Morphometric analysis of lava flow units: Case study over LIDAR-derived topography at Mount Etna, Italy

Journal of Volcanology and Geothermal Research 235–236 (2012) 11–22

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Morphometric analysis of lava flow units: Case study over LIDAR-derived topography at Mount Etna, Italy Simone Tarquini ⁎, Massimiliano Favalli, Francesco Mazzarini, Ilaria Isola, Alessandro Fornaciai Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Pisa, Italy

a r t i c l e

i n f o

Article history: Received 17 February 2012 Accepted 27 April 2012 Available online 10 May 2012 Keywords: LIDAR Lava flow unit Lava flow morphology High resolution DEM Etna

a b s t r a c t High resolution, LIDAR-derived digital elevation models of volcanic areas can significantly improve knowledge of lava flow morphology and emplacement mechanisms. Here we focus on single flow units, presenting a new semi-automatic procedure which provides a quantitative analysis of their shape. The method relies on the automatic processing of the elevation profiles obtained on transects orthogonal to the flow unit axis. The initial phase of the Mount Etna flank eruption from September 2004 is taken as test case, and the procedure is applied on an active lava flow, which was emplaced on the eastern flank of the volcano. The main topographic dataset used is a 2-m-resolution digital elevation model obtained from a LIDAR survey. Starting from the axis of a lava flow unit, our method yields morphometric data on the flow unit at a 2 m spacing, calculating parameters including flow width, channel width, the heights of the levees, inward and outward slope of levees, and estimating pre-emplacement slope along the axis. The procedure is embedded in a customized GIS, which allows easy processing, handling and displaying of data. The procedure has also been applied to another flow unit emplaced during the October–November 1999 overflow from the Bocca Nuova crater. Results show that the channel width seems to accommodate first‐order trends of the pre-emplacement slope along the flow unit axis, while it is little affected by high frequency changes in slope; in contrast, flow unit width and flow unit thickness are apparently influenced by small‐scale changes in slope. The different emplacement conditions of the two flow units are reflected by the overall contrasting morphologies, as shown by the different average thickness and by the different ratios between (i) flow width vs. channel width and (ii) flow unit section area vs. channel width. The new method provides an enhanced, systematic and thorough morphometric description of flow units, which may improve the understanding of the emplacement mechanisms of lava flows on Earth and other planets. © 2012 Elsevier B.V. All rights reserved.

1. Introduction At basaltic volcanoes such as Mount Etna (Italy), persistent effusive activity led to the formation of complex flow fields (Walker, 1971; Frazzetta and Romano, 1984; Guest et al., 1987; Calvari et al., 2002; Favalli et al., 2011; Tarquini and Favalli, 2011) whose global extent is a critical parameter for understanding the associated hazards (Kilburn and Lopes, 1988; Andronico and Lodato, 2005; Behncke et al., 2005). Lava flow fields grow mainly by iterative emplacement of newly forming flow units. According to Wadge (1978), a flow unit is a body of lava that flows and cools as a single entity. Flow units fed over a sufficient time interval invariably tend to construct a channel bounded by levees (Hulme, 1974). Sparks et al. (1976) explored this point providing the first classification of levees observed in lava flows. Once a channelized flow is established, it may experience a variety of dynamics such as overflow, blockage, levee breaching, bifurcation,

⁎ Corresponding author. E-mail address: [email protected] (S. Tarquini). 0377-0273/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2012.04.026

roofing, etc. (e.g. Calvari and Pinkerton, 1998; Harris et al., 2005; Bailey et al., 2006; Harris et al., 2009; Applegarth et al., 2010; Favalli et al., 2010a; James et al., 2012). As the effusive activity continues and flow units lengthen, these dynamics tend to transform/disrupt active channels until they are eventually abandoned (at least partially) and new flow units form increasing the complexity of the flow field. Lava flows consist of an unconfined multiphase and multicomponent stream whose temperature, rheology, and emission-rate all vary with time and space. Given the relevance of the evolution of flow units for hazard and risk assessment, their geometry and thermo-rheological properties have been modeled over years (e.g. Wilson and Parfitt, 1993; Tallarico and Dragoni, 1999; Harris and Rowland, 2001; Valerio et al., 2008). Because of the intrinsic complexity of these systems, similar studies need to be compared to the large spectrum of actual flow units to be validated. An accurate and systematic collection of a similar wealth of morphometric data has been lacking until recent years (e.g. Lipman and Banks, 1987; Calvari et al., 1994; Harris and Neri, 2002), when new technologies such as airborne LIDAR, started to be applied over volcanic areas, making very high resolution/high accuracy topographies available to volcanologists (Mazzarini et al., 2005; Pyle and Elliott, 2006; Csatho

