Mapping Volcanic Deposits of the 2011–2015 Etna Eruptive Events Using Satellite Remote Sensing

Abstract

Estimates of lava volumes provide important data on the lava flooding history and evolution of a volcano. For mapping volcanic deposits, including lava flows, the advancement of satellite remote sensing techniques offers a great potential. Here we characterize the eruptive events occurred at Mt Etna between January 2011 and December 2015 leading to the emplacement of numerous lava flows and to the formation of a new pyroclastic cone (NSEC) on the eastern flank of the South East Crater. The HOTSAT system is used to analyze remote sensing data acquired by the SEVIRI sensor in order to detect the thermal anomalies from active lava flows and calculate the associated radiative power. The time-series analysis of SEVIRI data provides an estimation of event magnitude and intensity of the effusive material erupted during each event. The cumulative volume estimated from SEVIRI images from 2011 to 2015 adds up to ~106 millions of cubic meters of lava, with a time-averaged rate of ~0.68 m³ s⁻¹. This estimate is independently supported and bounded using a topographic approach, i.e., by subtracting the last topography of Etna updated to 2005 from a 2015 digital elevation model (DEM), produced using tri-stereo Pléiades satellite images acquired on December 18, 2015. The total volume of products erupted from 2005 to 2015, calculated from topography difference by integration of the thickness distribution over the area covered, is about 287 × 10⁶ m³, of which ~55 × 10⁶ m³ is the volume of the NSEC cone. This 10-year volume is below the typical erupted volumes on decadal scale at Mt Etna, interrupting its stable and resilient output trend.

Introduction

Mapping the volcanic deposits constitute a critical component for constraining magma supply, understanding magma plumbing system, and controlling lava flow morphological parameters, and represents an important indication of the amount of magma the volcanic system is capable of transporting during an eruptive event (Wadge et al., 1975; Stevens et al., 1999). In particular quantifying the emitted lava volumes gives insights into emplacement processes and may aid computer simulations of lava flow paths for hazard assessment studies, providing both input data for predictive lava flow modeling, and tools for iterative validation and/or refining of parameters (Del Negro et al., 2013; Cappello et al., 2016). However, the calculation of the amounts of lava erupted is still source of uncertainty, and problems occur when comparing volume values that are measured using different approaches (Harris et al., 2007).

Generally, volcanic products can be mapped following two different approaches. The simplest method is based on field measurements by multiplying the covered area, which can be determined with a good precision, by an estimated average thickness that remains the major source of uncertainty (Behncke et al., 2014). The second approach requires accurate mapping of the pre and post-eruption Digital Elevation Models (DEMs). This topographic approach is advantageous because deposit thickness is mapped throughout the flow field, resulting in detailed three-dimensional measurements of the morphology and yielding a highly accurate volume estimate. The method is limited by the availability of topographic data of the surface underneath the volcanic deposit of interest, and such measurements are difficult, if not impossible, to be made retrospectively (Del Negro et al., 2016).

The recent availability of satellite images gives new opportunities for mapping volcanic products. Indeed high spatial resolution data acquired in stereo or tri-stereo configuration (e.g., Pléiades) can be used to frequently update the topography in volcanic areas and thus provides a new tool for measuring volcanic deposits by differencing successive topographies. In addition, high temporal resolution data provided by geostationary infrared remote sensing (e.g., SEVIRI) allow to estimate mean eruptive rates associated to the lava flow and to compute the related volumes.

Here we employed both Pléiades data to map the eruptive products and SEVIRI data to characterize the eruptive episodes occurred at Mt Etna between January 11th, 2011 and December 8th, 2015.

