week. In many cases, historical chronicles do not specify if ash or coarser material was ejected (Fig. 2c).

Ground inflation and deflation were also observed during the explosions, while no obvious seismic activity was described to precede or accompany the explosive events. Despite not causing damage to the inhabited area of Vulcano Porto, ash covered the island for days before being completely remobilised by rainfall and winds. Occasionally, prevailing winds carried the ash to the neighbouring islands of Salina and Lipari (Salino 1874; De Fiore 1922).

Between 1873 and 1879, at least six eruptive vents opened within the La Fossa crater, with four of them excavated during the 1873 event (Fig. 2d; Mercalli 1883; Mercalli and Silvestri 1891; De Fiore 1922).

Fieldwork focused on identifying and describing the stratigraphic, sedimentological, and physical features of the 1873 phreatic deposit in the north-western flank of the La Fossa cone, with additional investigations on the southern slopes. Stratigraphic logs were compiled at each site, correlating stratigraphies and collecting samples from the main stratigraphic units at the most complete outcrops. Samples were mechanically dry-sieved at half-phi intervals (phi=-log₂(d/d₀), where d is particle diameter in millimetres and d₀ = 1 mm) for the fraction ≥ 250 µm. The fraction below 250 µm was analysed using the Malvern Morphologi G3 particle analyser instrument. Grain-size distributions were obtained combining data from both techniques, assuming constant density across different classes. Abundances of particles with diameters of 355 and 250 µm identified by the particle analyser were combined with the size bin immediately below (i.e. ϕ = 2.0–2.5 or 177–255 µm).

Isopach maps were compiled for the different stratigraphic units. Given the complex isopach shapes and lack of a regular thickness attenuation with distance, we estimated the volume of the deposit using the trapezoidal rule (Froggatt 1982; Fierstein and Nathenson 1991), avoiding thickness extrapolation with distance implied in other methods. The uncertain eruptive vent location and unknown deposit distribution in the proximity of the source (tens of meters) complicated isopach contour tracing. Consequently, our calculations might underestimate thickness in very proximal areas. Volume determination considers two different extensions of the zero isopach: one based on measured data and another assuming a larger affected area up to approximately 1 km from the La Fossa north-western crater rim.

To model the explosion and dispersal of the ejected products, the pre-1888 morphology of the La Fossa crater and cone were reconstructed from the 1868 topographic map of Vulcano Island. The historical map, obtained from the Italian Istituto Geografico Militare (IGM), was geo-referenced using a recent model (1 m of resolution) of Vulcano Island as a reference, aligning areas with similar morphology. Contour lines were digitised at 5 m intervals for the La Fossa crater and 10 m for the cone flanks, approximating the vertical accuracy of the map. The QGIS open-source software was used to extract contour lines and generate two interpolated Digital Elevation Models (DEMs) with a 1 m grid, which were then merged into a unified DEM with a 2 m grid using an appositely written Python script to minimise altitude differences in overlapping zones.

The “Breccia De Fiore” deposit

Stratigraphy

A complex stratified breccia and tuff deposit rich in hydrothermally altered clasts, was found in the north-western flank of the La Fossa cone (Fig. 3a). This deposit, named here Breccia De Fiore (BDF) after Prof. Ottorino De Fiore, who extensively documented the activity of La Fossa in the 1873–1888 period, is interbedded between the deposits of the Pietre Cotte eruptive sequence and the products of the last magmatic eruption of 1888–1890 (Fig. 3b).

Based on eyewitness accounts reporting vent opening in the northern sector of the La Fossa crater (Mercalli 1883), the duration of the activity (a total of 44 days), and field observations, we confidently attribute the BDF to the 1873 eruptive phase, the most intense phreatic crisis between 1873 and early August 1888. Because of their stratigraphic features, further phreatic deposits below the 1888–1890 Vulcanian phase cropping out in the south-eastern flank and south-western crater rim (Fig. 3a) were not correlated with those in the western and north-western side, and they were associated to different eruptive episodes.

The BDF is a very poorly sorted lapilli tuff or tuff breccia, with centimetric to pluri-decimetric thickness (45 cm on average), composed of lithics ranging from fine ash to coarse lapilli and minor blocks. Clasts are primarily fragments of old lavas and tuffs showing variable alteration (Fig. 3c). No clear juvenile material was recognised in all the samples, according to the phreatic nature of the deposit. Despite its complexity, the BDF exhibits distinguishable characteristics, such as sedimentological, grain size, and colour variations, allowing the identification of five layers generally separated by planar surfaces, denoted as layer A to layer E.

Layer A (3–30 cm) is a grey, matrix-supported, fine lapilli-bearing layer at the base of the BDF sequence (Figs. 4 and 5). It usually lays on an erosive surface cut on a brownish ash deposit at top of the Pietre Cotte cycle or on reworked deposits related to lahars or slope grain flows. The amount of ash ranges between 35 and 90 wt.%, with the highest values corresponding to the more lateral and distal outcrops (Table S1). It varies from massive, weakly reversely graded, poorly sorted (e.g., sections 1, 3, 15; Figs. 4a, e, and 5) to stratified, with alternations of well-sorted, reversely graded, fine lapilli beds and coarse to fine ash beds (Fig. 4b, g). In its uppermost portion, it is often characterised by a concentration of coarse lapilli (2–5 cm in diameter) and minor blocks with average diameter of 11–15 cm (Fig. 4a, f). Layer A coarsens at section 10, where a laterally discontinuous, openwork-textured bed of blocks and coarse lapilli is present at the top (Figs. 4d and 5). At outcrops in drainage channels close to the base of the cone (i.e., section 14), this layer is thickened by reworked material forming alternations of parallel-bedded ash and lapilli layers (Figs. 4c and 5). At the most distal sector, layer A grades into a weakly laminated thin level of grey to yellowish, coarse ash (Figs. 4h and 5).