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et al., 2008; Ventura and Vilardo, 2008; Favalli et al., 2009, 2010a; Fornaciai et al., 2010). Appropriate methods to effectively collect and process reliable morphometric data from digital elevation models (DEMs) become crucial. Here we introduce a new semi-automatic method to improve and speed-up the acquisition of morphometric measurements of channelized lava flow units from high resolution DEMs. The early phase of the 2004 Mount Etna effusive activity imaged by the LIDAR survey presented by Mazzarini et al. (2005) is used as a test case. Afterwards, we apply the processing on a flow unit emplaced westward from the rim of the Bocca Nuova crater during the 1999 Mount Etna eruption (Harris and Neri, 2002), and discuss the results obtained in the two cases. 2. Case study 2.1. The Mount Etna effusive activity imaged on 16 September 2004 The 2004 Mount Etna flank eruption started on 7 September, when a fracture field opened on the lower eastern flank of the South East Crater (SEC), from an elevation of 3000 m asl toward ESE (Fig. 1). A first small lava flow formed emerging from a fissure at an elevation of 2920 m lasting two days (Burton et al., 2005; Allard et al., 2006). Afterwards, fractures propagated further down slope, and on 10 September new vents opened at 2620 and 2320 m asl. The effusive activity continued from these two vents and eventually ceased on 8 March 2005 (Allard et al., 2006). In the meantime, on 16th September 2004, an airborne LIDAR survey was carried out over a large portion of Mount Etna (Mazzarini et al., 2005), imaging the 2004 Mount Etna eruption a few days after the onset of the effusive activity, when two lava flows were emplaced simultaneously in the Valle del Bove from vents at 2620 and 2320 m asl (Mazzarini et al., 2005; Favalli et al., 2009). The northernmost of the two flows

already showed two major overflows originating from both sides of the main flow unit. 2.2. Topographic datasets used We use topographic data of the summit area of Mt. Etna acquired with an Optech ALTM 3033 laser altimeter on 16th September 2004, from 7:00 to 8:30 a.m. LT. The LIDAR device emits 33,000 pulses/s and warrants a horizontal accuracy of 1/2000× altitude and an elevation accuracy of ±15 and ±35 cm at 1.2 and 3.0 km flight altitude (respectively, nominal specifications of the instrument). Acquired data were processed to construct a 2 m-cell size DEM of the volcano in grid format with an elevation accuracy of ±0.40 m, a horizontal accuracy of ±1.5 m (Mazzarini et al., 2005; Fig. 1). For brevity, we call this LIDAR-derived DEM the 2004 DEM. We also made use of TINITALY/01 DEM (Tarquini et al., 2007, 2012), which covers the whole of Italy. For Mount Etna, TINITALY/01 is built upon the digital cartography derived from an aero-photogrammetric survey carried out on 1998 (Neri et al., 2008). This DEM was derived in a triangular irregular network format (TIN) improved by the DEST algorithm (Favalli and Pareschi, 2004). Here we used a DEM in grid format derived from the TINITALY database, which was found to have a root mean square error (RMSE) in elevation of 1.98 m (Neri et al., 2008). For brevity, we refer to this DEM as the 1998 DEM. It served as a pre-emplacement topography for a large portion of the northernmost flow unit (e.g. Allard et al., 2006). 2.3. Previous systematic morphometric analysis of a channelized lava flow unit Based on the above LIDAR survey, Mazzarini et al. (2005) presented a set of data describing the morphometry of the northernmost lava flow unit emplacing on 16 September 2004 at Mt Etna. Measurements

Fig. 1. Shaded relief of Mount Etna, Italy. Dark grey area shows the extent of the DEM obtained from the 16 September 2004 LIDAR survey. Thick black contours show the coverage of: (i) the flow field formed as a consequence of the overflow from the Bocca Nuova crater in October–November 1999, and (ii) the flow field formed by the September 2004–March 2005 flank eruption. SC indicates Summit Craters, and VdB indicates the Valle del Bove valley. Projection, UTM zone 32 N; Datum WGS84.

