# Effusion Rate Trends at Piton de la Fournaise: A Review of 24 Years of Space-Based Thermal Observation

Adele Campus, Nicolas Villeneuve, Oryaëlle Chevrel, Aline Peltier, Andrea Di Muro, Diego Coppola

► To cite this version:

Adele Campus, Nicolas Villeneuve, Oryaëlle Chevrel, Aline Peltier, Andrea Di Muro, et al. Effusion Rate Trends at Piton de la Fournaise: A Review of 24 Years of Space-Based Thermal Observation. Journal of Geophysical Research: Solid Earth, 2025, 130 (6), 10.1029/2024JB030962. [https://doi.org/10.1029/2024JB030962]

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# JGR Solid Earth

## RESEARCH ARTICLE

10.1029/2024JB030962

**Key Points:**

* 24-year study of Piton de la Fournaise volcano effusion rate trends with satellite infrared imagery. Identified five eruptive groups

* Long-term output rate shows steady-state behavior since 1998. Changes in the frequency of eruptive groups after the 2007 caldera collapse

* The different eruptive groups require a distinct assessment of associated hazards and risk

**Supporting Information:**

Supporting Information may be found in the online version of this article.

**Correspondence to:**

A. Campus,
adele.campus@unito.it

**Citation:**

Campus, A., Villeneuve, N., Chevrel, O., Peltier, A., Di Muro, A., & Coppola, D. (2025). Effusion rate trends at Piton de la Fournaise: A review of 24 years of space-based thermal observation. *Journal of Geophysical Research: Solid Earth*, 130, e2024JB030962. https://doi.org/10.1029/2024JB030962

Received 10 DEC 2024
Accepted 18 MAY 2025

**Author Contributions:**

**Conceptualization:** Adele Campus,
Diego Coppola

**Data curation:** Adele Campus,
Nicolas Villeneuve, Oryaëlle Chevrel,
Diego Coppola

**Formal analysis:** Adele Campus,
Diego Coppola

**Investigation:** Adele Campus,
Diego Coppola

**Methodology:** Adele Campus,
Diego Coppola

**Software:** Adele Campus

**Supervision:** Diego Coppola

**Visualization:** Adele Campus

**Writing – original draft:** Adele Campus,
Diego Coppola

© 2025. The Author(s).
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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## Effusion Rate Trends at Piton de la Fournaise: A Review of 24 Years of Space-Based Thermal Observation

Adele Campus¹ ID, Nicolas Villeneuve²,³,⁴ ID, Oryaëlle Chevrel³,⁴,⁵ ID, Aline Peltier³,⁴ ID, Andrea Di Muro⁵, and Diego Coppola¹ ID

¹Earth Science Department, University of Turin, Torino, Italy, ²Laboratoire GéoSciences Réunion, Université de La Réunion, Saint Denis, France, ³Institut de Physique du Globe de Paris, Université Paris Cité, Paris, France, ⁴Observatoire Volcanologique du Piton de la Fournaise, Institut de Physique du Globe de Paris, Bourg Murat, France, ⁵Laboratoire Magmas et Volcans, CNRS, IRD, OPGC, Université Clermont Auvergne, Clermont-Ferrand, France, ⁶Université Claude Bernard Lyon 1, Lyon, France

**Abstract** We combined thermal satellite imagery from Moderate Resolution Imaging Spectroradiometer (MODIS) and Visible Infrared Imaging Radiometer Suite (VIIRS) instruments to reconstruct the effusion rate trends in terms of Time Averaged Discharge Rate (TADR) of 37 eruptions (2000–2023) at Piton de la Fournaise (La Réunion, France). The analysis of quantitative data on durations, locations, final erupted volumes, and TADR trends reveal the occurrence of five groups of eruptions. Group 1: eruptions at or near the summit, low volumes, variable durations, and low, almost steady TADR. Group 2: eruptions located over the entire edifice, low volumes, short durations, and a single-pulse of effusion. Group 3: eruptions located in the S-SE sector, moderate volumes, long durations, and exponentially decreasing TADR. Group 4: eruptions in the N-E-S sectors, moderate volumes, long durations, and waning-waxing TADR trends. Group 5: eruptions located in N-E-S sectors, high volumes, and complex trends. Recognizing different groups unravels the complex eruptive patterns of Piton de la Fournaise, only partially compatible with the “pressure-cooker” model usually invoked in closed-vent basaltic contexts. Extending the analysis back to 1998 reveals that although the long-term lava output rate has remained nearly constant (~0.7 m³/s, steady-state condition), the occurrence and frequency of different eruptive groups changed after April 2007 summit caldera collapse. Our analysis underlines that: (a) the potential occurrence of Groups 4–5 events in inhabited areas makes them the most hazardous, (b) the 2007 collapse modified magma transfer from the deep storage zone to the surface, but not the steady-state behavior of the volcano.

**Plain Language Summary** Piton de la Fournaise (La Réunion, France) is one of the most active effusive volcanoes in the world. Our work analyzes 24 years of volcanic activity using satellite infrared imaging to categorize various eruptive trends. By examining attributes such as lava discharge rates, eruption volumes, durations, and vent locations, we identified five distinct groups of eruptions. Each group displayed unique characteristics: smaller eruptions exhibited simpler and shorter patterns, while larger eruptions demonstrated longer and more complex effusion trends. These differences may be linked to magma transfer dynamics which likely reflect variations in shallow magma paths and stress changes within the volcanic edifice. From a long-term perspective, we observed a stable rate of deep magma input into the shallow reservoir, supported by a stable mean output rate since 1998. By focusing on each eruption group, our findings suggest that the caldera collapse associated with the exceptional event in March 2007 altered the occurrence of the different eruptive groups. The significance of this study lies in the potential for a better understanding of the volcano's behavior from a long-term perspective. Furthermore, our results lead to the identification of groups of eruptions as the most dangerous, as they could extend their impact into inhabited areas.

## 1. Introduction

Effusion rate represents a key parameter for understanding the dynamics driving effusive volcanic activity and associated hazards. The effusion rate is the primary quantity controlling the advance rate, volume, length, and coverage of lava flows (Kilburn, 2000). It has direct control over the distance they can reach (Harris & Rowland, 2009; Walker, 1973) and is an essential parameter for hazard assessment and forecasting potentially inundated areas (Cappello et al., 2022; Chevrel et al., 2022; A. Harris et al., 2016, A. J. L. Harris et al., 2016; Zuccarello et al., 2022).
Writing – review & editing:

Adele Campus, Nicolas Villeneuve,
Oryaëlle Chevrel, Aline Peltier, Andrea Di
Muro, Diego Coppola

The variation in effusion rate reflects the pressure conditions within the magmatic system and may be linked to different eruptive mechanisms operating at basaltic volcanoes (Aravena et al., 2020; Harris et al., 2000, 2011; Harris & Neri, 2002; Lautze et al., 2004; Machado, 1974; Steffke et al., 2011; Wadge, 1981). An exponential waning trend has been commonly associated with the tapping of pressurized magma reservoirs (Aki & Ferraz-zini, 2001; Harris et al., 2011; Machado, 1974; Wadge, 1981). The same trend has been related to different eruptive mechanisms, such as a gravity-driven magma discharge model at Stromboli (Ripepe et al., 2015) or a piston-collapse model at Bardarbunga caldera (Coppola, Di Muro, et al., 2017; Gudmundsson et al., 2016). In the case of Etna and Krafla, a stable, low-flux lava effusion has been ascribed to an overflow of magma input rate from summit craters (Harris et al., 2000). An increasing flux toward the end of the eruption has been associated with (a) top-down mechanisms, such as the unloading of a deeper magmatic reservoir (Coppola et al., 2019; Harris et al., 2011), or (b) bottom-up mechanisms, such as the direct ascent of deep magma inputs (Di Muro et al., 2014), or (c) changes in the width of the feeding dike geometry (Aravena et al., 2020) till the closure of the dike (Journeau et al., 2023). Finally, a pulsatory pattern has been ascribed to either degassing processes with short-term oscillation in shallow magma supply (Lautze et al., 2004) or conduit processes associated with viscoelastic rheology (Piombo & Dragoni, 2021).

Measuring effusion rates in a consistent, continuous, and homogeneous way in real time remains a significant
challenge despite its critical importance. Ground measurements of effusion rates are still particularly difficult due
to at least three main different reasons. Firstly, hazardous conditions and logistical constraints often prevent field
visits. Additionally, there is no standardized approach for these measurements, and the inconsistencies across
different methods generate inaccuracies in the long-term measurements. Finally, it should be noted that the high
variability of in situ effusion rates over short time and large spatial scales further complicates the measurement
process. These difficulties are reflected in the limited availability of published information on effusion rate
measurements and trends (Tsang & Lindsay, 2020; Wilson et al., 2014). Consequently, there is a growing need for
improved techniques and methodologies for continuous and effective measurements of effusion rates.

Since the end of the 1990s, satellite thermal data have become an increasingly robust method for estimating effusion rates in near real-time (NRT), especially for basaltic lava flows (Ganci et al., 2012; Gouhier et al., 2016; Harris & Baloga, 2009; Harris et al., 1997; Wooster & Rothery, 1997). Based on the physical-theoretical framework developed by Pieri and Baloga (1986), Harris et al. (1997) first introduced the thermal approach, which determines the effusion rate of an expanding lava flow field from the amount of heat flux measured from space. This approach (often called thermal proxy) has been the subject of further theoretical and empirical treatments that have highlighted its strengths and limitations (Coppola et al., 2013; Dragoni & Tallarico, 2009; Garel et al., 2014; Harris & Baloga, 2009; Tarquini, 2017; Wright et al., 2001). A major outcome of these works is that this satellite-based method does not allow the measurement of an instantaneous effusion rate but rather a Time-Averaged lava Discharge Rate (TADR), defined as a measurement of the volumetric flux integrated over a specific period before the satellite overpass (Harris et al., 2007; Wright et al., 2001). Moreover, it has been pointed out that the proportionality between thermal and volumetric flux may vary for each volcano depending on variations in rheology, thermal insulation (mainly due to the tube development; Koeppen et al., 2013), and topography (Coppola et al., 2013; Harris & Baloga, 2009). Therefore, an accurate calibration is necessary case-by-case (Coppola et al., 2013).