Layer B (1–18 cm) is an ash rich, matrix-supported, laminated to stratified reddish to brownish bed (Table S1). It is formed by an alternation of well-sorted, fine to coarse ash up to fine lapilli beds (Fig. 4b and sections 8, 10 in Fig. 5). Locally, layer B shows weak laminations and reverse grading (Figs. 4a, e, f). It is absent in the most lateral sections (i.e., sections 4 and 9) and at the base of the cone (i.e., sections 5, 14, and 16; see Figs. 4c, g, h, and 5).

Layer C (2–35 cm) is grey, matrix-supported, and fine to coarse lapilli-bearing (Table S1), with an internal structure varying from massive to stratified (Fig. 4g, f). In most cases, stratification includes alternating levels of fine to coarse ash and fine to coarse lapilli. Coarse, lapilli-bearing lenses (e.g., section 1) and scattered clasts of 5–10 cm of diameter are locally present (Fig. 4f, g). At the foot of the north-western flank of the cone, this layer is represented by a 2-cm-thick massive bed of fine to coarse ash (Figs. 4h and 5). Layer C is absent in the easternmost outcrops (i.e., sections 10, 11).

Layer D (1–39 cm) is the most characteristic and coarsest bed of the sequence (Table S1). It is a yellow, clast- to locally matrix-supported layer composed of moderately-to-well-sorted, fine to coarse lapilli and scattered blocks. It is in all sections, except for the easternmost (section 10; Figs. 4d and 5). It is generally massive, with the exception of section 3 where it is organised in a clast-supported basal portion of moderately sorted, coarse lapilli, a central portion of fine to coarse lapilli with abundant ash, and a clast-supported, poorly sorted top portion composed of coarse to fine lapilli (Fig. 4e). It can also show reverse grading (Fig. 4b, e).

Layer E (3–18 cm) caps the whole BDF eruptive sequence in almost all outcrops (Figs. 4 and 5). It is a hardened, massive, matrix supported, grey to pinkish layer composed of fine to coarse lapilli and rare blocks in a vesicular, fine, cohesive ash matrix. The matrix content tends to increase moving down to the base of the cone. Vesicles are mostly concentrated in the matrix at the base of the layer.

Contrary to layers A, B, C, and D, which are thicker in the north-western sector of the cone and can show lateral thinning at the scale of the outcrop (Fig. 4), thickness of layer E is higher in proximal (i.e., sections 1, 3, 12) and distal (i.e., sections 14 and 15) zones compared to the central outcrops (see sections 5 and 8 in Fig. 5).

Sample grain-size and Total Grain Size Distribution (TGSD)

Eight sections located in the western (i.e., 1, 2, 3, 4), eastern (i.e., 10), central (i.e., 15) and distal (i.e., 14, 16) sectors of the BDF deposit were selected to investigate grain-size variations. Grain-size data and statistical parameters are listed in Table S1. Samples generally show multi-modal grain-size distributions that can be deconvoluted in 2 to 4 normal subpopulations (SP-A to SP-D, Table S2) of variable weight in each sample. The mean values of these subpopulations cluster into 4 groups and, on this basis, average mean (µ) and sorting (σ) values for each of the 4 subpopulations can be defined: SP-A, µ = -3.7 φ and σ = 0.7; SP-B, µ = -1.6 φ and σ = 1.1; SP-C, µ = 0.9 φ and σ = 0.14; SP-D, µ = 5.5 φ and σ = 1.0.

The coarsest products are usually associated with layers A and D, often presenting a very abundant mode (up to 25–30 wt.%) in the coarse lapilli fraction. Grain size distributions of layers C and E are generally mostly centred on the fine lapilli to coarse ash fractions. Layer B represents the finest grained bed, with an important presence of medium to fine ash.

In terms of SPs, the coarsest SP-A is never present in layer B samples, the latter being dominated by SP-C (around 70 wt.% of the total). Contribution of SP-A is instead important for A and D layers (on average 20 wt.% and 35 wt.%, respectively, for all the samples, except for the most distal ones), while values generally lower than 17 wt.% characterise layers C and E. The finer grained SP-D is always present in all the layers, although in a very low amount (generally lower than 5 wt.%).

Given the good areal coverage of the analysed samples, grain-size data from all the stratigraphic sections and layers were weighted by the relative thickness of each layer to estimate a total grain size distribution (TGSD) for the whole deposit. TGSD was estimated both directly averaging weighted grain size distributions of each sample or their deconvolution into the 4 normal subpopulations listed above (Table 1). Figure 6a shows a bimodal TGSD, with peaks at ∼-4 and 1 ϕ and a clear negligible contribution of fine ash.