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3. Method

orthogonal to the flow unit axis are then automatically traced every 2 m from the vent. A profile is derived from each section, and it is considered as a function in the form y = f(x) where y is the elevation and x is the coordinate along the section (plots in Fig. 3). The first and second derivatives (f′ and f″) are then calculated over a densified and smoothed version of the profile and points of relative minima and maxima in the second derivative are identified on the original profile (Fig. 3). All these points are candidate “tie-points”, and some of these are then selected to identify specific morphometric features on the considered profile. All the elevation profiles are processed as in the case illustrated in Fig. 3, and are aligned one below the other creating a customized reference system centered on the flow axis (Goff and Nordfjord, 2004; Fig. 4). The vertical shift between two successive profiles, as a rule, is constant, but locally it may increase to avoid intersections between profiles. The points of maxima and minima of the second derivative are reported on each profile and are assigned with a color table to rank absolute values of minima and maxima (Fig. 4c). These points are derived by a straightforward mathematical processing of each profile and are used to obtain an objective tracing of morphological features. Specific tools have been devised to easily draw such features by connecting a series of homologous points on successive profiles (Fig. 4d). Fig. 5 illustrates the definition of a first set of morphometric parameters measured through the described mathematical processing. The proposed procedure also collects the heights of the left and right levees, allowing the identification and quantification of asymmetry in the levee heights due, for example, to lateral overbanking at the flow bends (e.g. Harris and Neri, 2002). In addition, both the inward and the outward slope of levees have been collected (Fig. 5). Fig. 6 compares channel and flow widths measured by Mazzarini et al. (2005) with the ones collected here. The two methods provide similar results: channel widths essentially coincide, while flow widths show some local discrepancy, essentially due to the presence of two major overflows which disrupted the geometry of the main flow unit.

3.1. Mathematical processing of elevation profiles

3.2. Baseline interpolation

The initial step of the new procedure is the tracing of the axis of the flow unit, which is usually done by hand (Fig. 3). Sections

Some morphological parameters need a reference baseline (Fig. 2). Generally a pre-emplacement topography is not available making approximations necessary. In Mazzarini et al. (2005) the baseline was simply the straight line joining the left and right margins of the flow unit (Fig. 2). Here, instead, we derive more realistic baselines based on a reconstruction of the pre-emplacement profile from slope trends outside the flow (see Fig. 7). The nearest portions of the elevation profile immediately outside the flow unit (on both sides) are processed to draw straight segments below the flow. Reference points are then taken on these segments at given distances from the flow unit boundaries, and further geometric constructions are used to obtain additional reference points that are joined together to form a polyline (Fig. 7). This polyline is then smoothed using a spline function to obtain a final candidate baseline (Fig. 7). Further automatic geometrical checks guarantee the consistency of the result with respect to the input profile (e.g. intersections between the baseline and the profile are not allowed but at the flow unit boundaries). This baseline relies on the assumption of a relatively simple pre-emplacement topography. Obviously, a similar premise may not be suitable for all cases, but in our test case the new baseline gives more reliable results than the straight line used in Mazzarini et al. (2005). The baseline is used for the measurement of additional morphometric parameters as defined by Fig. 8. The envelope of all the baselines below the flow unit approximates a pre-lava flow emplacement surface on which it is possible to calculate the downflow slope along the flow axis, so that, in cases where a pre-emplacement topography is not available, this procedure can be used as a proxy for this crucial