Mt Etna is one of the most active volcanoes in the world, showing a persistent activity at the summit craters and frequent effusive eruptions from eruptive vents located on the volcano flanks (Cappello et al., 2013). Today five craters are active on the summit of Mt Etna (Neri et al., 2017): the North-East Crater (NEC), the Voragine (VOR), the Bocca Nuova (BN), the South-East Crater (SEC), and the New South-East Crater (NSEC). NEC, VOR, and BN formed inside the old Central Crater in 1911, 1945 and 1968, respectively. SEC, which has been the most active summit crater over recent decades, was built up during the 1971 eruption. Finally, the NSEC has formed in 2007 as a pit crater at the South-east base of the SEC (Acocella et al., 2016), but rapidly grew during the lava fountains occurred since January 2011 (Vicari et al., 2011). After the last flank eruption in 2008–2009 (Ganci et al., 2012b), the volcanic activity of Etna has shown a more explosive behavior, with an increasing number of paroxysmal events at the summit craters (Bonaccorso and Calvari, 2013; Cappello et al., 2013). Indeed, from 2011 to 2015, the SEC area has been the source of 53 eruptive episodes, most of them characterized by the emission of lava fountains, pyroclastic material, and short-lasting lava flows that mostly spread within the Valle del Bove (VdB).

In the following we estimated the effusion rate and hence the lava flow volume from SEVIRI data and computed some morphometric parameters (area, maximum length, thickness, and volume) from the three-dimensional distribution of the volcanic deposits obtained by DEM difference using a new 2015 digital topography of Mt Etna derived from Pléiades data. The volcanic deposits obtained from DEM difference provide an independent maximum bound to the SEVIRI-derived lava flow volume and possibly an estimate of loose materials.

Materials and methods

Thermal-infrared satellite data sources and analyses

Volcanic hotspots (i.e., high temperature areas or pixels) were detected and processed from SEVIRI images (sample time: 15 min, pixel resolution: 3 km at nadir) using the HOTSAT system for thermal monitoring of volcanoes (Ganci et al., 2011a,b). HOTSAT takes as input mid-infrared (MIR) and thermal infrared (TIR) channels acquired by the Spinning Enhanced Visible and Infrared Imager (SEVIRI), and provides cloud mask and thermal anomalies maps (if applicable). Since clouds can obscure thermal anomalies, we used an algorithm based on textons (Ganci et al., 2011a) to avoid underestimations in satellite-derived results. The detection of thermal anomalies associated to active lava flows is based on a contextual algorithm (Harris et al., 1995), which compares the “volcanic” area (where the thermal anomaly is expected) with a “non-volcanic” area (close to the volcano, where the thermal anomaly is not foreseen). The lava thermal flux is computed considering all those pixels classified as “hot” by using the mid-infrared radiance approach introduced by Wooster et al. (2003), which allows estimating the radiant heat flux radiated by a sub-portion of the thermally anomalous pixels approximating the Planck's radiation law to a fourth order power law. This approach is particularly efficient with low spatial resolution sensors, such as SEVIRI, or moderate spatial resolution sensors such as Moderate Resolution Imaging Spectroradiometer (MODIS), given that the extent of lava flow fields is often subpixel in size (Ganci et al., 2013). During an eruption, the system computes an estimation of the thermal flux radiated by lava for each image, giving as result a heat flux curve describing the event. This heat flux curve can be converted in time-averaged discharge rate (TADR in m³/s—the effusion rate averaged over a specified period) adopting a straightforward relationship between the radiant heat loss from an active lava flow and the effusion rate (e.g., Harris et al., 1997; Wright et al., 2001):

where Q is the total radiant heat loss obtained summing up the radiant heat flux computed for each hot spot pixel, ρ is the lava density, C_p is the specific heat capacity, ΔT is the eruption temperature minus temperature at which flow stops, C_L is the latent heat of crystallization, and Δφ is the volume percent of crystals that form while cooling through ΔT. HOTSAT computes a minimum and maximum estimates for TADR by considering the respectively largest and smallest values that each lava parameter can take, resulting in a volume variation of ±25% (Ganci et al., 2016).