This study focuses on Piton de la Fournaise volcano, located on La Réunion island (French overseas territory). It is one of the most active effusive volcanoes in the world, with an average of two eruptions per year (Chevrel et al., 2021; Roult et al., 2012; Staudacher et al., 2016). The recent activity of this volcano has been the object of numerous studies (e.g., Bachelery et al., 2016), including a preliminary analysis of the TADR trends of 15 eruptions that occurred between 2000 and 2007 (Coppola et al., 2009). This previous work was based on the analysis of satellite thermal infrared data elaborated by the Middle Infrared Observation of Volcanic Activity (MIROVA) system and provided the first systematic analysis of the effusive rates and patterns of the Piton de la Fournaise's eruptions. Since 2015, the operational MIROVA system has been used in the framework of a multinational response to the effusive crisis of Piton de la Fournaise (Chevrel et al., 2022, 2023; Harris et al., 2017, 2019) whereby satellite-retrieved TADR are delivered to the Observatoire Volcanologique du Piton de la Fournaise of the Institut de Physique du Globe de Paris (OVPF-IPGP) in NRT.

In this paper, we revise the previous work of Coppola et al. (2009) and update the database of effusive eruptions monitored by MIROVA, performing an analysis of selected events occurring from 2000 to 2023. This led to the
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Map of the recent activity at Piton de la Fournaise. (a) Location of La Réunion Island. (b) Color-coded map of the lava flows emplaced between 1998 and 2023 at Piton de la Fournaise (OVPF-IPGP database, 10.18715/REUNION.OVPF). Gray areas represent all the flows since 1998 not analyzed in this work, while the colored ones represent the 37 analyzed eruptions used for the classification discussed in Section 4. Each color defines a Group (see Section 4). The approximate boundaries of the rift zones were taken from Peltier et al. (2018) and Boudoire et al. (2017). The white triangles represent the locations of the eruptive vents of the 37 analyzed eruptions. The red dashed line delimits the edge of the Enclos.</figcaption></figure>

detailed reconstruction of the TADR trends and measurement of emitted lava volumes of 37 eruptions (Section 3). These data were used to classify these eruptions into distinct groups based on the analysis of specific eruptive attributes (Section 4). We then extended our classification to all the eruptions occurring between 1998 and 2023 (Section 5) in order to analyze the evolution and the volumetric contribution of each group during the last 26 years. Finally, we discuss the eruptive groups in light of hazard assessment, underlining some implications for monitoring and risk mitigation (Section 6).

## 2. Geological Setting and Recent Activity

Piton de la Fournaise (2,632 m asl) is a shield volcano characterized by dominantly effusive basaltic eruptions. The volcanic edifice began to grow about 530 kyrs ago on the southeastern flank of the Piton des Neiges, the largest dormant volcano on the island (Chevallier & Bachelery, 1981; Gillot & Nativel, 1989). During its evolution, different phases of activity were recognized, which led to the formation of several calderas. Among these, Enclos Fouqué is the last horseshoe-shaped caldera, which hosts the main terminal cone of Piton de la Fournaise (Figure 1). This volcanic center has been active for about 4,500 years and is characterized by a 1.1 × 0.8 km summit caldera called Dolomieu (Chevallier & Bachelery, 1981; Lénat et al., 1989; Staudacher & Allègre, 1993), which has been formed by recurrent collapses, the last one occurring in April 2007 (e.g., Michon et al., 2009; Staudacher et al., 2009). Bory Crater is a smaller crater (0.38 × 0.23 km) located westward of the Dolomieu (Figure 1).
Most erupted lavas are aphyric or cotetic (cxp + pl ±ol) basalts, with a narrow and relatively evolved compositional range, while olivine-rich lavas (oceanites) and plagioclase-rich lavas are less frequent (Albarède et al., 1997; Boivin & Bachèlery, 2009; Villemant et al., 2009; Welsch et al., 2013). Recent eruptions appear to be fed by a shallow magma reservoir located just above the sea level (Peltier et al., 2009), whose volume has been estimated at 300–500 × 10⁶ m³ (Peltier et al., 2007, 2008; Sigmarsson et al., 2005). Several geophysical, geochemical, and petrographic evidence suggest the existence of a shallower network of partially interconnected magma pockets (Lénat & Bachèlery, 1990) located between the roof of this reservoir and the Dolomieu caldera (Coppola, Ripepe, et al., 2017; Di Muro et al., 2014, 2015; Gurioli et al., 2018; Michon et al., 2016; Thivet, Gurioli, & Di Muro, 2020; Vlastélic & Pietruszka, 2016).

Eruptions generally originate from a vertical magma injection under the central cone, rise quite high in the massif, and usually progress by dikes or sills (Peltier et al., 2005). Fissures then open mainly inside the Enclos Fouqué caldera following the shallow structural discontinuity paths described by three main rift zones: the southeast rift zone (SERZ), the northeast rift zone (NERZ), and the 120N rift zone (N120). Since the 18th century, only seven confirmed eruptions over more than 200 occurred outside the Enclos (namely "Hors Enclos") along the main rift zones (Chevrel et al., 2021; Michon et al., 2015). Of these Hors Enclos events, only three eruptions occurred in the last century: 1977, 1986, and 1998. Notably, all these events started inside the Enclos and then progressed outside it (Villeneuve & Bachèlery, 2006). Generally, the eruptive style begins with small-sized (usually<100 m high) Hawaiian lava fountains that feed channel-fed lava flows (Bachèlery, 1981; Hibert et al., 2015; Staudacher et al., 2016). As eruptions progress, emissions concentrate in one or, more rarely, several cones formed by lava fountain fallout. This often leads to the development of a temporary lava lake, Strombolian activity, and tube-fed lava flows (Chevrel et al., 2023; Coppola, Ripepe, et al., 2017; Harris et al., 2019; Hibert et al., 2015; Soldati et al., 2018).

In the last century, there have been more than 130 effusive eruptions divided into periods (also called cycles) of frequent eruptions lasting 12–24 years each, usually alternating with periods of low activity and/or complete rest, lasting 3–6 years (Derrien et al., 2019; Peltier et al., 2009; Roult et al., 2012; Staudacher et al., 2016 and references therein, Stieltjes & Moutou, 1989; Vlastélic et al., 2018). The period between 1998 and 2007 was characterized by intense activity (26 eruptions from March 1998 to May 2007 or 3,6 eruptions/year) also involving the formation of a pit crater and a caldera collapse, and almost continuous inflation of the edifice, testifying to a persistent recharge of the magma reservoir located approximately at sea level (Peltier et al., 2009). The effusive activity in the period March 1998 to February 2007 (before the large March–May 2007 eruption) was mainly located inside the Enclos Fouqué (except for a phase of the March 1998 eruption; Villeneuve & Bachèlery, 2006), and produced ~240 × 10⁶ m³ of lava (bulk volumes; Table S1.1), which resulted in a mean output rate of ~21.2 × 10⁶ m³/year (~0.67 m³/s). Distinct sub-cycles were recognized within this period, each one represented by a sequence of eruptions migrating from the near-summit zone to proximal distance and then to distal, low-altitude eruptions (Peltier et al., 2009). The intense 1998–2007 period ended with the voluminous March–May 2007 distal eruption, which drained most of the Piton de la Fournaise shallow plumbing system and produced the collapse of the reservoir at the origin of the Dolomieu crater collapse (320 ± 20 m depth; Staudacher et al., 2009). After the collapse, the volcano entered a 7-year-long period of lower activity characterized by a sequence of short-lived, small-volume eruptions occurring between 2008 and 2010 and 41 months of complete rest between 2011 and 2014. The eruptive activity resumed in June 2014 with a new period/cycle of frequent eruptions (Coppola, Di Muro, et al., 2017; Peltier et al., 2016). Between 2014 and 2023, 25 eruptions occurred within the Enclos and produced ~170 × 10⁶ m³ of lava, with a mean output rate of ~20.9 × 10⁶ m³/year (~0.66 m³/s), a value comparable with the 1998–2007 period before the collapse.

Geophysical data reveal that the Dolomieu crater collapse in 2007 disturbed the stress regime of the entire volcanic edifice, making the seismic and geodesic precursor signals that characterized the pre-collapse eruptions different from the post-collapse ones (Peltier et al., 2010, 2018; Roult et al., 2012).

### 3. Methods

The thermal activity of Piton de la Fournaise was reconstructed through the analysis of the Middle Infrared data acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Visible Infrared Imaging Radiometer Suite (VIIRS). These data were processed using the MIROVA algorithm, first developed by Coppola et al. (2016) for MODIS acquisitions and subsequently implemented for VIIRS granules (Campus et al., 2022). In
this work, we re-analyze the eruptions of Piton de la Fournaise between 2000 and 2007, observed only by MODIS (Coppola et al., 2009), and add the most recent eruptions occurring between 2008 and 2023. Additionally, since 2012, we have integrated the eruptions covered by the VIIRS data set following a process of data fusion as described in Campus et al. (2022). In the following sections, we briefly explore the workflow for obtaining TADR and lava flow volumes. A deeper description of the full satellite-retrieved data set and the applied approaches is given in Supporting Information S1.

### 3.1. TADR Computation

The process for retrieving the TADR begins with the quantification of the Volcanic Radiative Power (VRP, in Watt) from satellite images. The MIROVA system processes MODIS and VIIRS images in NRT to identify, locate, and calculate the intensity of the thermal activity in terms of VRP (Campus et al., 2022; Coppola et al., 2016, 2020). The VRP is calculated by applying the Middle Infrared (MIR) method described by Wooster et al. (2003):

$$VRP = k_{\text{MIR}} * A_{\text{PIX}} * \sum_{i=0}^{n} L_{\text{MIRhot}}(i) - L_{\text{MIRbk}}(i) \quad (1)$$

where $A_{PIX}$ is the surface pixel size, corresponding to 1 km² for MODIS and 0.5625 km² for VIIRS acquisitions. The constant $k_{MIR}$ (in µm sr) represents a wavelength-dependent coefficient with values of 19.2 and 19.7 µm sr, for MIR bands of MODIS and VIIRS respectively (Aveni et al., 2023). $L_{MIRhot}$ represents the total MIR radiance (in W m⁻² µm⁻¹ sr⁻¹) of the pixels containing the thermal anomalies, and $L_{MIRbk}$ is the total background radiance. This approach allows the quantification of the VRP time series of all the detected eruptions occurring since 2000 (53 events; refer to Table S1.1).