were collected every 10 m downhill; widths were measured along a section orthogonal to the flow axis, and heights as elevation differences. An elevation profile was traced for each orthogonal section to visualize the morphology and support the analysis of collected results (Fig. 2a, see also Figs. 2–3 in Mazzarini et al., 2005). Measurements were based on the morphological features manually traced on shaded relief images and on the intensity map of backscattered LIDAR pulses (Mazzarini et al., 2007; Tarquini and Favalli, 2011). The latter map, however, can help in locating flow unit boundaries only under favorable conditions, and it cannot support the detection of other features such as the top of the levees. The subjective interpretation of morphological features was a significant source of uncertainties. In Fig. 2, the hand-derived points of Mazzarini et al. (2005) are compared with selected points of relative maximum curvature of the profile (either positive or negative, i.e. concavities or convexities). This comparison suggested setting up a new procedure, which uses relative maximum curvature points of elevation profiles as key points for the identification of morphological features. These points are mathematically derived, hence they are not subjective by definition and their accuracy depends essentially on the resolution of the DEM. Levees could have been identified also by local maxima of the elevation profile, but the analysis of a large set of different profiles revealed that, in most cases, the local maximum curvature is a more effective marker (e.g. even in case of a brimful channel). In addition, curvature variations (in this case the minimum) also work for the determination of flow width. It was also evident that to collect, store, retrieve and analyze adequately a database of similar morphometric measurements, the procedure had to be embedded in a customized geographic information system (GIS). In order to tackle the above issues, we introduce an improved methodology which automatically traces profiles across the flow providing an extended set of morphometric measurements and creating a GIS database to dynamically handle/process data for many flow units.

Fig. 2. Conceptual scheme and definitions of the flow unit morphological quantities. (a) Definitions after Mazzarini et al. (2005): arrows show the hand-traced points identifying the boundaries of the flow and levee positions. Wc: distance between levee apexes; Dc: channel depth; Hl: levee heights over the baseline; Tf: thickness of lava flow below the channel surface. (b) Conceptual scheme and definitions introduced in this work. Points of maximum curvature, either concave (V) or convex (A) are selected on the profile. These points are used as key points for morphological measurements instead of the hand-traced points of scheme (a).

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Fig. 3. From the flow axis to processed elevation profiles; (a) the hand-traced axis of the flow unit (red line); (b) the whole set of sections traced orthogonally to the flow axis (every 2 m along flow); (c) zoom in the selected section “s”; (d) profile: “p” (black line) derived from the section “s” in the form of a function y = f(x), f’ (blue-light blue line), f” (pink-red line); (e) relative maxima (yellow points) and minima (green points) of f” are set on the second derivative (red line) and then projected onto the original profile (black line). Due to the noise intrinsically introduced by the derivative operator, the derivative has been applied on a densified, splined profile. This explains why a small shift at times exists between derivative peaks and their projection onto the original nodes of the profile.

parameter. Fig. 9 compares the downflow slope considering the 1998 DEM as pre-emplacement topography (as in Mazzarini et al., 2005) with the downflow slope obtained by using the envelope of the baselines. Although the results are consistent, the two plots are not coincident, because, in the first part of the diagram, the 1998 DEM is not actually the pre-emplacement topography, owing to the emplacement of the 1999 and 2000–2001 flows (Allard et al., 2006). Fig. 10 compares the area of flow unit sections obtained by using the baselines calculated here and the same area calculated by comparing the elevation profiles over the 1998 DEM and the 2004 DEM. Fig. 10 also shows 14 elevation profiles with relative baselines. By comparing profiles over 1998 DEM and 2004 DEM, a vertical offset (up to 3–4 m) between the two DEMs is observed, especially in the terminal segment of the flow. The offset can be explained either as a local bias of the 1998 DEM or as an effect of local accumulation of tephra between the two topographic surveys. This offset explains the globally higher values of the flow section areas measured from DEM comparisons with respect to the ones measured considering the baselines. This latter method provides some peaks where the baselines overestimate the flow thickness (hence increasing the section area). In the first quarter of the diagram, the two plots are far apart, due to the local inadequacy of 1998 DEM as a pre-emplacement topography. A substantial inconsistency is also obvious where the two overflows originate. Taking all this into account, the overall pattern of the two plots is quite consistent, with a substantial conformity of the broad plot outline and of local peaks. This evidence suggests that, in cases similar to the one considered here, the baseline method may provide a reliable first-order approximation of the flow unit section when only the post-emplacement topography is available.