During short-lived paroxysmal events the volcanic hotspot is often affected by the tephra/ash column accompanying the climax, which can attenuate or obscure the thermal anomaly; vice-versa, when not obscured, due to the huge radiated power released, it can easily saturate the sensor. As a consequence, the TADR cannot always be calculated during the peak fountaining phase. Moreover conversion between radiant heat flux and TADR is possible only if a thermal steady state is reached (Garel et al., 2012) and this is not true for very short and transient events, like lava fountains at Mt Etna. A way to provide an estimation of the event magnitude, i.e., mass in kg or volume in m³, and intensity, i.e., mean output rate (MOR) in kg/s or m³/s, has been introduced by Ganci et al. (2012a) by modeling the plume- and saturation-free cooling curve apparent in the heat flux time series. In fact during short-lived eruptive events, the SEVIRI-derived heat flux curve is characterized by three phases: a slight increase in the heat fluxes (first phase), a rapid increase to higher heat fluxes during the main fountaining activity, often marked by a trough and/or scattered behavior, due to plume obscuration (second phase); a waning flux during the cooling of the emplaced lava (third phase). In this case, volumes are inferred by minimizing the difference between the modeled and the SEVIRI-derived cooling curve. By considering the R-squared index, we computed the uncertainties on the estimated volumes ranging within ±20% (Ganci et al., 2012a).

Here we applied the technique of Ganci et al. (2012a) to catalogue the short lived events occurred during the 2011–2013 period, for which the duration of the three phases is <2 days (Figure 1). For the longest effusive events occurred during 2014–2015 (Figure 2), we used the standard HOTSAT approach (Ganci et al., 2011b), and computed the total lava volumes by integrating the TADR curves. In this way we obtained the cumulated volume of lava emitted from 2011 to 2015.

Figure 1

Figure 2

Mapping of lava flows and pyroclastic deposits

To complement the SEVIRI-derived results, we analyzed the distribution of the 2011–2015 eruptive products thanks to the availability of pre-eruptive and post-eruptive DEMs of Etna volcano. For the pre-eruptive base, we used the last topography of Etna covering the whole deposits of the 2011–2015 eruptions, which goes back to 2005 (Gwinner et al., 2006). It is a DEM at 1 m spatial resolution resulted from a digital photogrammetric mapping, applying the airborne HRSC-AX sensor and photogrammetric processing system (Gwinner et al., 2006). After this, only local updates of the summit area have been derived from Light Detection And Ranging (LIDAR), digital photogrammetry and/or terrestrial laser scanning in 2007, 2010, 2012, 2014, and 2015 (Neri et al., 2008, 2017; Caracciolo D'Ajello et al., 2014; De Beni et al., 2015; Behncke et al., 2016). As post-eruptive base, we considered a topography produced using satellite images from the Pléiades constellation acquired on 18 December 2015.

The best result obtained with the Pléiades images of 18 December 2015 is a 4-meter resolution DEM spanning an area of about 16 × 15 km, which covers the summit craters and a portion of the south-east flank of the volcano, including the upper part of the Valle del Bove (Figure 3). According to our 2015 DEM, the highest point of 3,367 m is on the south-west rim of the NEC, while the SEC and NSEC reach an elevation of 3,336 and 3,325 m, respectively. By subtracting the 2005 DEM from the 2015 one, we obtained the topographic changes due to the volcanic deposits emplaced during the eruptive activity of Etna occurred between 2006 and 2015.

Figure 3

Results

Analyzing the thermal activity, we were able to characterize 46 of 53 eruptive events occurred at Etna between 2011 and 2015. Table 1 reports event number (En), date of the first thermal anomaly detected by HOTSAT, duration, maximum value of the radiative power, MOR and lava volume for each event. In spite of the ash interference and/or cloudy conditions that affected 14 events (gray rows in Table 1), volume estimations were computed for 6 of them. Eventually only 7 events were not characterized at all (En: 2, 14, 29, 30, 32, 40, 50), while duration and maximum value of the radiative power were retrieved for the event occurred on 2 December 2013 (En: 44).

We observed that 38 of the 39 events detected between 2011 and 2013 follow the typical waveform associated with lava fountains, characterized by three phases (Figures 1A,B). In particular, during 2013 we noticed an increase in explosive behavior visible in the shape of the radiant heat flux curve of some of the events (Figures 1C,D), more peaked with a shorter climax and a very rapid cooling, with a loss of more than 10 GW in 15 min, possibly related to the cooling of pyroclastic material rather than lava. From December 15th, 2013 the general radiative heat flux waveform changes, the duration of the events increases and the cooling phase is not as easy to isolate (Figure 4).