From the time series data provided by MIROVA, we selected the best images to accurately reconstruct eruption trends. This process removed cloudy and bad geometry-viewed images and was undertaken separately for the MODIS and VIIRS data sets. After acquiring more reliable time series, we applied the trapezoidal integration method to calculate the cumulative Volcanic Radiative Energy (VRE, in Joule) released by each eruption. The associated standard error for these measurements is ±30% (Coppola et al., 2016). We then merged the VIIRS data for eruptions since 2012, enabling the integration of VIIRS and MODIS acquisitions into a robust, unified data set since 2012. A detailed explanation of this data fusion process, including calibration, along with the figures for the picked MODIS and VIIRS VRP and VRE time series of all eruptions since 2012, can be found in Supporting Information S1. From the 53 eruptions detected since 2000, 48 of them were characterized by at least one good-quality image (Table S1.1). After reconstructing the VRP and VRE time series for these 48 events, we chose the best-sampled eruptions, excluding those with fewer than three good acquisitions (refer to Text S1.3 in Supporting Information S1). Following this filtering step, over 55 events since 2000, only 37 had sufficiently suitable images for reconstructing and analyzing the effusive trends during the eruptions (Table S1.1). For these 37 “analyzed” eruptions (Table 1), we calculated the satellite-retrieved TADR and volumes (VOL_SAT), along with the eruption attributes detailed in the next sections. To convert VRP time series into TADR for each eruption, we employed the empirical method suggested by Coppola et al. (2009), which assumes a linear relationship between the satellite-measured thermal energy (VRE) and the independently measured erupted volume (VOL_GROUND):

$$c_{\text{rad}} = \text{VRE}/\text{VOL}_{\text{GROUND}} \quad (2)$$

where $c_{rad}$ is an empirical coefficient affected by specific rheological, topographical, and insulation conditions of the observed case. By using the VRE obtained for the 37 eruptions and the volumes already published or reported by OVPF-IPGP as the ground truth (VOL_GROUND, Table S1.1 for the references), we retrieve a best-fit value of $c_{rad}$ of 2.1 × 10⁸ J/m³, hereby assumed to be representative of the PdF eruptions (see Supporting Information S1 for details).

For all the analyzed eruptions, we then calculate TADR values for each VRP measurement according to:

$$TADR = VRP/c_{\text{rad}} \quad (3)$$
<caption>**Table 1**</caption>

Summary of the Satellite-Retrieved Attributes of the 37 analyzed Eruptions

<table><thead><tr><th>Eruption</th><th>Duration Days</th><th>VOL<sub>SAT</sub> ×10<sup>6</sup> m<sup>3</sup></th><th>TADR<sub>max</sub><sup>start</sup> m<sup>3</sup>/s</th><th>TADR<sub>max</sub><sup>end</sup> m<sup>3</sup>/s</th><th>Effusive Trend</th><th>Eruptive GROUP</th></tr></thead><tbody><tr><td>October 2000</td><td>32.7</td><td>13.27</td><td>13.29</td><td>15.86</td><td>Waning—waxing</td><td>4</td></tr><tr><td>March 2001</td><td>7.6</td><td>4.39</td><td>13.28</td><td>2.86</td><td>Exponential waning</td><td>3</td></tr><tr><td>June 2001</td><td>26.0</td><td>13.96</td><td>54.32</td><td>1.46</td><td>Exponential waning</td><td>3</td></tr><tr><td>November 2002</td><td>19.5</td><td>14.27</td><td>34.43</td><td>42.57</td><td>Waning—waxing</td><td>4</td></tr><tr><td>May 2003 (June 2003)<sup>a</sup></td><td>38.3</td><td>1.42</td><td>4.37</td><td>1.01</td><td>Steady</td><td>1</td></tr><tr><td>August 2003</td><td>5.1</td><td>4.72</td><td>16.09</td><td>28.72</td><td>Waning—waxing</td><td>4</td></tr><tr><td>September 2003</td><td>0.5</td><td>0.71</td><td>34.86</td><td>21.95</td><td>Single pulse</td><td>2</td></tr><tr><td>December 2003</td><td>18.2</td><td>0.60</td><td>1.02</td><td>0.34</td><td>Steady</td><td>1</td></tr><tr><td>October 2005</td><td>12.5</td><td>1.19</td><td>2.29</td><td>2.24</td><td>Steady</td><td>1</td></tr><tr><td>December 2005</td><td>23.3</td><td>10.64</td><td>34.98</td><td>28.08</td><td>Waning—waxing</td><td>4</td></tr><tr><td>July 2006</td><td>25.6</td><td>3.36</td><td>8.29</td><td>0.99</td><td>Exponential waning</td><td>3</td></tr><tr><td>August 2006</td><td>123.6</td><td>15.48</td><td>11.49</td><td>4.09</td><td>Complex</td><td>5</td></tr><tr><td>March 2007<sup>b</sup></td><td>42.0</td><td>150.27</td><td>400.00</td><td>117.27</td><td>Complex</td><td>5</td></tr><tr><td>September 2008</td><td>10.5</td><td>0.51</td><td>1.56</td><td>0.26</td><td>Steady</td><td>1</td></tr><tr><td>December 2008</td><td>51.8</td><td>1.11</td><td>2.28</td><td>0.15</td><td>Steady</td><td>1</td></tr><tr><td>January 2010</td><td>9.6</td><td>0.79</td><td>1.47</td><td>0.15</td><td>Steady</td><td>1</td></tr><tr><td>October 2010</td><td>16.4</td><td>5.16</td><td>15.82</td><td>2.86</td><td>Exponential waning</td><td>3</td></tr><tr><td>June 2014</td><td>0.8</td><td>0.16</td><td>6.71</td><td>0.88</td><td>Single pulse</td><td>2</td></tr><tr><td>February 2015</td><td>11.5</td><td>0.59</td><td>1.63</td><td>0.79</td><td>Steady</td><td>1</td></tr><tr><td>May 2015</td><td>13.3</td><td>5.54</td><td>27.95</td><td>3.12</td><td>Exponential waning</td><td>3</td></tr><tr><td>July 2015</td><td>2.1</td><td>1.25</td><td>19.95</td><td>5.75</td><td>Single pulse</td><td>2</td></tr><tr><td>August 2015</td><td>70.6</td><td>39.74</td><td>47.94</td><td>31.64</td><td>Complex</td><td>5</td></tr><tr><td>May 2016</td><td>1.1</td><td>0.08</td><td>1.59</td><td>1.00</td><td>Single pulse</td><td>2</td></tr><tr><td>September 2016</td><td>6.8</td><td>5.04</td><td>29.74</td><td>13.23</td><td>Waning—waxing</td><td>4</td></tr><tr><td>January 2017</td><td>27.0</td><td>4.64</td><td>9.14</td><td>2.04</td><td>Exponential waning</td><td>3</td></tr><tr><td>July 2017</td><td>45.1</td><td>8.32</td><td>20.73</td><td>1.66</td><td>Exponential waning</td><td>3</td></tr><tr><td>April 2018</td><td>34.6</td><td>5.64</td><td>26.40</td><td>1.71</td><td>Exponential waning</td><td>3</td></tr><tr><td>July 2018</td><td>0.8</td><td>0.16</td><td>3.72</td><td>5.06</td><td>Single pulse</td><td>2</td></tr><tr><td>September 2018</td><td>47.0</td><td>12.85</td><td>30.01</td><td>2.39</td><td>Exponential waning</td><td>3</td></tr><tr><td>February 2019</td><td>18.6</td><td>12.96</td><td>30.78</td><td>40.96</td><td>Waning—waxing</td><td>4</td></tr><tr><td>June 2019</td><td>2.2</td><td>1.08</td><td>16.81</td><td>5.11</td><td>Single pulse</td><td>2</td></tr><tr><td>August 2019</td><td>3.5</td><td>1.73</td><td>13.82</td><td>15.49</td><td>Single pulse</td><td>2</td></tr><tr><td>October 2019</td><td>2.1</td><td>0.87</td><td>12.79</td><td>3.83</td><td>Single pulse</td><td>2</td></tr><tr><td>December 2020</td><td>1.1</td><td>0.35</td><td>15.80</td><td>1.76</td><td>Single pulse</td><td>2</td></tr><tr><td>December 2021</td><td>25.9</td><td>11.34</td><td>7.28</td><td>13.26</td><td>Waning—waxing</td><td>4</td></tr><tr><td>September 2022</td><td>16.1</td><td>10.71</td><td>9.24</td><td>24.19</td><td>Waning—waxing</td><td>4</td></tr><tr><td>July 2023</td><td>38.9</td><td>12.88</td><td>29.86</td><td>3.70</td><td>Exponential waning</td><td>3</td></tr></tbody></table>

<footnote><sup>a</sup>The first phase of the eruption, occurring on 30 May 2003, has not been detected by satellite. The satellite volume is computed starting from 4 June 2003. In the text, the authors refer to this eruption as “May–June 2003.”<sup>b</sup>The first phase of the eruption, occurring between 30 and 31 March 2007, has not been detected by satellite. The satellite volume was calculated from 2 April. This eruption reached the sea: MIROVA volumes are estimations and may be imprecise. In the text, the authors refer to this eruption as “March–May 2007.” The $c_{rad}$ used for this eruption is $1e8 J/m^3$.</footnote>
<figure><img src="image_4.png" /><figcaption>**Figure 2.** (a) Histograms of VOL<sub>SAT</sub> (log-transformed) of the 37 analyzed eruptions (light blue bars) and VOL<sub>GROUND</sub> of all the eruptions between 1998 and 2023 (gray bars). The bimodal distribution highlights two families (low and moderate-to-high volume) intersecting at 2.5 × 10<sup>6</sup> m<sup>3</sup> (dark red dashed line). (b) Distribution of the durations (log-transformed) of the analyzed and all eruptions since 1998 (light blue and gray bars, respectively). The red dashed line divides two families with durations <4.5 days (short-lasting) and ≥4.5 days (moderate-to long-lasting). (c) Percentage distribution of the TADR<sub>max</sub><sup>start</sup> (light blue bars) and TADR<sub>max</sub><sup>end</sup> (dark red bars) for the analyzed eruptions. (d) Distributions of the distance from the summit and elevation of the vents for the analyzed eruptions (dark red and light blue bars, respectively).</figcaption></figure>

Then, we reconstruct the TADR time series to obtain the satellite-derived volume (VOL<sub>SAT</sub>; Table 1). The average error to both TADR and associated VOL<sub>SAT</sub> is ±35% as detailed in Supporting Information S1.