3.3. Presentation of results and comparison with field-based measurements The new procedure yields a wealth of morphometric data. To support data analysis and the interpretation of results, tools to plot data have been embedded in the customized GIS environment. At first, a list of parameters to be plotted is selected, then the corresponding measurements are automatically plotted in diagrams juxtaposed one above the other above the rectified image of the flow. The image helps in linking data to features observed on the flow unit. An example of the display of the data is provided in Fig. 11, which shows two series of data obtained from two flow units: the northernmost 16 September 2004 flow unit and a flow unit belonging to the 1999 flow field emplaced at Mount Etna in October–November 1999 westward from the Bocca Nuova crater (Wright et al., 2001; Calvari et al., 2002; Harris and Neri, 2002; Tarquini and Favalli, 2011; see also Fig. 1). The 1999 flow unit has been identified and processed exactly in the same way as the already discussed 2004 flow unit, using the 2004 DEM. In Fig. 12 a comprehensive picture of the processed 1999 flow unit is shown according to observations of Wright et al. (2001), Harris and Neri (2002) and Tarquini and Favalli (2011). It is clear that Harris and Neri (2002) worked hard to glean a few morphometric measurements on the field (“under intolerable conditions”) along the same flow unit processed here, belonging to the flow field segment emplaced in 17 h on 28 October 1999. The first 450–500 m of this flow unit, downhill from the western rim of the crater, have probably been hidden by late evolutions of the flow field, and are not

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Fig. 5. Morphometric parameters measured from the mathematical processing of the elevation profile: Wc = channel width; Wf = flow width; Dc_r = channel depth (from the right levee); Dc_l = channel depth (from the left levee); So_r = outward slope of the right levee; Si_r = inward slope of the right levee; So_l = outward slope of the left levee; Si_l = inward slope of the left levee. Left and right are identified with respect to flow direction. Slopes are calculated as the absolute value of arctan (Δy/Δx), where Δy and Δx are the elevation difference and the abscissa difference, respectively, between the two tie-points which limit the relative profile's segment (different gray level in figure).

4.1. Channel width Mazzarini et al. (2005), for the 2004 lava flow unit, found an average downhill increase of the channel width together with opposite average decrease of the pre-emplacement slope. Given the consistency of the two datasets, this pattern is confirmed here for the 2004 flow unit. A similar pattern is consistent with a general negative correlation between slope and channel width. This model is consistent also with the 1999 flow unit morphometry, especially in the first (measured) portion of it (Fig. 12): between 100 and 500 m along flow, the channel width decreases from 30–40 m to ~10 m, while the pre-emplacement slope increases from ~10° to 30°; afterwards, in the remaining ~ 1000 m of channelized flow, channel width (while fluctuating by an amplitude of about 10 m) gradually increases up to 20–30 m, whereas the slope roughly decreases from an average of 30° to an average of 25°.

4.2. Flow unit width

Fig. 4. An example of the customized downflow reference system used to work with profiles: (a) all the profiles obtained from a flow unit are aligned and displaced one below the other; (b) points of relative maxima (yellow) and minima (green) of the second derivative are set on profiles; (c) the points identified in (b) are assigned with a color code ranked according to maxima (concavity, values increase from yellow to red) and minima (convexity, values increase from green to blue) of the second derivative; (d) morphometric features of the flow unit are obtained by connecting together selected tie-points on adjacent profiles. No vertical exaggeration is used.

In the second half of the 2004 flow unit, local maxima in flow width and local minima in pre-emplacement slope are aligned, as well as local minima in flow width and local maxima in pre-emplacement slope (e.g. relative maxima in slope at 1000, 1300, 1550 m; relative minima in slope at 1150, 1400, 1620 m downflow). To a lesser extent, this pattern is also apparent in the final 500–600 m of the 1999 flow unit (e.g. relative minima in slope at about 900 and 1250 m downflow), but also at the beginning of the same flow unit (the minimum in slope at about 200 m downflow). The average thickness of flow units is rather well correlated with its width, with main (and at times even secondary) peaks located at the same downflow distance.