Table 1

TGSD of the BDF deposit. Data derived from the weighted sum (based on relative thickness) of the grain size distribution of each sample (left) and from the weighted contributions of each normal sub-population (right)
Phi	Wt.%	SP	Mean	Sorting	% of the total
-6	0.00	A	-3.7	0.7	17.8
-5	2.39	B	-1.6	1.1	21.0
-4	9.24	C	0.9	1.4	59.9
-3	12.13	D	5.5	1.0	1.3
-2	10.38
-1	13.21
0	16.46
1	15.10
2	10.57
3	5.92
4	1.98
5	1.32
6	0.76
7	0.38
8	0.14
9	0.02
10	0.00

Isopach maps and volume estimation

Figure 6b displays the isopach map for the total BDF deposit, obtained from the integration of thickness data (see Materials and Methods section). The absence of the deposit along the most proximal slopes of the cone is possibly due to non-deposition related to the high steepness of the upper part of the cone slope, as well as to the intense post-eruption erosion of the pyroclastic cover in this sector of the volcano (Ferrucci et al. 2005). The deposit shows a thickness of 50 cm at about 130 m from the north-western crater rim. Thickness decreases to 5 cm at the base of the cone, about 400 m from the north-western crater rim. Isopach maps for layers A, B, C, and D are reported in Fig. S3.

From isopach maps, we estimated the volume of the total BDF deposit at 3.7– 4.6×10⁴ m³ andindividual layers between 8.5×10³ m³ and 1.9×10⁴ m³, with layer A representing the most dispersed and voluminous layer (Table 2). These values are importantly affected by the extension of the zero isopach contour (Table 2). Assuming a deposit density of 1300 kg/m³ and a porosity of 35–40%, we estimated an erupted mass of 4.8–6×10⁷ kg for the whole BDF deposit, with a maximum value of 8×10⁷ kg considering potential losses. Our estimates may on one hand underestimate the deposit volume, as they may not encompass: i) fine-grained distal material that could have been dispersed by convection/diffusion (the low relative weight of the fine-grained SP4 in the TGSD however suggests a limited effect of this process); ii) the material that might have been lost due to remobilization phenomena, erosion, and elutriation of fine particles (the latter however not clearly described in the chronicles); iii) the material resedimented inside the La Fossa crater and not found in PDCs deposits (at least 20%). On the other hand, the estimation method potentially overestimates the volume, especially with a limited number of thickness data (Fierstein and Nathenson 1992).

Table 2

Volume estimated for the total BDF and for layers A, B, C, and D
	Case^a	Volume [m³]
Total BDF deposit	1	3.66×10⁴
Total BDF deposit	2	4.59×10⁴
Layer A	1	1.57×10⁴
Layer A	2	1.94×10⁴
Layer B	1	6.37×10³
Layer B	2	8.52×10³
Layer C	1	1.24×10⁴
Layer C	2	1.48×10⁴
Layer D	1	6.46×10³
Layer D	2	1.01×10⁴
^a Cases 1 and 2 represent volume calculations considering a smaller (1) and a greater (2) extension of the zero isopachs

Physical and numerical modelling

Field evidence suggest that the BDF was emplaced through PDCs and ballistic fallout (see the Discussion section). Our numerical modelling is aimed at assessing whether a small volume (i.e., < ∼10⁵ m³), shallow phreatic explosion could produce PDCs and ballistic fallout matching field observations. We also investigate the role of vent geometry and initial pressure and temperature conditions on the eruptive cloud and product distribution.

We simulate an explosion of magnitude in the estimated range for either the total BDF deposit or a single layer. We assume a scenario involving gas accumulation and overpressure build-up at shallow depths beneath the La Fossa crater, exceeding the tensile strength of rocks (Browne and Lawless 2001; Stix and De Moor 2018), followed by explosive decompression of a mixture of gases and fragmented material in the atmosphere.

The simulations use the transient, three-dimensional, multiphase flow model OpenPDAC (see S4 for model; Neri et al. 2003; Esposti Ongaro et al. 2012), describing the explosion dynamics of a gas and fine particles mixture (Eulerian-Eulerian description) one-way coupled with coarser ballistic particles (Lagrangian model; de’ Michieli Vitturi et al. 2010). The model simulates the sudden decompression of high-temperature, high-pressure fluids (water vapour) and fragmented rocks, resulting in a multiphase mixture ejected into the atmosphere.

Initial conditions for simulations

We explore a range of initial conditions (e.g., erupted volume/mass, explosive energy, temperature, pressure) to reproduce some of the main observed deposit feature. To set initial and boundary conditions, we identified a set of observables constrained by field and volcanological data to define the eruptive scenarios. We imposed geometric boundary conditions (crater and volcano morphology), total ejected solid mass, pressure of the water vapour, and temperature of the erupted material. We also defined parameters for the physical properties of the eruptive mixture, including gas composition and particle size and density. Input parameters for eruption scenarios are summarized in Table 3.