# 4. Results

## 4.1. Eruption Attributes

The 37 eruptions were analyzed quantitatively to define a series of attributes that macroscopically characterize their main features and trends (Figure 2). These attributes are:

1. The final volumes of erupted lava calculated with the satellite approach (VOL<sub>SAT</sub>, in m<sup>3</sup>);

2. The eruption duration (in days);

3. The maximum TADR values recorded in the first half of the eruption (TADR<sub>max</sub><sup>start</sup> in m<sup>3</sup>/s);

4. The maximum TADR values in the second half of the eruption (TADR<sub>max</sub><sup>end</sup> in m<sup>3</sup>/s);

5. The location of the eruptive vents.
The VOL<sub>SAT</sub> varies significantly across the analyzed eruptions ranging from ~0.08 × 10<sup>6</sup> m<sup>3</sup> (May 2016) to ~150 × 10<sup>6</sup> m<sup>3</sup> (March-May 2007), and averages at ~10.4 × 10<sup>6</sup> m<sup>3</sup> (Table 1). Notably, the histogram of log-transformed VOL<sub>SAT</sub> shows a bimodal distribution (Figure 2a), which has been modeled through a Gaussian mixture model (mean, μ, and covariance values, σ, are reported in Figure 2). Accordingly, we recognize two main families: (a) low-volume eruptions, with VOL<sub>SAT</sub> ≤ 2.5 × 10<sup>6</sup> m<sup>3</sup> (Log VOL<sub>SAT</sub> ≤ ~6.4), and (b) moderate-to high-volume eruptions, with VOL<sub>SAT</sub> > 2.5 × 10<sup>6</sup> m<sup>3</sup> (Log VOL<sub>SAT</sub> > ~6.4). The comparison of the selected eruptions (VOL<sub>SAT</sub>) with the distribution of the full data set represented by VOL<sub>GROUND</sub> (see Supporting Information S1) confirms the representativeness of our data set (Figure 2a), showing roughly the same ranges of values as well as the bimodality of the distribution. Notably, 50% of the Piton de la Fournaise eruptions emit less than 5 × 10<sup>6</sup> m<sup>3</sup>, and 25% emit more than 1 × 10<sup>7</sup> m<sup>3</sup>.

The duration of the satellite-analyzed eruptions varies widely (Figure 2b), with the shortest eruption lasting only ~0.5 days (September 2003) and the longest lasting 123.5 days (August 2006). The distribution of duration is also bimodal and highlights two different families (short- and moderate-to-long-lasting eruptions) characterized by (a) durations shorter than 4.5 days (Log duration ≤0.65) and (b) longer than 4.5 days (Log duration >0.65). Statistical analysis reveals that 24% of the selected eruptions are short-lived (less than 4.5 days), and about 25% last more than 1 month. By comparing the duration of the analyzed eruptions with the duration of all eruptions since 1998, we confirm that our selected data set is representative of the broad activity occurring since 1998 (Figure 2b).

The TADR<sub>max</sub><sup>start</sup> range from a minimum of 1 m³/s (December 2003) to a maximum of 400 m³/s (March-May 2007). This maximum value represents an estimation during the March-May 2007 eruption, computed as the mean lava flux of 6 April 2007, as reported in Duputel & Rivera (2019; Figure 2c), and for which a peak was approximated at 800–1,000 m³/s (Duputel & Rivera, 2019; Michon et al., 2011). According to our data set, ~75% of the analyzed eruptions have TADR<sub>max</sub><sup>start</sup> higher than 5 m³/s and ~20% higher than 30 m³/s. No particular distribution is noticed for this attribute.

The TADR<sub>max</sub><sup>end</sup> ranged from a minimum of 0.15 m³/s (December 2008) to a maximum of 117 m³/s (March-May 2007). This attribute exhibits a skewed distribution toward high values (Figure 2c) with 60% of the eruptions ending with TADRs lower than 4 m³/s, while ~30% of them show values higher than 15 m³/s.

The spatial distribution of the eruptive vents feeding the effusive activity is also analyzed (Figures 1b and 2d). The eruptive vents are located at distances from the center of the Dolomieu caldera ranging from a few hundred meters (summit and proximal) to a maximum of ~7,000 m (March-May 2007), involving distal areas (>3,000 m from the summit; Peltier et al., 2008). Particularly, less than 10% of eruptions occurred in distal areas, while ~40% of them are located at less than 1 km from the summit (Figure 2d). The vents opened at elevations ranging between ~700 and ~2,600 m asl, with ~50% located between 2000 and 2,500 m asl.

Although the spatial limits of the rift zones are not well constrained, we note that most of the eruptions since 1998 occurred along the NE and SE rift zones (Figure 1). Some of the remaining eruptions occurred inside the Dolomieu caldera, and others were fed by the opening of eruptive fissures across the eastern flank (Figure 1b). These last could be attributed to the N120 rift zone (Michon et al., 2009, 2016) or could be related to flank sliding without a link with the main rift zones (Dumont et al., 2022).

## 4.2. Grouping the Eruptions

The attributes described above have been used for identifying and classifying groups of eruptions that share similar behaviors.

Accordingly, we identified 5 main groups (Figure 3) subdivided into 3 main categories:

* Small volume eruptions (≤2.5 Mm³):

* Group 1 (7 eruptions): located within the Dolomieu caldera, or in its very proximity, and characterized by long duration (≥4.5 days) with low TADR<sub>max</sub><sup>start</sup> (<5 m³/s), and low TADR<sub>max</sub><sup>end</sup> (<5 m³/s);

* Group 2 (9 eruptions): occurring in all the sectors of Enclos Fouqué and characterized by short-lived (<4.5 days), single effusion pulse, with variable TADR<sub>max</sub><sup>start</sup>, and variable TADR<sub>max</sub><sup>end</sup>.
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Boxplots summarizing the distribution of the attributes chosen for the eruptive groups. (a) VOL~SAT~; (b) duration; (c) TADR~start~; (d) TADR~end~; (e) Distance of the vent from the summit; (f) elevation asl of the vent. In each boxplot, the central red lines are the median values, the edges of the boxes are the 25th and 75th percentiles, the dashed black lines comprise the range of values from maximum to minimum, and the red crosses are related to the outliers. The mean values for each parameter are displayed with the red diamonds.</figcaption></figure>
* Moderate volume eruptions (2.5–15 Mm³):

Group 3 (10 eruptions): located on the southern and south-eastern sectors of Enclos Fouqué and characterized by moderate duration (≥4.5 days) with a clear TADR exponential waning trend from high TADR<sub>max</sub><sup>start</sup> (>10 m³/s) to low TADR<sub>max</sub><sup>end</sup> (<10 m³/s);

Group 4 (8 eruptions): occurring in all the sectors of Enclos Fouqué except the western flank of the cone, characterized by moderate duration (≥4.5 days), and a waning-waxing TADR trend with high TADR<sub>max</sub><sup>start</sup>, (>10 m³/s) and high TADR<sub>max</sub><sup>end</sup> (>10 m³/s).

* Large volume eruptions (>15 Mm³):

Group 5 (3 eruptions): occurring in all the sectors of Enclos Fouqué and characterized by complex TADR trends, moderate to long duration and with attributes that can be shared with the other groups.

In Figure 3 we present how the analyzed attributes are distributed across the 5 eruptive groups. The VOL<sub>SAT</sub> boxplot (Figure 3a) allows us to clearly distinguish the two small-volume eruption groups (Groups 1 and 2) from the two moderate-volume eruption groups (Groups 3 and 4), in agreement with the bimodal distribution observed in Figure 2a. Notably, Group 5 includes outlier values significantly higher than those from other groups.

The analysis of eruption duration reveals that Groups 1, 3, and 4 include eruptions characterized by similar durations (ranging between a week and 2 months), while for Group 2, eruptions last less than 4 days, and for Group 5 eruption durations are always longer than 1 month (Figure 3b). We note that only eruptions of Group 1 start with a low TADR (<5 m³/s), while only eruptions of Group 4 end with TADR<sub>max</sub><sup>end</sup> higher than 15 m³/s (Figures 3c and 3d). The analysis of the locations helps identify Group 1, located very close to the summit of the volcano, both in terms of distance and elevation (Figures 3e and 3f). The other Groups occur at similar distances from the summit, while Groups 3 and 4 fissures open at elevations higher than 1,700 m asl.

Finally, we also analyzed the temporal sequence of all the groups: in general, the full set of eruptions since 1998 does not reveal any seasonality (Figure S1.64 in Supporting Information S1) in the occurrence of the events. This conclusion differs from the conclusion of Roult et al. (2012), who suggested a correlation between rainy season and fortnightly Earth tides on the seismic crisis triggering the magma intrusions.

# 5. Discussion

## 5.1. Characterization of the Eruptive Groups

The analysis of eruptive attributes reveals that the Piton de la Fournaise eruptions follow at least 5 distinct eruptive patterns (Figure 4).

### 5.1.1. Group 1: Low Volume, Low Flux Summit Eruptions

Group 1 eruptions occurred inside the Dolomieu caldera except for the February 2015 eruption, which occurred southwest of the Bory crater (Figure 4a). They are small-volume eruptions with variable durations, persistent low-flux, and TADR trends almost constant or gently declining (Figure 4b, Table 2). The eruptions of this group are usually characterized by a decreasing SO₂ trend and produce rather evolved lavas (Hibert et al., 2015; Staudacher et al., 2016).

The low lava flux makes them resemble the “Type II” eruptive trends of Harris et al., 2000, which have been ascribed to the overflow of the magma supply. Indeed, the mean output rate that feeds these summit eruptions (averaging around 0.61 m³/s; Table 2) is roughly comparable to the long-term magma output rate of the last decades (~0.66 m³/s; see Section 2 and Figure 7), suggesting that these events are poorly pressurized and may be favored by the presence of a weakened central rock pathway, with eruption of residual magma stored in pockets (Di Muro et al., 2015; Staudacher, 2010). Some of these are effectively associated with the resumption of central activity after a collapse, such as the case of the eruptions of May–June 2003 (the first eruption after the December 2002 pit-crater collapse) and September 2008 (the first eruption after the April 2007 Dolomieu caldera collapse). They hence represent small eruptive episodes associated with the development or restoration of a central and
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Clusterization of the eruptions. (a) Maps of the 5 eruptive groups and the associated lava flows for the 37 analyzed eruptions listed in Table 1. (b) Plots of the time averaged discharge rate (TADR) and VOL_SAT of some eruptions belonging to Groups 1 to 5 (from the upper to the lower panels). The TADR picked values are displayed with the blue triangles and the blue dashed line; a smoothed trend is represented by the dark blue lines with an estimated error of ±35%. The dark red lines display the cumulative erupted VOL_SAT with respective ±35% error (thinner red lines). The yellow star denotes the final VOL_SAT.</figcaption></figure>

shallow magmatic system (Peltier et al., 2007, 2010; Steffke & Harris, 2011; Staudacher, 2010) with evolved
magma ascending along already open paths of the fractured system (low strength).