4.3. Flow unit average thickness and flow section area recognizable on the basis of our DEM analysis. In Fig. 12e and f we selected the 10 elevation profiles that can be compared with measurements and sketches presented by those authors. This comparison highlights a substantial agreement between the two datasets. 4. Results Fig. 11 provides an inclusive illustration of flow units’ morphometry. The presented layout of data allows straightforward analyses of the relationships between different parameters measured in the same flow unit as well as of the relationship of the same parameters measured in different flow units. In this section the main features emerging from the whole dataset are outlined.

The flow section area can be obtained multiplying flow unit width times its average thickness, hence it is not surprising that, given the above-mentioned correlation between average thickness and width, the plot of the flow section area looks like a stretched version of the plot of the flow average thickness (with minor exceptions). In any case, we show both plots, given the relevance of both parameters. As expected from the above, the second half of the flow section area diagram for the 2004 unit shows positive peaks aligned with negative peaks in slope. A similar relation holds for the 1999 flow unit (i.e. peaks at 150, ~750, ~ 900, ~1250 m). Observing in more detail, it seems the positive peaks in the section area somewhat precede the negative peaks in the slope by 20–30 m, with few exceptions.

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Fig. 6. Channel and flow widths measured by Mazzarini et al. (2005) compared with the values calculated here. To support the interpretation of the plots, a shaded relief image of the flow unit obtained in an along-the-flow coordinate system is aligned with the plots. Colors show the thickness of the flow unit calculated over the flow baselines (Section 3.2); s1, s2, etc. refer to the position of the profiles of Fig. 10. Data is shown without any averaging or smoothing.

4.4. Levee heights

4.5. Levee slopes

Levee heights measured over the flow baseline have a pattern that mimics the one of the average flow unit thickness. Due to the typical flow unit geometry, when levees are well formed and the lava stream is well constrained inside the channel, the tops of the levees tend to be the points of maximum thickness, as occurs (with few exceptions) in the two cases shown. The record of heights of left and right levees permits flow asymmetry investigation. As an example, a narrow bending at about 150 m downflow in the 1999 flow unit originates a higher right levee, due to the “pile of overflow units capping the levee at a bend” described by Harris and Neri (2002, see also Fig. 12). Similarly, between 1300 and 1500 m along the 2004 unit, the higher right levee is probably linked to the local gentle leftward bending. The measurements of levee heights, with reference to the height of the channel, are too sparse and the method and/or the resolution of the DEM need to be improved to provide sound interpretations.

The outward and inward average slopes of the levees are measured for each profile (Fig. 5). The levee slopes appear to be particularly noisy; however, maximum values are very similar in both cases (~30°). Flow dynamics generate variations in this value, for example where small overflows widen the flow locally (e.g. at ~ 1400 m downflow in the 2004 flow unit). When the levee heights are comparable to the pixel size, the noise becomes more important. This also explain the very noisy trend of inward slopes. In the latter case, in order to collect more accurate results, a higher resolution DEM is necessary. In any case, our measurements suggest that the inward slope is higher than the outward slope (Fig. 11). 5. Discussion We will start the discussion by outlining the general morphometric patterns emerging from our data, then we will highlight the different circumstances that led to the formation of the two flow units, we will then relate morphological evidence to the emplacement history of the two cases, and finally we will discuss the relevance of this technique in supporting lava flow modeling on Earth and other planets. 5.1. General morphological patterns in flow units Overall, our data supports an inverse relation between slope and channel width (i.e. a decrease in slope is associated with an increase

Fig. 7. Scheme of baseline interpolation starting from an actual elevation profile (s7 in Fig. 9). Arrows mark flow unit margins (e1 and e2). From e1 and e2, straight lines are drawn, towards the center of the flow unit, with the angular coefficient calculated from the best fit of the nearest portions of the profile (i.e. from e1 and e2 outwards; (a)). The points k1 and k2 are taken over these straight lines at 1/3 the distance between e1 and e2, from the two edges inwards. Additional points a1 and a2 are the midpoints of segments e1–k1 and e2–k2 (b). A further point a3 is taken as the midpoint of the segment k1–k2 lowered to the minimum elevation between k1 and k2. The polyline e1, a1, k1, a3, k2, a2, and e1 is a preliminary baseline, which is then splined and, optionally, further modified to derive the final baseline (c).