Table 3

Input parameters and results of numerical modelling. ID: simulation number; R/D: vent radius and diameter; V, M: ejected volume and mass of particles (considering particle density of 2000 kg/m³); ɑ_p: fine particle volume fraction within the eruptive vent; P, T initial pressure and temperature of the gas-particle mixture in the vent; d_p: fine particle diameter; E, E_m: total explosive energy and specific energy (considering a bulk density of 600 kg/m³); F, Cr, Co: percentage of fine particles within PDCs, the La Fossa crater, and the eruptive column, respectively.
ID	R/D	i, d	V [m³]	M [kg]	α_p	P [MPa]	T [°C]	d_p [mm]	E [×10¹¹J]	E_m [kJ/kg]	F [%]	Cr [%]	Co [%]
1	20/100	0°	2.5×10⁴	5×10⁷	0.3	5	250	1	5.8	12	21	31	48
2	20/100	21°, W	2.5×10⁴	5×10⁷	0.3	5	250	1	5.8	12	22	22	56
3	20/100	21°, NW	2.5×10⁴	5×10⁷	0.3	5	250	1	5.8	12	17	12	71
4	15/150	21°, W	2.1×10⁴	4.2×10⁷	0.3	5	250	1	5.8	12	20	40	40
5	20/100	21°, W	2.5×10⁴	5×10⁷	0.3	7	250	1	8.5	17	7	17	76
6	20/100	21°, W	2.5×10⁴	5×10⁷	0.3	3	250	1	3.2	6	53	35	12
7	20/100	21°, W	5×10⁴	1×10⁸	0.6	5	250	1	3.3	3	38	60	2
8	20/100	21°, W	2.5×10⁴	5×10⁷	0.3	5	400	1	5.8	12	9	8	83
9	20/100	21°, W	2.5×10⁴	5×10⁷	0.3	5	250	0.177	5.8	12	35	6	59
10	10/100	0°	6.3×10³	1.3×10⁷	0.3	5	250	1	1.5	12	6	18	76
11	10/100	21°, W	6.2×10³	1.2×10⁷	0.3	5	250	1	1.5	12	11	13	75
12	10/100	21°, NW	6.3×10³	1.3×10⁷	0.3	5	250	1	1.5	12	18	10	72
13	10/100	21°, W	1.3×10⁴	2.5×10⁷	0.6	5	250	1	0.83	3	47	52	1

Pre-eruption topography, vent position and geometry were based on available historical data and field evidence. The 1888–1890 eruption significantly modified the La Fossa cone topography and the crater area (Fig. 7), which was filled by approximately one-third (Mercalli and Silvestri 1891; De Fiore 1922). To simulate an eruption in a topographic setting like that before 1888, we used a reconstructed DEM from the digitisation of the 1868 contour line map of Vulcano Island (see Materials and Methods section). Given the absence of any residual structure related to the phreatic vents formed in 1873 and according to the descriptions of Mercalli (1883), we positioned the explosion vent inside the La Fossa crater, near the northern crater wall. This vent was modelled as a prolate hemispheroid shape with specific radius (semi minor axis) and depth (semimajor axis). Historical descriptions (Mercalli 1883) suggest the explosion vents possibly measured up to 20 meters in diameter. However, to accommodate a predetermined volume of particle and gas, we assumed vent diameters ranging from 20 to 40 m, with depths of 100 to 150 meters. In some scenarios, the geometry of the source area was inclined at a 21° angle with respect to the vertical axis by translating horizontally the base of the hemispheroid.

Erupted solid mass was constrained by field data (see Isopach maps and volume estimation section), thus all our eruption scenarios fall below the maximum value of hypothesized erupted mass (i.e., 8×10⁷ kg).

The initial temperature of the gas-particle mixture was set at 250°C as minimum value, based on stable mean values recorded at the fumaroles at the summit of La Fossa before the 2021 unrest (Diliberto 2021). Although higher values cannot be excluded, we considered a maximum initial value of 400°C, in the range of the peak temperature registered throughout the 2021–2023 crisis (Federico et al. 2023).

We assumed homogeneous pressure within the eruptive vent, exploring a range of 3–7 MPa (Table 3) based on lithostatic pressure and rock fragmentation thresholds. At a depth of 100 to 150 m below the La Fossa crater, the lithostatic pressure ranges between 1.9 and 2.9 MPa (considering an average rock density of 2000 kg/m³). The initial pressure in the vent must be higher than these values. Typical fragmentation thresholds (Spieler et al. 2004; Scheu et al. 2006; Koyaguchi et al. 2008; Mayer et al. 2016) are between 2 and 5 MPa (e.g., Mayer et al. 2015, 2016; Montanaro et al. 2016). Because we do not model fragmentation and the energy spent in fragmentation is typically in the order of 10–15% of the total energy (Montanaro et al. 2016), we assumed that the initial overpressure is of the same order of magnitude of the rock fragmentation threshold.

The total explosive energy (E), expressed in Joules, was calculated using an ideal gas model, considering adiabatic and reversible expansion of water vapour (Montanaro et al. 2016):

$$E=\left[\frac{{P}_{0}\times {V}_{wv}}{\gamma -1}\right]\times \left[1-{\left(\frac{{P}_{atm}}{{P}_{0}}\right)}^{\frac{\gamma -1}{\gamma }}\right]$$

where P₀ represents the initial pressure (Pa) within the vent prior to the explosion, V_wv is the initial volume of water vapour (m³) in the vent, P_atm is the atmospheric pressure (Pa), and γ is the ratio of specific heats (i.e., about 1.3 for water vapour). From the average density per unit volume (J/m³), the initial particle volume fraction, and fine particles average density (2000 kg/m³) we calculate the average specific energy (energy per unit of mass) available in the explosion in the range 3–17 J/kg (Table 3).

The initial porosity of the host rocks determines the volume fraction of gas in the explosion vent, thus it defines the specific energy available in the explosion (Montanaro et al. 2016). To account for this factor, we set a fixed volume fraction of gas, and consequently of particles, within the eruptive vent. However, this value does not directly correlate with the actual initial porosity of source rocks, as it represents, in our simulations, the amount of gas and hence the energy available for expansion after fragmentation (fragmentation energy is not considered in the calculation). As initial conditions, we either assumed a porosity of 40%, based on studies on rocks involved in phreatic/hydrothermal explosions (Mayer et al. 2015, 2016), and a higher porosity of 70% considered as the porosity after fragmentation and just before the initial expansion.