5.1.2. Group 2: Low Volume, Short Duration, Single-Pulse Eruption

Group 2 low-volume eruptions are characterized by a single and short TADR pulse (Table 2). These eruptions are proximal to distal as they occurred in all flanks of the volcano at a variable distance from the summit and elevation (Figure 4). Some of these events (September 2003, July 2018, and December 2020) occurred in the western area of the caldera, which was rarely affected by eruptions during the last century (Chevrel et al., 2021).
<table><caption>**Table 2**

Summary Table With the Mean Values and Standard Deviation (σ) of the Attributes of Each Group of Eruptions</caption><thead><tr><th></th><th></th><th>GROUP 1</th><th>GROUP 2</th><th>GROUP 3</th><th>GROUP 4</th><th>GROUP 5</th></tr></thead><tbody><tr><td>N events</td><td></td><td>7</td><td>9</td><td>10</td><td>8</td><td>3</td></tr><tr><td>VOL<sub>SAT</sub> [×10<sup>6</sup> m<sup>3</sup>]</td><td>Mean</td><td><strong>0.89</strong></td><td><strong>0.71</strong></td><td>7.67</td><td>10.37</td><td><strong>68.50</strong></td></tr><tr><td></td><td>σ</td><td>0.35</td><td>0.57</td><td>4.05</td><td>3.62</td><td>71.85</td></tr><tr><td>Duration [days]</td><td>Mean</td><td>21.74</td><td><strong>1.58</strong></td><td>28.16</td><td>18.49</td><td><strong>74.56</strong></td></tr><tr><td></td><td>σ</td><td>16.57</td><td>0.97</td><td>13.29</td><td>9.28</td><td>47.11</td></tr><tr><td>TADR<sub>max</sub><sup>start</sup> [m<sup>3</sup>/s]</td><td>Mean</td><td>2.09</td><td>14.01</td><td><strong>23.58</strong></td><td><strong>21.98</strong></td><td>153.14</td></tr><tr><td></td><td>σ</td><td>1.11</td><td>9.98</td><td>13.62</td><td>11.65</td><td>214.56</td></tr><tr><td>TADR<sub>max</sub><sup>end</sup> [m<sup>3</sup>/s]</td><td>Mean</td><td>0.70</td><td>6.76</td><td><strong>2.28</strong></td><td><strong>25.86</strong></td><td>50.99</td></tr><tr><td></td><td>σ</td><td>0.75</td><td>7.20</td><td>0.85</td><td>11.60</td><td>59.02</td></tr><tr><td>Distance from summit [m]</td><td>Mean</td><td><strong>453.57</strong></td><td>2,275.56</td><td>1,522.00</td><td>1,604.37</td><td>3,030.00</td></tr><tr><td></td><td>σ</td><td>154.80</td><td>1,767.56</td><td>519.73</td><td>980.73</td><td>3,830.20</td></tr><tr><td>Elevation asl [m]</td><td>Mean</td><td><strong>2,477.43</strong></td><td>2,079.56</td><td>2,247.20</td><td>2,242.12</td><td><strong>1,890.30</strong></td></tr><tr><td></td><td>σ</td><td>85.29</td><td>462.55</td><td>182.58</td><td>259.54</td><td>1,030.98</td></tr><tr><td>Eruptive trends</td><td></td><td>Steady</td><td>Single pulse</td><td>Exponential waning</td><td>Waning—waxing</td><td>Complex</td></tr></tbody></table>

Note. The bold values in the table represent the key attributes that are specific for classifying each group.

For these eruptions, the SO₂ release and seismic tremor intensity usually follow the TADR trend, with a decrease after a peaking onset, in some cases associated with the occurrence of gas-piston events at the end (e.g., Coppola, Di Muro, et al., 2017; Journeau et al., 2023; Peltier et al., 2007; Thivet, Gurioli, & Di Muro, 2020).

The single-pulse trends could have originated from an initial overpressure of the magmatic system (as proposed for the initial phases of Groups 3 and 4, Table 2). However, the short durations and the small volumes involved suggest that Group 2 eruptions might be fed by (a) a small-volume pulsed magma supply quickly disconnected from its source (Dragoni & Piombo, 2020; Lautze et al., 2004; Smittarello et al., 2019) or (b) single, small batches of magma, already isolated from their pressure source (Gurioli et al., 2018; Journeau et al., 2023; Sundermeyer et al., 2020). Focusing on the second mechanism, previous researches ascribe some of these eruptions to the tapping of shallow, small-volume magma pockets induced by deeper magma injections (Sundermeyer et al., 2020), or by second-boiling processes (Coppola, Ripepe, et al., 2017; Gurioli et al., 2018).

### 5.1.3. Group 3: Moderate Volume, Waning Trend

Group 3 eruptions present a well-defined, exponentially decreasing TADR trend (Figure 4b). All the eruptions occurred in the southern sector of the Enclos Fouqué caldera at 1,800–2,170 m asl and distances from the Dolomieu crater between 670 and 2,000 m. This eruptive group is characterized by moderate volumes and variable duration of more than 1 week but less than 2 months (Table 2). The maximum discharge rate is always reached in the first 15 hr (i.e., less than 20% of the entire eruption duration), and is followed by a gradual decline of TADR.

Group 3 eruptions are commonly accompanied by the initial deflation of the volcanic edifice (Coppola, Ripepe, et al., 2017; Journeau et al., 2023), by a decrease in degassing (Coppola, Ripepe, et al., 2017; Verdurme et al., 2022) and by a gradual transition from fountain-fed lava flows, to strombolian activity at the vent, often accompanied by gas-piston events during the final phases (Journeau et al., 2023).

All of these observations are consistent with a pressurized eruption model (Aki & Ferrazzini, 2001; Anderson & Segall, 2011, 2013; Aravena et al., 2020; Harris et al., 2000, 2011; Huppert & Woods, 2002; Machado, 1974; Segall et al., 2001; Stasiuk et al., 1993; Wadge, 1981; Woods & Huppert, 2003), whereby the time evolution of the erupted lava volume V(t) can be described by:

$$V(t) = V_{ex} \left[ 1 - e^{-t/\tau} \right] \quad (4)$$
<figure><img src="image_7.png" /><figcaption>**Figure 5.** (a)–(l) Plots of Group 3 eruptions displaying the cumulative VOL<sub>SAT</sub> throughout the events (blue dots). The red lines represent the modeled curves obtained by applying Equation 4. In each plot are reported the final modeled volume V<sub>ex</sub> in m<sup>3</sup> and the τ value in days.</figcaption></figure>

where V<sub>ex</sub> is the initial excess of volume within the magma reservoir and τ is the decay constant (in days). This type of trend was described as a “Type I” trend by Harris et al. (2000, 2011) indicating the tapping of a pressurized magma volume (Harris et al., 2011; Wadge, 1981). All Group 3 eruptions correlate very well with this model (Figure 5; average R<sup>2</sup> = 0.99) and are characterized by V<sub>ex</sub> ranging from 3.4 × 10<sup>6</sup> m<sup>3</sup> to 13.9 × 10<sup>6</sup> m<sup>3</sup> and τ comprised between 3.5 and 13.1 days (Table 3). In this type of pressurized eruptions, the final erupted volume
<table><caption>**Table 3**
Table Summarizing the Model Parameters for Group 3 and Group 4 Eruptions Displayed in Figures 5 and 6</caption><thead><tr><th rowspan="2">Eruption</th><th rowspan="2"></th><th colspan="2">Measured</th><th colspan="3">Model</th></tr><tr><th>VOL<sub>SAT</sub><br>×10<sup>6</sup> m<sup>3</sup></th><th>Duration<br>days</th><th>V<sub>ex</sub><br>×10<sup>6</sup> m<sup>3</sup></th><th>τ<br>days</th><th>R<sup>2</sup><br>-</th><th>VOL<sub>phase2</sub><br>×10<sup>6</sup> m<sup>3</sup></th></tr></thead><tbody><tr><td rowspan="10">GROUP 3</td><td>March 2001</td><td>4.39</td><td>7.62</td><td>5.62</td><td>4.52</td><td>0.991</td><td>N.D.</td></tr><tr><td>June 2001</td><td>14</td><td>26.05</td><td>13.9</td><td>4.20</td><td>0.999</td><td>N.D.</td></tr><tr><td>July 2003</td><td>3.36</td><td>25.62</td><td>3.44</td><td>6.71</td><td>0.995</td><td>N.D.</td></tr><tr><td>October 2010</td><td>5.16</td><td>16.44</td><td>5.80</td><td>7.25</td><td>0.998</td><td>N.D.</td></tr><tr><td>May 2015</td><td>5.54</td><td>13.29</td><td>5.59</td><td>3.55</td><td>0.997</td><td>N.D.</td></tr><tr><td>January 2017</td><td>4.64</td><td>26.99</td><td>4.98</td><td>9.90</td><td>0.999</td><td>N.D.</td></tr><tr><td>July 2017</td><td>8.32</td><td>45.09</td><td>8.49</td><td>11.63</td><td>0.993</td><td>N.D.</td></tr><tr><td>April 2018</td><td>5.64</td><td>34.61</td><td>5.54</td><td>7.58</td><td>0.994</td><td>N.D.</td></tr><tr><td>September 2018</td><td>12.9</td><td>46.98</td><td>13.2</td><td>13.09</td><td>0.998</td><td>N.D.</td></tr><tr><td>July 2023</td><td>12.9</td><td>38.85</td><td>12.0</td><td>10.74</td><td>0.983</td><td>N.D.</td></tr><tr><td rowspan="7">GROUP 4</td><td>October 2000</td><td>13.3</td><td>32.69</td><td>6.08</td><td>8.70</td><td>0.986</td><td>8.05</td></tr><tr><td>November 2002</td><td>14.3</td><td>19.46</td><td>5.68</td><td>2.82</td><td>0.995</td><td>9.15</td></tr><tr><td>August 2003</td><td>4.72</td><td>5.08</td><td>1.15</td><td>1.35</td><td>0.993</td><td>3.77</td></tr><tr><td>December 2005</td><td>10.6</td><td>23.28</td><td>6.63</td><td>3.31</td><td>0.99</td><td>4.46</td></tr><tr><td>September 2016</td><td>5.04</td><td>6.82</td><td>3.26</td><td>1.90</td><td>0.997</td><td>2.51</td></tr><tr><td>February 2019<sup>a</sup></td><td>12.96</td><td>18.56</td><td>N.D.</td><td>N.D.</td><td>N.D.</td><td>N.D.</td></tr><tr><td>December 2021<sup>a</sup></td><td>11.34</td><td>25.94</td><td>N.D.</td><td>N.D.</td><td>N.D.</td><td>N.D.</td></tr><tr><td>September 2022</td><td>10.7</td><td>18.56</td><td>3.66</td><td>6.41</td><td>0.999</td><td>8.20</td></tr></tbody></table>