Fig. 8. Main morphometric parameters based on the presence of the baseline: Hl_r = right levee height; Hl_l = left levee height; Tf = thickness of the flow section below the lower point inside the channel; gray area shows the whole section of the flow unit (between the baseline and the profile itself).

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Fig. 9. Downflow slopes computed along the flow axis over the 1998 DEM (Mazzarini et al., 2005) compared with the slopes calculated here by using the obtained baselines (averaged values). See caption of Fig. 6 for the image at the bottom and related color table; s1, s2, etc. refer to the position of the profiles of Fig. 10.

in channel width). But this relation seems to hold only for the large scale trend, while sudden changes in slopes within 100–200 m (e.g. at half length in 1999 flow unit and in the distal portion of 2004 flow unit), which can attain up to 10° (Fig. 11), do not affect channel width. On the other hand, similar high frequency changes seem to affect flow unit width noticeably, which shows an inverse relation with slope especially in the second half of the flow. The short wavelength component of the slope also shows an inverse relationship with flow unit average thickness, flow unit section area, and levee heights. This relationship holds, with few exceptions, throughout the whole flow unit.

This evidence suggests that the stream of lava, which builds up the channel and then flows inside it, has enough inertia to overcome small-scale variations of boundary conditions such as slope. Therefore, the “flow unit system” reacts to a sudden decrease in slope by increasing the height of levees, which in turn results in widening the flow unit (the repose angle of levees being roughly constant). The viscosity of the lava stream, when the downstream portion of the flow decelerates due to a lower slope, propagates a deceleration wave in the flow uphill, which may explain why the positive peaks in the section area tend to precede, by some tenths of meters, the negative peaks in the slope.

Fig. 10. Flow unit's section areas obtained by using baselines and DEMs comparison. The plot shows the area of flow unit sections identified by the baselines calculated here (averaged values) and the area of the flow section inferred from the elevation profiles over the 1998 and 2004 DEMs (i.e. the area between the two profiles, dashed areas shown in profiles s1 and s7). See caption of Fig. 6 for the image below the main plot and related color table; s1, s2, etc. refer to the position of the sections used to obtain the 14 profiles shown below. See main text for discussion.

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Fig. 11. Summary-plot of data obtained from the two processed flow units. See caption of Fig. 6 for the images at the bottom and related color table.

5.2. Different effusive styles in 1999 Bocca Nuova and 2004 flank eruption The processed 2004 flow unit represents the very early phase of a flank eruption which quietly emplaced some 40 × 10 6 m 3 of lava over about six months (Allard et al., 2006). The relative effusive regime is moderate and relatively constant, yielding on average some 2.5 m 3 s − 1 (Tarquini and Favalli, 2011). Burton et al. (2005) explained this eruption as a passive emission of degassed lava from the very shallow plumbing system of the volcano. The 1999 effusive

activity from the Bocca Nuova crater, instead, is completely different, being characterized by 11 separate overflows spaced in time with pauses in lava emission (Harris and Neri, 2002). The emplacement of single overflows lasted from hours to three days and was often accompanied by Strombolian paroxysms. Overflows from the crater were sustained by vigorous lava supplies (up to ~90 m 3 s − 1, Harris and Neri, 2002). Each overflow formed its own relatively thin, short-lived flow units, which cumulatively originated a fairly thin flow field, even when flow units overlapped each other (average thickness ~6 m, Tarquini and Favalli, 2011). The 2004 flow unit

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Fig. 12. The 28 October 1999 flow unit illustrated in previous works. (a) Landsat 7 ETM + redrawn after Fig. 1 in Wright et al. (2001). (b) The 1999 Bocca Nuova flow field mapped by Tarquini and Favalli (2011, magenta line) is overlaid to the previous image, and the yellow arrows fA1, fA2, fA31, fB1, fB2 point to the fronts of five different units identified in (a) by reddish colors (due to a distinctive SWIR radiance); the thick white line identifies the axis of the 1999 flow unit selected for processing in this work. (c) Zoom on the DEM difference map (2004 DEM–1998 DEM) presented by Tarquini and Favalli (2011), margins of the flow unit obtained here are in green. (d) Slope map obtained from the 2004 DEM, red dotted line is the flow unit axis; sections a–k (white dashed lines) identifies the profiles shown in (e). (e) Sketches redrawn after Harris and Neri (2002) compared with elevation profiles over the 2004 DEM (red lines), green circles show the position of the channel margins derived by applying the presented procedure. (f) Zoom in the first part of the channel and flow widths plots presented in Fig. 11, where data from Harris and Neri (2002) have been added for direct comparison (red circles).