To simplify the numerical computation and focus on the large-scale eruptive dynamics, we have used the two-phase (i.e., gas plus monodispersed solid particles) version of the Eulerian-Eulerian flow model, which requires the use of a single, hydraulically equivalent grain size and density (Carcano et al., 2014). This can be computed as the first-order moment of the TGSD. According to TGSD calculations (refer to Sample grain-size and Total Grain Size Distribution section) we chose a single class of 1 mm in diameter and 2000 kg/m³ of density as the fine material composing the erupted mixture. Future studies will address this modelling aspect more specifically.

Modelling results

Simulations describe the generation and propagation of an eruptive mixture of water vapour, ash (1 mm in diameter), and coarser particles or ballistics (5, 10, and 20 cm in diameter) over time in three-dimensional space. Three main stages are identified: i) an initial decompression phase (5 to 7.5 s) involving the expansion of the eruptive mixture and acceleration of ballistics; ii) a column formation phase where the gas-ash mixture is driven vertically by buoyancy and coarser particles follow quasi-ballistic trajectories; and iii) a column collapse and PDCs propagation phase.

Run 1 is our first-guess scenario, with a vertical vent of 20 m radius and 100 m depth, hosting a total solid mass of 5×10⁷ kg at 250°C and 5 MPa, resulting in a specific energy of 12 kJ/kg (Table 3). In the initial phase (∼5 s), the gas particle-mixture expands up to 550 m above the vent at 191 m/s (average between particle and gas velocity; Fig. 8a), driven by gas decompression. Ballistics are ejected at maximum angle of about 55° (with respect to the vertical axis) and accelerated to 140 m/s by the gas-particle drag force (de’ Michieli Vitturi et al., 2010; Fig. 8a). After this phase, convection drives the mixture upwards, mixing with the surrounding atmosphere. In the north-east side, this mixture expands (see Carcano et al. 2014) forming a PDC climbing over the crater rim (Fig. 8b) but remaining confined within the outer C-PC crater rim (Fig. 9a). At 20 s, the mixture starts partially collapsing from 1.1 km asl (Fig. 8c), generating two PDCs, one east-directed and a smaller one travelling northward (Fig. 8d) and reaching 700 m of distance after 60 s. The remaining part of the mixture reaches 1.8 km asl (i.e., the maximum vertical size of the domain).

Ballistics reach a maximum height of 850 m above the vent and decouple from the mixture at about 13 s. By 30 s, nearly all of them have landed, largely preceding the PDCs phase. They distribute radially around the eruptive vent, up to 700 m of distance (Fig. 10a). It is important to remark that the number of ballistics on the ground is not representative of the real number observed on the field, as this number results from the combination of the defined vent size and an assumed number density of ballistics within the vent.

We evaluated about 48% of the erupted mass going into convection, 21% feeding PDCs, and 31% settling into the La Fossa crater (for method see S5).

Using a westward-inclined vent (21° to the vertical axis, azimuth 270°) with the same initial conditions as Run 1 (i.e., Run 2 in Table 3), produces similar maximum height and velocity of both the ejected mixture and ballistics (Fig. 8e), but the material distribution changes, affecting the western and north-western flanks of the cone (Figs. 8e–h, 9c, d and 10b). The expansion phase (∼5 s) produces a PDC overcoming the north-western flank of the cone, propagating at 15–25 m/s, keeping a temperature of up to 220°C along the propagation axis, and of 150°C on the flow edges. The eruptive column develops vertically, not significantly influenced by vent inclination (Fig. 8f). It partially collapses at 20 s (Fig. 8g) forming a second PDC impacting the western flank, partially overlapping the first flow (Fig. 8h). By 60 s, the flow runout extends to 1 km from the eruptive vent (or 700 m from the north-western rim of the La Fossa crater).

Ballistics reach a maximum height of ∼850 m above the vent following more directional trajectories, likely due to the complex decompression dynamics and the effect of the non-isotropic crater geometry. This results in an asymmetrical ground distribution: up to 900 m in the W-E direction and 700 m in the N-S (Fig. 10b). The timing of the ballistic landing and the invasion of the first PDC is similar, with the PDC at times preceding the ballistic shower (as observed in the deposit), particularly near the crater rims.

By the end of Run 2, 22% of the total erupted mass is conveyed into PDCs, 56% into convection, while 22% settles within the La Fossa crater (Fig. 11, Table 3).

Since the areal dispersion of the erupted material in the westward-inclined vent scenario (i.e. Run 2) more closely agrees with the observed distribution of the BDF compared to the vertical vent scenario (i.e. Run 1), Run 2 was used as a reference to examine the influence of different vent geometries, initial and boundary conditions, and erupted volumes. A vent inclined to the northwest (21° to the vertical, azimuth 315°; Run 3 in Table 3), produces a faster PDC directed to the north-western flank of the cone (Fig. 9e, f). The partial collapse of the column generates a second PDC confined to the north, on the Pietre Cotte lava flow (Fig. 9f). The ground distribution of ballistics is asymmetrical, elongated in a NW-SE direction according to the direction of vent inclination (Fig. 10c). The percentage of collapse is significantly lower compared to Run 2, with 71% of the mass going into convection, only 17% into PDCs, and 12% settling within the La Fossa crater (Table 3).

A different vent aspect ratio (15 m radius, 150 m depth; Run 4, Table 3) does not significantly vary PDCs and ballistics distribution with respect to Run 2. However, it affects the expansion stage (Figs. 9g, h and 10d) increasing the decompression time to ∼7.5 s and lowering the maximum mixture velocity to 180 m/s. Consequently, the percentage of collapse is higher, with 40% of the mass going into convection, 20% feeding PDCs, and 40% falling back into the La Fossa crater (Table 3).