corresponds to $V_{ex}$, which in turn is linked to the initial excess pressure ($P_{ex}$, in MPa; the primary pressure driving the eruption) and the capacity of the reservoir (C, in m³/Pa; Wadge, 1981; Aki & Ferrazzini, 2001):

$$V_{ex} = P_{ex} \times C \qquad (5)$$

In the simplest hypothetical case in which all Group 3 eruptions would be fed by the elastic response of the sea level magma reservoir (with total volume $V_{ch}$ = 0.3 km³; Peltier et al., 2007), the capacity C will be given by Gudmundsson (2016):

$$C = V_{ch}(\beta_{m} + \beta_{r}) \qquad (6)$$

Where $\beta_m$ is the magma compressibility, and $\beta_r$ is the host-rock compressibility (both with the unit of Pa⁻¹). Assuming $\beta_m$ and $\beta_r$ equal to 1 × 10⁻¹⁰ and 3 × 10⁻¹¹ Pa⁻¹, respectively (Gudmundsson, 2016 and references therein), Equation 5 gives $P_{ex}$ in the range of 90–350 MPa, which is much higher than the values generally associated with magma chamber rupture (Gudmundsson, 2012). More reliable values of the $P_{ex}$ in basaltic volcanoes (1–9 MPa, Gudmundsson, 2012; Peltier et al., 2008) can be obtained only by taking into account either a larger magma reservoir or a much more compressible magma. While Group 1 eruptions occur when magmas ascend along already fractured central paths (low strength), we might speculate that Group 3 eruptions result from the opening of new fractures in a less damaged sector of the edifice; this might be consistent with tapping a closed part of the reservoir, where fluids can accumulate, and higher $P_{ex}$ is required to open the dykes. In this regard, Thivet, Gurioli, and Di Muro (2020) have suggested the occurrence of foam accumulation in the top part of the reservoir for some eruptions of the 2014–2015 sequence. Even if the eruptions studied by the authors cannot be ascribed to Group 3 (but are consistent with Group 2), we recognize that the presence of a foam layer could

<footnote><sup>a</sup>Eruptions for which the exponential fit (Equation 4) doesn't give any results. We consider an estimated error of 35% in the VOL<sub>SAT</sub> and VOL<sub>phase2</sub> values.</footnote>
produce much higher compressibility, more compatible with our results. However, we may not exclude that other mechanisms than the simple elastic response of the reservoir must contribute to the genesis of this eruptive group.

Recently, Journeau et al. (2023) have highlighted that some eruptions of this group show an inflation of the volcanic edifice during the decreasing effusive activity. They interpreted this syn-eruptive inflation phase as the resumption of magmatic recharge following the initial drainage of the shallow magma reservoir. Our data suggest this new pressurization does not perturb the vent effusive dynamics.

The pressurized eruption model (Equation 4) also allows the estimation of the critical effusion rate $Q_{crit}(t_{end}) = Q_0 \times e^{-t_{end}/\tau}$ (in m³/s) below which the eruption will end. Our modeling suggests that Group 3 eruptions end when the $Q_{crit}$ falls below an average threshold of 0.6 m³/s. This happens on average when the eruption lasts from $1.7\tau$ to $7.2\tau$, suggesting that some portion of $V_{ex}$ (typically <10%) may remain unerupted (Table 3).

### 5.1.4. Group 4: Moderate Volume, Waning-Waxing Trend

Group 4 is characterized by an initial exponential decrease of TADR, followed by a subsequent increase during the late stages of the eruptions, and an abrupt end (Figure 4b). These eruptions occurred on all flanks of the edifice at altitudes of 2,600–1,700 m asl and distances from the summit of 500–3,400 m. All the eruptions of this group produce moderate volumes of lava over a variable duration (5–~33 days) slightly shorter than Group 3. The VOL<sub>SAT</sub>, duration, and TADR<sub>max</sub><sup>start</sup> as well as declining TADR trend during the first phase are very similar to Group 3 (Table 3) but eruptions of Group 4 have a second phase of increasing effusion rates, called waxing phase (TADR<sub>max</sub><sup>end</sup> = 25.9 ± 11.6 m³/s, Table 2) before the sudden end of the eruption. The two TADR peaks often reach similar values, and are reached on average within the first 10% and over 60% of the eruption duration, respectively. This particular pattern indicates a two-stage eruptive dynamic also evident in the plots of the cumulative volumes (Figure 6). Notably, the first phase of 6 of these eruptions can be modeled in a similar way as the eruptions of Group 3, using Equation 4 (only 2 eruptions were not modeled; see Text S1.6.2. in Supporting Information S1).

We found that the initial stage is characterized by $V_{ex}$ ranging between $1.1 \times 10^6$ m³ to $6.6 \times 10^6$ m³, and $\tau$ from 1.3 to 8.7 days (Table 3 and Figure 6). Compared with Group 3, the average $V_{ex}$ is slightly smaller, and the average decay is a little faster (Table 3). A clear deviation from the exponential waning trend is visible starting more or less from the middle of each eruption (i.e., between 37% and 54% of total duration) when the erupted volume grows rapidly during the second waxing trend phase (Figure 6). The volume of erupted magma during this second stage (VOL<sub>phase2</sub>) ranged from $2.5 \times 10^6$ to $9.1 \times 10^6$ m³ representing, on average, 55% of the total volume erupted by Group 4 eruptions (Table 3). This result implies a rough volumetric balance between the two phases of the eruptions (waning and waxing, respectively) also revealed by comparable peaking TADR values and comparable durations of the two phases.

The increase in effusion rates during the second phase is often accompanied by a deflation of the volcanic edifice, an increase in gas flux and tremor amplitude, and the late emission of olivine-rich lavas (Coppola et al., 2009; Famin et al., 2009; Hayer et al., 2023; Journeau et al., 2023; Longpré et al., 2007; Peltier et al., 2005, 2007, 2009; Staudacher et al., 2016; Verdurme et al., 2022; Vlastélic et al., 2005, 2007). Previous studies have suggested that this second phase may be associated with an unloading of the shallow magmatic system giving rise to basaltic effusive paroxysms at the end of the eruptions (Harris et al., 2011; Journeau et al., 2023; Steffke et al., 2011; Verdurme et al., 2022). Other models suggest that the unusual, intense ending of these eruptions could be enhanced by the formation of a plug in the conduit and pressurization by a new magma input (Thivet, Gurioli, Di Muro, et al., 2020), or by variations in the geometry of the dyke, for example, due to conduit enlargement (Aravena et al., 2020) or dike closure (Journeau et al., 2023), followed by a collapse-dominated eruption shutdown (Longprè et al., 2007).

We remark here that the volumetric balance and the observed symmetrical TADR trends and values of the two phases point toward the existence of a link between the two consecutive phases, eventually involving different processes acting on the same pressurized source, or the same processes on different sources. Further detailed multiparametric and systematic analysis (seismic, deformation, degassing, petrography) will surely provide new clues on the process at the origin of the observed patterns and whether this group's tapped magmatic source is the same as Group 3.
<figure><img src="image_8.png" /><figcaption>**Figure 6.** (a)–(h) Plots of the eruptions of Group 4 displaying the cumulative VOL<sub>SAT</sub> throughout the events (blue dots). The red lines represent the modeled curves obtained by applying Equation 4. In each plot, it is reported the final modeled volume V<sub>ex</sub> in m<sup>3</sup> is associated with the first phase of the eruptions and the τ value in days. (f) and (g) eruptions are those for which the application of the model fails (see in the text).</figcaption></figure>

5.1.5. Group 5: Large Volume, Complex Trend

Group 5 eruptions are distinguished by their long durations (>30 days), very high volumes (>15 × 10⁶ m³), and complex effusive trends, making them difficult to classify within previous groups. However, they share some attributes with the other groups, albeit on a different scale. For example, the eruption of August 2006 occurred inside the Dolomieu caldera, typical of Group 1, but it was characterized by volumes and durations atypical for this group. It also began with a pulse of 10 m³/s, similar to Group 2, and increased its strength toward the end, similar to Group 4 (even if with a lower intensity; Figure 4b). This complexity makes it anomalous, or potentially ascribable as an end-member of Group 1, due to its low effusion rates and prolonged activity in the Dolomieu caldera. Such dynamics are consistent with the occurrence of low-pressurized summit eruptions where magma rise is favored by a weak pathway (Coppola et al., 2009). We remark that this eruption completely filled the Dolomieu caldera by adding more than 20 m of lava thickness. Eventually, the associated increase in the lithostatic load on the roof of the magma reservoir could have partially favored the collapse of the Dolomieu caldera, which occurred a few months later. The March–May 2007 event is classified in this group, because it emitted an exceptionally high volume of lava and was accompanied by the Dolomieu crater collapse (starting on 5th April 2007). In this case, the satellite-retrieved volumes, estimated at ~150 × 10⁶ m³, are likely underestimated due to persistent cloud coverage at the beginning of the eruption (the first satellite acquisition was on 2 April 2007) and a large amount of lava entering the ocean that could not be detected by satellites. Apparently, the general TADR trend seems compatible with an exponential waning trend, as for Group 3 (Figure 4b), but resulting in a much higher initial TADR peak and final erupted volume. If we apply the pressurized eruption model (Equation 4) to this eruption, we obtain V<sub>ex</sub> equal to ~160 × 10⁶ m³ and a τ of ~10 days. By assuming the same parameters applied for Group 3 (V<sub>ch</sub> = 0.3 km³, β<sub>m</sub> and β<sub>r</sub> equal to 1 × 10<sup>-10</sup> and 3 × 10<sup>-11</sup> Pa<sup>-1</sup>, respectively), Equation 5 results in a P<sub>ex</sub> higher than 4 GPa, which appears not realistic. This confirms that a simple elastic model is not able to explain this eruption, as proved by the occurrence of the caldera collapse. In this regard, we remark that caldera collapse regulated by lateral magma discharge can produce a similar exponential but inelastic trend, as observed at Bardarbunga (Coppola, Di Muro, et al., 2017; Gudmundsson et al., 2016). For these considerations, the March–May 2007 eruption remains an exceptional case that cannot be classified in any other eruptive group.
<figure><img src="image_9.png" /><figcaption>**Figure 7.** Cumulative VOL<sub>GROUND</sub> for the last three periods of activity and each group. (a) Cumulative VOL<sub>GROUND</sub> between 1998 and 2023 (black line). The star represents the total cumulative VOL<sub>GROUND</sub> since 1998 corresponding to ~660 x 10<sup>6</sup> m<sup>3</sup> (the references of the plotted volumes in Table S1.1). The vertical bars indicate the eruptions, and each color corresponds to a group (details in the legend). The red arrows indicate the formation of the pit crater in December 2002 and the caldera collapse during April 2007. The dashed gray lines are the best fit between the cumulative VOL<sub>GROUND</sub> and the duration of the eruptions, representing the Q<sub>SS</sub> (reported on the plot) for each period; (b)–(f) cumulative VOL<sub>GROUND</sub> of Group 1 to Group 5 eruptions. The vertical dashed lines in panels (b)–(f) delimit the three periods.</figcaption></figure>
The August 2015 eruption started with a peaking TADR<sub>max</sub><sup>start</sup> = 47.9 m<sup>3</sup>/s that decreased in a couple of days, similar to Group 2. This peak was followed by a long period of stable and almost constant TADR with values around 5–10 m<sup>3</sup>/s, similar to Group 1 but with higher TADR (Figure 4b). Then, the eruption ended with the occurrence of three exceptional “effusive paroxysms” (TADR<sub>max</sub><sup>end</sup> = 31.6 m<sup>3</sup>/s) interspersed with two cessation intervals before ending definitively. These final lava pulses involved new juvenile and volatile-rich magma, accompanied by an increase in SO<sub>2</sub> flux and summit deflation of the edifice (Coppola, Ripepe, et al., 2017; Sundermeyer et al., 2020; Verdurme et al., 2022). The “effusive paroxysms” phase is very similar to Group 4, but the August 2015 eruption lasted longer, produced a higher volume, and displayed a more complex trend, excluding it from Group 4.