Figure 4

Volumes, durations and MOR of the events occurred between 2011 and 2015, excluding the two long-lasting effusive periods of 2014, are reported in Figure 5. Considering only cloud-free events, erupted lava volumes (Figure 5A) vary between a minimum volume of 0.49 × 10⁶ m³ (erupted on 28 November 2013, En: 43) and a maximum of 2.86 × 10⁶ m³ (erupted on 12 August 2011, En: 10). The median and standard deviation are respectively 1.4 × 10⁶ and 0.6 × 10⁶ m³, for a total volume expressed as dense-rock equivalent (DRE) of 64 × 10⁶ m³ erupted during 43 events. Durations range from 2.5 h occurred on 23 October 2011 (En: 17) and 12 April 2012 (En: 24) to 3.5 days occurred on 14 June 2014 (En: 48), while MOR (Figure 5B) varies between 4.2 (on 14 June 2014, En: 48) and 260 m³ s⁻¹ (on 19 February 2013, En: 26). MOR for each event was obtained by dividing the total volume by the event duration, where the latter was computed by analyzing the radiant heat flux curves from the first thermal anomaly to the beginning of the cooling phase. To highlight the presence of trends or patterns characterizing the temporal distribution of eruption intensities, we calculated the moving average of the MOR time series with a window size set to 5 (orange line in Figure 5B).

Figure 5

In order to compute the total volume, we assumed that the eight missing events (En: 2, 14, 29, 30, 32, 40, 44, 50) had median volumes (1.4 × 10⁶ m³) obtaining an estimate of 75 × 10⁶ m³ over 51 events. Applying a correction of 20% for vesicles (according to Behncke et al., 2014) and including the two effusive periods, the cumulative volume, from January 2011 to December 2015, reaches ~106 ± 22 million of cubic meters (Figure 6). Of these, 50.6 ± 4 million of cubic meters, emitted between May 2012 and October 2014, are well in agreement with the 56.4 million of cubic meters derived from DEM difference (De Beni et al., 2015). Comparing the SEVIRI-derived results with field data (Behncke et al., 2014; De Beni et al., 2015), we found a good fit, generally within the measurement error, with the volume difference ranging from 0.12 × 10⁶ m³ (En: 35) and 1.49 × 10⁶ m³ (En: 36) with an average difference in the volumes of 0.62 × 10⁶ m³.

Figure 6

We also explored the chance to provide an upper bound to the SEVIRI-derived cumulative volume by using a topographic approach, i.e., by subtracting the 2005 DEM from the 2015 one. In this way we estimated the distribution and volume of volcanic deposits emitted between 2006 and 2015. To minimize the errors introduced by the fact that the two DEMs were derived from different data sources, we first resampled the 2005 DEM to the same spatial resolution of the 2015 DEM (4 m) and improved their coregistration so that the histogram of the elevation differences was centered on zero (Figure 7). Positive values of the bar chart represent the cells where the volcanic deposits emplaced.

Figure 7

The volcanic deposits emplaced between 2006 and 2015 cover an area of ~12.6 × 10⁶ m², with a maximum length of ~6.8 km due to the lava flow emplacement of the 2008–2009 flank eruption (Figure 8). The minimum, median, and average thicknesses, computed from DEMs difference, are 1.01, 13.62, and 22.84 m, respectively. The maximum thickness of 214 m is obtained at the NSEC cone.

Figure 8

The total volume of products, calculated by integration of the thickness distribution over the area covered, is 287 × 10⁶ m³, with an uncertainty of ~37 × 10⁶ m³, corresponding to 13% of the entire volume. The uncertainty was computed by multiplying the area (~12.6 × 10⁶ m²) by the residual vertical accuracy outside the margins of the deposits, i.e., the standard deviation (~2.95 m) of the height difference between the DEMs in the area not covered by deposits. The NSEC cone covers an area of ~0.53 × 10⁶ m² with a volume of 55 ± 1.5 × 10⁶ m³.