In Runs 5 and 6, we investigated the effects of different initial pressures because pressure is one of the main parameters controlling the total energy available in the expansion process, therefore the mixture and ballistic behaviour. A higher initial pressure of 7 MPa (Run 5, Table 3) increases the mixture expansion velocity up to 230 m/s, ejection velocity of ballistics to 180 m/s and their maximum height to 1.2 km. The specific energy increases to 17 kJ/kg, significantly reducing the percentage of collapse (7% of the material in PDCs, 17% into the crater, 76% into convection; Table 3, Fig. 11c). The affected area is wider compared to Run 2 (Fig. 10e). Conversely, a lower pressure of 3 MPa (Run 6) reduces the specific energy to 6 kJ/kg, lowering the exit velocity of the mixture (140 m/s), ballistics ejection velocity (100 m/s) and maximum height (550 m above the vent). The percentage of collapse significantly increases to 83% (53% of the material in PDCs, 35% settled within the La Fossa crater, 12% into convection; Table 3, Fig. 11b).

The results described above refer to an initial gas volume fraction of 70%. An initial gas volume fraction of 40% (i.e., Run 7, Table 3) results in almost total column collapse (98%; Fig. 11d) due to the reduced amount of gas available for expansion. The specific energy drops to 3 kJ/kg (Table 3), with reduced mixture expansion velocity (120 m/s), ballistic ejection velocity (94 m/s), and ballistic maximum height (400 m above the vent). Most of the ejected mixture collapses around the vent area (60% of the material settling within the La Fossa crater), and a single PDC carrying 38% of mass affects the north-western and western flanks of the cone. Only 2% of mass goes into convection (Table 3, Fig. 11d).

Increasing the initial temperature of the gas-particle mixture to 400°C (Run 8 in Table 3) does not significantly vary the expansion energy, therefore, the jet and initial PDC dynamics are very similar to Run 2. However, column buoyancy increases, reducing the collapse percentage (9% of the material in PDCs, 8% settling within the La Fossa crater, and 83% in convection; Table 3).

A smaller average grain size of 0.177 mm (Run 9 inTable 3) extends the decompression phase to 6 s, increasing the mixture expansion velocity to 200 m/s and ballistics ejection velocity to 160 m/s. The percentage of collapse is higher than Run 2, with 35% of the mass forming PDCs, 6% settling within the La Fossa crater, and 59% in the convective column (Table 3).

Runs 10–13 involve erupted masses not exceeding the values estimated for single layers of the BDF (Table 3). Runs 10 and 11 test respectively vertical and westward-inclined vents (both 10 m in radius and 100 m deep) under the same initial conditions as Run 2, resulting in a specific energy of 12 kJ/kg. Both scenarios produce similar mixture expansion (180 m/s) and ballistic ejection (130 m/s) velocities. They also generate a single main PDC that, for the vertical vent scenario (Run 10), is confined to the northern and north-eastern flanks of the cone (Figs. 8i–l and 9i, j), while the westward-inclined vent (Run 11) directs it northwest. In the latter case, part of the flow is initially confined between the La Fossa Modern and PC-C crater rims, then splits into two lobes, one flowing above the Pietre Cotte lava and the other to its west (Figs. 8m-p and 9k, l). Ballistics are asymmetrically distributed on the ground: up to 800 m to the north-west and 500 m to the south. Some of them land simultaneously with the passage of the flow. The percentage of collapse is lower compared to Run 2, with 11% of the material feeding PDCs, 13% settling within the La Fossa crater, and 75% forming the eruptive column (Run 11, Table 3; Fig. 11e).

Run 12, with a northwest-inclined vent, confines the PDC to the northern and north-western flanks of the cone (Fig. 9m, n), elongating the ballistic ground distribution in the NW direction (Fig. 10k). The exit velocity of the mixture and maximum column and ballistic height reduce compared to Run 11, resulting in a slightly higher percentage of collapse (17% of the material in the PDCs, 10% within the La Fossa crater, and 73% into convection; Table 3, Fig. 11g).

Finally, run 13, with a lower initial gas fraction of 40% (Table 3), results in nearly total collapse (99%; Table 3, Fig. 11h), impacting the north-western to south-western flanks. Ballistics are confined to the La Fossa crater rims, with many falling within the La Fossa crater (Fig. 10l).

In summary, eruption dynamics are influenced mainly by erupted mass, explosion energy, and vent geometry. Higher initial pressure increases the mixture ejection velocity and ballistic range, decreasing collapse percentage. Ballistic landing time is around 30 s but reduces with less erupted mass and lower initial volume fraction of gas. Using non-spherical ballistic shapes does not significantly change the landing time, but varies the ground distribution, resulting in shorter distances from the vent due to the larger drag coefficients. Coarse ballistics (20 cm) reach longer distances than finer clasts (5, 10 cm), due to stronger inertia of coarser blocks, allowing to reach greater distances from the eruptive vent. Vent inclination strongly controls ballistic distribution and PDC propagation. Initial temperature has a smaller impact, mainly enhancing mixture buoyancy and reducing collapse percentage.