We point out that these three high-volume peculiar eruptions seem to occur near the end or at the beginning of renewed periods of intense activity of Piton de la Fournaise (Peltier et al., 2010; Figure 7). It is plausible that they are governed by different processes, or exasperated dynamics from those that characterize the 4 main groups described previously.

## 5.2. Volumetric Output Rate of Piton de la Fournaise Between 1998 and 2023

The different eruption dynamics observed at Piton de la Fournaise make the interpretation of short- and long-term effusive activity non-trivial and more complex than expected for a single, pressurized magma reservoir model, typically invoked to explain closed-vent basaltic volcanism (Gudmundsson, 2016; Wadge, 1982). Our results demonstrate that in the short-term (from daily to monthly scale) it is possible to identify distinct mechanisms corresponding to various effusive trends on the surface, that can be grouped based on similar attributes. In this section, we tentatively extend the classification to all the 59 eruptions occurring since 1998 to date (Table S1.1), thus also considering the 22 eruptions previously excluded for the satellite data analysis. For this purpose, we used the OVPF-IPGP observations and the available literature data to constrain some of the attributes and assign at each eruption a specific group.

### 5.2.1. Steady-State Output Rate

Despite a clear variability in eruption behavior, the long-term cumulative volume curve of lava erupted between 1998 and 2023 (Figure 7a) is typical of a “steady-state” volcano where magma is supplied to the plumbing system and erupted at an almost constant rate, even though the detailed eruptive history may consist of separate events (Wadge, 1982). The long-term magma output rate, namely the steady-state output rate ($Q_{ss}$, in m<sup>3</sup>/s), is defined as the best fit of the volumes throughout the eruptions, that is, the linear slope of the cumulative volume (Wadge, 1982). Based on the cumulative volume curve plotted in Figure 7a, we recognize three distinct periods of activity:

1. The 1998–2007 period (ending after the eruption of February 2007)

2. The 2007–2014, including the March–May 2007 eruption, and the following period of low-level activity, and

3. The 2014–2023 period (not concluded a the time of writing).

Accordingly, we calculate the $Q_{ss}$ for these 3 periods of activity and find very similar values (~0.67, ~0.71, and ~0.66 m<sup>3</sup>/s, respectively) suggesting that the steady-state behavior of the overall plumbing system has not been disturbed by the Dolomieu collapse in 2007 (Figure 7a).

According to the steady-state model proposed by Wadge (1982), the sawtooth sequence of erupted volumes characterizing the periods of frequent activity that occurred in 1998–2007 and 2014–2023, may originate from two primary processes: (a) the partial or complete discharge of an upper magma reservoir continuously supplied from depth, or (b) the intermittent arrival of unbuffered magma batches to the shallower reservoir.

For the 1998–2007 period, the observed constant inflation of the edifice (Peltier et al., 2010, 2018) and cyclic change of magma compositions (Vlastélic et al., 2018) are inconsistent with the arrival of unbuffered, discrete magma batches. More likely during this period the steady-state behavior could have been regulated by the partial discharge of the shallow magma reservoir continuously supplied by deeper magma. In the case of a single shallow magma reservoir, this model would imply that excess pressure increases until it surpasses a critical threshold, triggering eruptions (e.g., Aki & Ferrazzini, 2001; Gudmundsson, 2012, 2016) that decay exponentially, accompanied by the deflation of the volcanic edifice. This pressure-cooker model only partially explains the variety of eruptive groups we observed at Piton de la Fournaise, suggesting a much more complex dynamic.
On the contrary, for the 2014–2023 period, Peltier et al. (2018) observed a discontinuous inflation trend that they ascribed to the arrival of discrete deep magma batches. Therefore, the steady-state trend observed after 2014 could be related to a discontinuous magma supply, which, however, maintained the same long-term $Q_{ss}$ observed during the previous periods (Figure 7a). In this regard, it's important to note that although $Q_{ss}$ from the 2007–2014 eruptive period is similar to the other two periods (Figure 7a), the cumulative volume curve between 2007 and 2014 did not display a sawtooth pattern but is composed of only two different phases, one including the voluminous March 2007 eruption (and the associated collapse), and a second one characterized by low effusive activity (2008–2010) and a 41-month pause (2011–2014, before the June 2014 eruption). During this very low-activity phase (characterized by 8 eruptions and 8 intrusions; Roult et al., 2012) and throughout the 2011–2014 rest period, an almost constant contraction of the edifice was observed and attributed to the edifice destabilization following the 2007 caldera collapse, coupled with no significant deep magma refilling (Di Muro et al., 2015; Peltier et al., 2010, 2018; Staudacher & Peltier, 2016). Hence, the low activity occurring after the collapse could be considered as a “repose period” whose duration was proportional to the volume erupted by the March–May 2007 eruption, ensuring that after the very high-volume event, the cumulative volume curve is brought back to the typical steady-state long-term linearity (Wadge, 1982).

The above considerations support the hypothesis of a steady-state condition since 1998 but contrast with the large fluctuations in the composition of the mantle source, its melting degree, and the cyclic evolution of the erupted volume evidenced by Vlastélic et al. (2018) at a decadal scale since 1940. This implies that the variability in source composition and partial melting degree since 1998, documented by Vlastélic et al. (2018), produces an almost constant magma supply from the mantle. Moreover, the magma output rate for the three periods identified in the 1998–2023 period is higher than that reported for the 1972–1992 period (~0.28 m³/s; Vlastélic et al., 2018) and could suggest an increase in the magma supply rate possibly since 1998 (Vlastélic et al., 2018), or a decrease of the ratio of magma erupted to intruded and crystallized in the crust (Takada, 1999).

### 5.2.2. Cumulative Volumes of Eruptive Groups

The backward classification of all the eruptions occurring since 1998 allowed us to estimate the cumulative VOL<sub>GROUND</sub> curves for each group as reported in Figures 7b–7f. The total volume erupted between 1998 and 2023 of about $660 \times 10^6$ m³ has mostly erupted during the large-volume events of Group 5 (Figure 7f). Based on literature data and additional observations, we can add to this group the 1998 sequence of events, which was not considered before due to a lack of satellite data at that time. These eruptions mark the beginning of a new cycle after 5.5 years of quiescence and emitted a large volume ($>60 \times 10^6$ m³) over a long duration (196 days), which fit perfectly with the attributes of Group 5 (Figure 7b). The overall contribution of Group 5 is $340 \times 10^6$ m³, corresponding to more than half (54%) of the total volume erupted during the 25-year-long time window considered.

The volumetric contributions of the low-volume eruptions from Groups 1 and 2 are ~14 × $10^6$ m³ (2.5%) and ~23 × $10^6$ m³ (4%), respectively, while for the moderate-volume Groups 3 and 4, the volume of emitted lava is ~110 × $10^6$ m³ (17%) and ~140 × $10^6$ m³ (22%), respectively.

Figure 7 also shows how the magma output rate of each group changed over time. Group 1 showed an almost constant relative $Q_{ss}$ during the first and the second period (~0.03 m³/s), but decreased to almost zero since the beginning of the third period (2014–2023), mirroring the quasi-disappearance of this group in the last 10 years (Figure 7b). On the contrary, Group 2 eruptions have become more frequent since 2014 (Figure 7c), resulting in a 7-step increase of the $Q_{ss}$ (from 0.01 m³/s to 0.07 m³/s). Group 3 shows an elevated $Q_{ss}$ (0.50 m³/s) from 1998 to 2002 which drastically decreased to 0.03 m³/s after the pit-crater collapse of December 2002 (Figure 7d). The resumption of this group occurred with the renewal of the activity in 2014, with a $Q_{ss}$ of 0.30 m³/s, slightly lower than the 1998–2002 period. Another clear variation of the slope of the cumulative volume is noticeable for Group 4 eruptions (Figure 7e). In this case, the relative $Q_{ss}$ before the 2007 collapse was 0.50 m³/s, a value that decreased to 0.22 m³/s through the 2014–2023 period.

The observed patterns of the groups may indicate a significant perturbation in the shallow magma transport following the 2007 collapse, as already reported by previous authors (e.g., Got et al., 2013; Peltier et al., 2010, 2018; Staudacher, 2010). For example, Peltier et al. (2010, 2018) found significant variations in the pre-eruptive signals characterizing the eruptions before and after the 2007 collapse. Moreover, Got
et al. (2013) suggested that this event strongly influenced the arrangement of the stress field around the volcano, resulting in a variation in the elastoplastic response of the edifice during the transfer of magma to the surface.