Discussion and conclusions

The 5 years spanning from January 2011 to December 2015 at Etna were analyzed through SEVIRI images. Thanks the frequent revisit time, i.e., 15 min, SEVIRI represents the unique space sensor able to detect and characterize the short-lived activity (a few hours for each event) as that occurred at Etna during 2011–2013.

Radiant heat flux curves apparent in SEVIRI images permitted to compute durations, volumes, and intensities, cataloguing almost all the events (46 of 53) and to discriminate among fountaining activity (e.g., Figures 1A,B) with an information about the degree of explosivity (e.g., Figures 1C,D), Strombolian activity (e.g., Figure 4) and long effusive periods (Figure 2). A statistical analysis on SEVIRI-derived results, excluding the two longest effusive events of 2014, shows a Gaussian distribution slightly right skewed of the estimated volumes peaked on 1.4 million of cubic meters (Figure 9A), while the durations follow the typical features of a Pareto distribution with a large portion of events (76%) lasting around 500 min (Figure 9B). Distribution of MOR underlines the temporal non-stationarity of eruption intensity, mainly decreasing from February 2013, and reaching the minimum values since 2014 (Figure 5B). This highlights that eruptive events occurred with a higher intensity in the first period (2011–2012) than in the following 3 years of activity (2013–2015). It should be noted that our analysis does not take into account the eruptive events occurred on 28 December 2014 (because of the presence of a thick layer of clouds that prevented the observation from satellite) and the eruptive activity occurred at the VOR crater on 3–5 December 2015 (Bonaccorso and Calvari, 2017).

Figure 9

By differencing the 2005 and 2015 DEMs and excluding the volume of the NSEC cone, we obtained a 10-year volume of ~232 × 10⁶ m³, which is below the recent erupted volumes on decadal scale at Mt Etna (~300 × 10⁶ m³ from Harris et al., 2011), interrupting its stable and resilient output trend. By further subtracting published products retrieved from LIDAR analysis in 2007 and 2010 (Neri et al., 2008; Behncke et al., 2016), we found ~131 million of cubic meters of eruptive products emitted since 2011, which is still about ~25 × 10⁶ m³ higher than the volumes derived by SEVIRI. This difference, ranging between 10 and 30% of the total volume, is most likely due to loose materials as the pyroclastic fallout deposits in the same area where the lavas were emplaced and/or to the presence of tephra-covered snow.

Our study revealed a decreasing trend in eruption intensity (according to the volume of erupted products) beginning from 2013 and reaching lower values (around 5 m³ s⁻¹) in 2014 and 2015. This trend was confirmed also in the growth of the NSEC cone that gained only 5 million of cubic meters from December 2014 to December 2015, against 50 × 10⁶ m³ observed from January 2011 to October 2014 (De Beni et al., 2015). In particular, the plot of cumulative volume (Figure 6) allows us to identify five different phases in the observed period: (i) an initial phase, from 12 January to 9 July 2011, with a low TADR (0.41 ± 0.08 m³ s⁻¹); (ii) a growth phase, from 18 July 2011 to 24 April 2012, with a TADR of 1.53 ± 0.31 m³ s⁻¹; (iii) a peak phase, from 19 February 2013 to 27 April 2013, with a TADR of 3.68 ± 0.74 m³ s⁻¹; (iv) a waning phase, from 26 October 2013 to 7 July 2014, with a TADR of 1.00 ± 0.20 m³ s⁻¹; (v) a final phase, from 28 December 2014 to 8 December 2015, with an extremely low TADR (0.15 ± 0.03 m³ s⁻¹).

In terms of long-term output, we find that 106 ± 22 × 10⁶ m³ of lava was erupted in almost 5 years, which results in a time-averaged rate of 0.68 ± 0.14 m³ s⁻¹. This estimate falls within the range between 0.6 and 0.9 m³ s⁻¹ that characterizes the high eruption rate phase of Etna beginning in 1971 (Harris et al., 2011). However it interrupts the steady state output rate of 0.9 m³ s⁻¹ started in 2000 (Harris et al., 2011; Ganci et al., 2012a).

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