The BDF documents the phreatic activity at La Fossa di Vulcano in 1873. The absence of clear juvenile material excludes a phreatomagmatic origin, consistent with historical reports (Mercalli 1883; Mercalli and Silvestri 1891). Despite its variability, the BDF can be well correlated along the north-western flank of the La Fossa cone. Its diversity in the structure and thickness provides insights into different depositional mechanisms. Layers A to D were likely formed by PDCs, with layers A, C, and D showing also clear evidence of ballistic fallout. The local presence of cross- to plane-parallel alternations of lapilli and ash beds, mainly in layers A, B, and C, is interpreted to record oscillations in velocity and bulk density within the flow. These oscillations are independent of source variations and affect the transport and sedimentation capability of the flow as observed in natural pyroclastic flows and large-scale experiments (Brosch et al. 2021). Ballistics found at the top of layer A and within layers C and D suggest PDC deposition before and during ballistic impacts. The formation of layer E might involve a syn-eruptive lahar possibly fed by liquid/condensed phreatic water erupted during the final part of the activity, as observed in recent events like the 2014 Ontake eruption, Japan (Kaneko et al. 2016; Sasaki et al. 2016). This hypothesis is supported by its similar composition compared to the other layers and its constant association with them.

The 1873 explosive phase involved multiple explosions occurring at intervals from 30 minutes to a few days, and the opening of four closely spaced vents (< 100–150 m between the easternmost and westernmost vents) within the La Fossa crater (Mercalli 1883; De Fiore 1922; Mercalli and Silvestri 1891). The homogeneity of the BDF components and lack of both significant internal reworking and clear erosive surfaces between layers, suggest a short emplacement interval. Thus, the complex activity described for the 1873 cycle would well support the occurrence of multiple (major) explosions responsible for the emplacement of the stratified deposit described here.

Simulation outcomes suggest that a phreatic explosion with an inclined eruptive vent, positioned close to the northern wall of the La Fossa crater as described by Mercalli (1883) would produce an asymmetrical deposit consistent with field observations. These scenarios involve a total ejected mass of 1×10⁷ to 5×10⁷ kg, and a specific explosion energy around 12 kJ/kg or 7×10⁶ J/m³ (assuming a mixture bulk density of 600 kg/m³). This value is below the lower bound (~ 1×10⁷ J/m³) explored by Montanaro et al. (2020) for explosions driven by the expansion of water vapour. However, consideration of the minimum energy spent for fragmentation (~ 1×10⁶ J/m³; Montanaro et al. 2020) would lead to a comparable specific energy.

Scenarios with 5×10⁷ kg of ejected mass, within the range estimated for the total BDF deposit, impact a wide area on the north-western and western flanks of the cone, with ballistics reaching distances up to 700 m, that is about 200 m farther than observed in the field survey. A single explosion of this magnitude could produce two PDCs phases: the first from the lateral expansion of the mixture, the second from column collapse. This might explain the formation of at least two distinct beds of the BDF sequence, such as layers A and B. However, mass proportions calculated for the two currents (less than 5% for the first current, up to 22% for the other) does not match field observations, as layer A is often thicker than B. On the contrary, erupted masses around 1×10⁷ kg, in the range of those calculated for single layers of the BDF, affect a smaller area on the north-western flank of the La Fossa cone, with ballistics up to 500 m of distance, aligning with field observations. We therefore suggest that each layer of the BDF results from a single, smaller explosion. Each explosion might have had different specific energy, influenced by the water vapour available for expansion. Lower gas volume fractions (implying a specific energy as low as 3 J/kg) might better explain the emplacement of the massive, coarse-grained layers A and D. This general reconstruction is sensitive to the original volume and distribution of the deposit, which might have been modified by erosion and remobilization, as well as the subsequent 1888–1890 activity.

The relatively small mass involved in the eruptions, together with the mixture and ballistic dynamics, suggest a shallow explosion source, likely not exceeding 150 m (in the simulated scenarios). This is consistent with a model of bottom-up explosions driven by pressurisation of fluids at shallow depths following increased heat and gas fluxes released from a deeper reservoir. This interpretation agrees with historical accounts of increased fumarole activity in the months leading up to and during the 1873–1888 period, evidenced by continuous, intense degassing in the crater area. Picone, who witnessed the 1873 explosions, described the pre-eruption conditions as characterised by “a gas pressure so strong that the vent could not adequately dissipate it” (De Fiore 1922). Each explosion in 1873 was shortly preceded by repeated strong subterranean noise, likely accompanied by changes in fumarole emissions, suggesting that the system underwent rapid changes preluding eruptive activity (De Fiore 1922).

Comparison with recently observed events

The total volume of the BDF is estimated at 5×10⁴ m³, falling into typical values for phreatic eruptions. Larger events, like the 2014 Ontake and the 2012 Te Maari/Tongariro eruptions, had volumes around 10⁵ m³ (Lube et al. 2014; Breard et al. 2014; Kaneko et al. 2016; Maeno et al. 2016). Smaller eruptions include the 2016 Whakaari/White Island (~ 1.2×10⁴ m³), the 2015 Hakone (~ 10² m³), and the 2018 Kusatsu-Shirane (3.4–4.9×10⁴ m³; Montanaro et al. 2022).

Phreatic eruptions typically produce columns below 10 km of height (Kaneko et al. 2016; Kilgour et al. 2019; Montanaro et al. 2022). Historical accounts from La Fossa di Vulcano in the 1873–1888 period do not mention this data. Our simulations suggest columns reach at least 2 km asl, which is the vertical boundary of the domain. However, it is important to remark that the maximum column height is influenced by fine ash content, which was minimal (less than 5 wt.%) in the 1873 Vulcano activity. In turn, the abundance of ash in the mixture is strictly related to the lithology of the rocks involved in the explosion (Montanaro et al. 2022), indicating that column height could vary for explosion of similar energy but different remobilized material.