Our results suggest that the 2007 extraordinary caldera collapse produced a substantial change in the way the magma transfers from the shallow plumbing system to the surface because we see changes in the frequency and occurrences of the groups. This is likely due to the varying stress regime and implies changes in the volumetric output rate of the groups. However the average magma supply from the mantle remains constant throughout the whole period (Figure 7).

## 6. Implications for Hazard and Risk Assessment

The recognition of different eruptive groups has potential implications for the assessment of hazards and risks associated with the activity of Piton de la Fournaise. Group 1 eruptions, due to their features (low-flux summit eruptions), do not represent an important source of risk when they occur inside the Dolomieu caldera (unless they trigger partial collapses of the caldera cliff), but they can impact the tourist trails if they occur near the crater rim. Group 2 eruptions produce short-lived, low-volume lava flows that can reach lengths of 3–4 km in a few hours or days. In our data set, none of the Group 2 eruptions were able to reach the national road, and only one (October 2019) arrived at 270 m of the road due to the opening of a distal, low-altitude vent. The single pulse and low volume prevent the development of complex lava fields and severely limit the propagation downstream. Group 3 eruptions produce lava flows with larger final aerial extension and length due to the higher effusion rates (Figure 4). The relatively long duration, coupled with a high initial effusion rate that decreases over time, favors the formation of compound tube-fed flow characterized by the superposition of multiple flow units through breakouts and resurfacing (Harris et al., 2019). This emplacement style may cause a final stage of flow lengthening even during decreasing effusion rates (Harris et al., 2019), which may complicate the application of lava flow propagation models (Chevrel et al., 2022). On the other hand, all the flows of this group occurred inside the Enclos Fouqué caldera, and none of them reached the national road. The modeling of the effusive trend remains a useful tool to estimate the end of the eruptions, which seem to occur when the flux becomes lower than a critical value of ~0.6 m³/s. For Group 4 eruptions, the increasing TADR characterizing the second stage of the eruption (starting on average after ~7 days of eruption) always causes a renewed advancement of the flow (Chevrel et al., 2023). In fact, three eruptions (January 2002, November 2002, and February 2005) of this group have reached the national road during their second phase (Staudacher et al., 2016) and hence may present a risk of infrastructure destruction that would affect the population Moreover, according to Villeneuve and Bachèlery (2006), two of these events (January 2002 and February 2005, not analyzed here but attributed to Group 4), involved lateral dike propagation during the second phase that went beyond the caldera wall, but it still opened fissures inside the Enclos Fouqué without affecting inhabited areas. Finally, the eruptions of Group 5, although all occurred inside the Enclos, represent extraordinary events. The long durations and high volumes may allow these eruptions to produce lava flows that can reach distances close to 10 km. Three eruptions out of the five (March 1998, August 2004, August 2006, March–May 2007, August 2015) associated with this group reached the national road (March 1998, August 2004, March–May 2007), while one accumulated over 20 m of stacked lava inside the Dolomieu (August 2006).

The highest-risk eruptions of Piton de la Fournaise for the population are those *Hors Enclos* because the fissures can open in inhabited centers or immediately upstream (Chevrel et al., 2021). The last two eruptions of this type, having invaded villages, occurred in 1977 and 1986 and are, therefore, not part of our data set. However, our results may be used to tentatively “classify” these eruptions by attributing them to one of the different eruptive groups based on erupted lava volume and duration. A simple plot of the erupted volume versus duration (Figure 8) enables the differentiation of distinct fields corresponding to each group and shows that both the April 1977 and March 1986 eruptions fall into the field of Group 4 (Figure 8). These two eruptions also share other interesting features with Group 4, such as the emission of olivine-rich lavas in 1977, during its final phase (Kieffer et al., 1977), or the formation of a pit crater in the summit area in 1986 (Delorme et al., 1989). We can also infer that, in these *Hors Enclos* eruptions, the effusion rate increased during the final phase. This is suggested by the late-stage propagation of the eruptive fissures outside the Enclos Fouqué caldera, which was associated with a final rise in effusive activity and an increase in shallow seismic events, as reported by Delorme et al. (1989). The fact that both the 1977 and 1986 eruptions belong to Group 4 advises that this group, together with Group 5, produces a higher risk for the population and needs to be studied in more detail. The occurrence of Group 4
<figure><img src="image_10.png" /><figcaption>**Figure 8.** VOL<sub>GROUND</sub> versus duration relationships for all the eruptions that occurred between 1998 and 2023. Symbols refer to the eruptive groups defined in this work, while dashed lines mark the approximate limits between them. (a) Plot displaying all five groups; (b) plot displaying Groups 1 to 4 eruptions. The two stars represent the last two *Hors Enclos* eruptions, which can be associated with Group 4.</figcaption></figure>

eruptions is not an exclusive feature of the recent, highly productive cycles of activity (~0.70 m³/s between 1998 and 2023; Figure 7) but is probably typical of past cycles as well. It is worth noting that the two *Hors Enclos* Group 4 eruptions (1977, 1986) occurred during a cycle of relatively low magma output rate (~0.28 m³/s between 1972 and 1992; Vlastélic et al., 2018), suggesting a complex correlation between the occurrence of these hazardous events and the long-term magma supply rate.

The prompt recognition of the group during an ongoing eruption is certainly a valuable tool for better refining hazard and risk scenarios. The field separation shown in Figure 8 highlights its potential for real-time application, as it suggests that after approximately 5 days of eruption, it becomes possible to identify the field associated with a specific group, thereby ensuring the timely implementation of appropriate risk mitigation strategies in case of more hazardous eruptions. While encouraging, this approach should be implemented with due attention to the evolving dynamics of the eruption.

**7. Conclusions**

In this work, we presented a satellite analysis conducted on 24 years (2000–2023) of eruptive activity on Piton de la Fournaise. This analysis focused on the reconstruction of TADR trends from satellite thermal-infrared imagery using the MIROVA algorithm on MODIS and VIIRS images. By considering specific eruptive attributes (general TADR trends, TADR<sub>max</sub><sup>start</sup> and TADR<sub>max</sub><sup>end</sup>, VOL<sub>SAT</sub> of lava erupted, durations, and locations of the eruptions) we could define 5 different groups of eruptions with specific characteristics. Groups 1 and 2 contain low-volume (≤2.5 × 10<sup>6</sup> m³) eruptions with Group 1 having low TADR trends over variable durations while Group 2 are short and single-pulsed. Groups 3 and 4 include eruptions of moderate volume (2.5–15 × 10<sup>6</sup> m³). Group 3 eruption shows an exponentially decreasing TADR trend until the eruption ends, while Group 4 presents first a
decreasing TADR trend similar to 3 that will gradually increase during a second phase. Group 5 is formed by outstanding eruptions presenting very high volume (>15 × 10⁶ m³), very long durations, and complex TADR trends, occurring in some cases at the start or the end of eruptive cycles.

For Groups 3 and 4, it has been possible to apply a pressurized eruption model to explain the exponential decrease of TADR over the initial phase. This allowed calculations of overpressures, which are difficult to reconcile with the involvement of a single pressurized reservoir with constant volume and properties.

Our analysis suggests that, between 1998 and 2023, the cumulative volume curve is consistent with a steady-state condition characterized by a long-term magma output rate of ~0.7 m³/s, which remained essentially unperturbed before and after the 2007 collapse (with a temporary pause between 2011 and 2014). On the contrary, each group displays a distinct pattern of the cumulative volume curve, which reflects the impact of the 2007 caldera collapse in the way the magma is transferred to the surface. We thus suggest that in addition to the classical pressurized eruption model, Piton de la Fournaise eruptions are affected by additional shallow processes (i.e., destabilization of the conduit, dike closure, or geometry variations) possibly induced by the stress regime affecting the edifice. The influence of the evolution of stress patterns together with the role of deep inputs ascending from deeper parts of the plumbing system must be considered to interpret the short- and long-term effusive dynamics of Piton de la Fournaise's eruptions. With this work, we propose that shallow processes like summit caldera collapses affect the alternation of eruptions belonging to the different groups but not the long-term magma output rate, which seems to be controlled by the deep feeding system.

Finally, we underline that the recognition of these eruptive groups and, in general, the satellite approach can be applied to past and future eruptions to provide new elements useful for the evaluation of hazard and risk.

## Conflict of Interest

The authors declare no conflicts of interest relevant to this study.

## Data Availability Statement

The VIIRS and MODIS Level 1B products data are available through the Level-1 and Atmosphere Archive & Distribution System Distributed Active Archive Center (LAADS-DAAC) (https://ladsweb.modaps.eosdis.nasa.gov/). MODIS MIR data (2000–2020) processed by MIROVA algorithm are available in Coppola et al. (2023; https://osf.io/zm62w/). VIIRS MIR data are freely available under request, following the guidelines in https://www.mirovaweb.it/?action=contact. MIROVA processed time series of TADR and VOL_SAT of the Piton de la Fournaise eruptions analyzed in this work are available in Supporting Information S1 and stored in an Open Science Framework repository (https://osf.io/4cqrf/). The algorithms used to detect, locate, and measure MIR thermal anomalies in MODIS and VIIRS data sets are described in Coppola et al. (2016, 2023) and Campus et al. (2022).

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## Acknowledgments

We thank Matt Patrick for revising this work and the Editor and Associate Editor for their suggestions and kind availability, their comments helped the improvement of this work. The volume estimation, lava flow contours, and eruptive vent locations presented in this paper were collected by Observatoire Volcanologique du Piton de la Fournaise, Institut de Physique du Globe de Paris (OVPF-IPGP, 10.18715/REUNION.OVPF). We thank the Service National d'Observation en Volcanologie (SNOV-CNRS-INSU) for the use of the database. OC acknowledges the ClerVolc Program of Excellence of the International Research Centre of Disaster Science and Sustainable Development, contribution n° 671. MIROVA (https://www.mirovaweb.it/; accessed 29 April 2025) is a collaborative project between the Universities of Turin and Florence (Italy). AC acknowledges the MIROVA team of the University of Turin for the constant support. We acknowledge the LAADS data systems (http://lance-modis.eosdis.nasa.gov/; 27 November 2024) for providing MODIS and VIIRS products. Open access publishing facilitated by Universita degli Studi di Torino, as part of the Wiley - CRUI-CARE agreement.
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## References From the Supporting Information

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