Ballistic fallout and PDCs, responsible of the BDF deposit formation, have been directly observed in other phreatic eruptions, such as the 2016 and 2019 Whakaari/White Island and the 2014 Ontake eruptions (Kaneko et al. 2016; Maeno et al. 2016; Kilgour et al. 2019; Walsh et al. 2019). Depending on event size and topography. Phreatic eruption PDCs can travel up to 2.5 km, with velocities generally not exceeding 45 m/s (Nakada et al. 2005; Kaneko et al. 2016; Geshi and Itoh 2018). In our simulations, PDC velocities range from 10 to 25 m/s, with temperatures between 145 and 200°C. The temperature of phreatic eruption PDCs are generally lower than those associated with magmatic eruptions, often indicated by the absence of charred trees (Kaneko et al. 2016; Geshi and Itoh 2018).

Historical and observational evidence show that phreatic eruptions can be characterised by multiple explosions as observed in 1873 at La Fossa and other eruptions, such as the 1976/1977 La Soufrière de Guadeloupe (Heiken et al. 1980; Feuillard et al. 1983; Hincks et al. 2014) and the 2016 Whakaari/White Island (Kilgour et al. 2019; Walsh et al. 2019). The occurrence of multiple explosions can be caused by pressurised fluid pockets within the porous or fractured rocks, characterised by significant lateral and/or vertical gradients of permeability. These pockets may activate sequentially as the system undergoes decompression after an explosion in adjacent areas.

Our study shed light on small-magnitude (< 10⁷ kg or < 5×10⁴ m³) phreatic explosions at La Fossa di Vulcano. Through historical records, field data, and numerical modelling, we identified and described the Breccia De Fiore deposit from the intense 44-day-long explosive phase in 1873.

Characterising the stratigraphic sequence provided essential input parameters for simulations (i.e., grain size, volume, distribution, dynamics), despite the challenges related to the poor preservation and complexity of the deposit. Our simulations suggest explosion scenarios consistent with field observations, including those with (W-)inclined vents producing asymmetrical deposits, and smaller erupted masses that may account for the formation of individual layers of the BDF.

Our research indicates small-scale phreatic explosions generating PDCs and ballistic showers can impact the slopes of the La Fossa cone and nearby inhabited areas. The shape and orientation of the eruptive vent determine the final distribution of the ejected material, with initial pressure, gas content, and erupted volume influencing eruption dynamics and energetics, thus the relative proportion of the material feeding the convective column, and PDCs, and the ballistic range. It is important to remark that the present numerical analysis is limited to the general explosion phenomenology, PDC formation, and ballistic distribution. For more quantitative information on the emplacement dynamics of PDCs, future studies will require higher resolution (below metre scale) near the ground. Further improvements to the numerical model should also consider multiple particle classes, which would affect PDC dynamics (e.g., Neri et al., 2003; Esposti Ongaro et al., 2020).

Given the significant impact of phreatic explosions on Vulcano due to the proximity of the La Fossa cone to inhabited areas and growing tourism, understating the conditions leading to them is crucial for hazard assessment and mitigation. Our findings suggest these explosions may be triggered by pressure build-up in shallow porous rocks due to increased magmatic gas flux. The Breccia De Fiore case shows this process may precede a magmatic eruption by years or decades. However, quantifying the probability of occurrence of these events remains challenging due to the limited knowledge and constraints on related deposits. Further research is therefore necessary to understand and reconstruct the conditions for high-pressure formation in the shallow porous rocks at La Fossa. This research may also extend to other locations prone to vent opening, such as Forgia Vecchia or Baia di Levante (Selva et al. 2020).

Acknowledgements

SG was supported by a PhD fellowship founded by the Istituto Nazionale di Geofisica e Vulcanologia – sezione di Pisa, and partly by the Ministero dell’Università e della Ricerca (MUR) through the PRIN “HEATED” grant 2022FAWX8C. RC, MP, and MdMV were supported by the Ministero dell’Università e della Ricerca (MUR) through the PRIN “HEATED” grant 2022FAWX8C. TEO and MdMV were partially supported by the MUR project Pianeta Dinamico – WUnderVul and by the Euro-HPC Joint Undertaking of the European Union, ChEESE-2P project, Grant Agreement No. 101093038. The authors would also thank A. Neri and the members of the INGV working groups during the 2021 Vulcano hydrothermal crisis for their valuable contributions to the discussions on phreatic activity at Vulcano.

Conflict of interest

The authors declare that they have no competing interests.

Availability of data and material

All data needed to evaluate the conclusions in the paper are present in the paper and/or in the Supplementary Information.

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Dynamics of the 1873 CE “Breccia De Fiore” phreatic eruption at Vulcano (Aeolian Islands, Italy) through historical chronicles, physical volcanology, and numerical modelling

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Abstract

Figures

Introduction

Geological background and last 1000 years of activity at La Fossa

The 1873–1888 activity from contemporary chronicles

Materials and Methods

The “Breccia De Fiore” deposit

Stratigraphy

Sample grain-size and Total Grain Size Distribution (TGSD)

Isopach maps and volume estimation

Physical and numerical modelling

Initial conditions for simulations

Modelling results

Discussion

Comparison with recently observed events

Conclusions

Declarations

Acknowledgements

Conflict of interest

Availability of data and material

References

Supplementary Files

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Version 1