imaged on 16 September, instead, was successively completely buried by the following activity which locally piled up a series of lava flows up to 70 m thick (average thickness ~ 14 m, Tarquini and Favalli, 2010, 2011). Overall, the two flow units processed here are a picture of two markedly different effusive styles: (i) the mild discharge of lava drained from a lateral opening of the volcano imaged at the beginning (September 2004), and (ii) the vigorous, intermittent overflow from a summit crater (October–November 1999 Bocca Nuova activity) imaged after their complete emplacement. 5.3. Emplacement styles and morphological evidence Wright et al. (2001) studied in detail the flow field segments emplaced on 27–28 October 1999 from a remote-sensing perspective. These authors analyzed the short-wave infrared radiance (SWIR) of a Landsat 7 Enhanced Thematic Mapper+ (ETM+) image acquired on 28 October 1999 at approximately 10:00 a.m. LT (Fig. 12). According

to their work, in this image there are two distinctive high temperature signals above the background: a moderately high value can be attributed to fairly cooled flow units which already ceased to move (red hues in Fig. 12a, flows A and B), while very high to saturated values can be ascribed to very hot, still emplacing flow units (green hues and black in Fig. 12a, flow C). According to Harris and Neri (2002), A and B in Fig. 12a refer to episode 6, and emplaced on 27 October, while C refers to episode 7, and was still emplacing at the time of image acquisition. The comparison between the Landsat image and the map of the final 1999 flow field presented by Tarquini and Favalli (2011) shows that all the five flow unit fronts identified by the moderately high SWIR values had actually ceased to move at the time of image acquisition (Figs. 12a, b; 13), confirming the correctness of the SWIR radiance analysis. Wright et al. (2001) further expanded the implication of the observed cooling, and assuming a sustained lava supply, they argued that it could have forced flow fronts to stop. But the sole evidence of

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cooling is not enough to determine a cooling-limited flow. In fact, several authors, for the above-mentioned Bocca Nuova effusive activity, described a pulsed (as opposed to sustained) lava supply (Calvari and Pinkerton, 2002; Calvari et al., 2002; Harris and Neri, 2002; Behncke et al., 2003). Harris and Neri (2002) estimated the lava supply for the 28 October effusive episode in 13–16 m 3 s − 1, and Calvari et al. (2002) proposed that the observed effusive style led to the emplacement of volume-limited flow units. Harris et al. (2007), instead, implicitly assumed that the 2004 flow unit processed here is cooling limited. Strictly speaking, as the flow was still active at that time, it was not even certain that the front had already ceased to advance reaching its maximum extension downhill, either because of cooling or because of an interrupted lava supply. Certainly, flow morphology may provide a variety of clues to unravel the emplacement conditions (Heslop et al., 1989; Wilson and Parfitt, 1993; Kauahikaua et al., 2002; Ventura and Vilardo, 2008; Zimbelman et al., 2008; Harris et al., 2009; Favalli et al., 2010b; Stevenson et al., 2012). With respect to cooling- or volume-limited flows, Pinkerton and Wilson (1994) pointed out that volume‐limited flow units show a drained channel, while cooling-limited ones are

characterized by a thickened front. According to this view, both the processed flow units seem limited by an interrupted lava supply, as the channels are well drained (apart from minor sectors of brimful channel) and the flow fronts are not thickened (Fig. 11). Although data for the first ~500 m of the 1999 flow unit is missing, Fig. 11 highlights interesting differences between the two flow units. The 1999 flow is considerably thinner than the 2004 one (3.8 and 7.2 m thick on average, respectively). Examining the data in greater detail, in the 2004 flow the thickness tends to increase downflow up to 1000–1100 m downhill, and then it tends to decrease up to the flow front 700–800 m ahead. The thickness of the 1999 flow, instead, shows no clear trend. As discussed above (Section 5.1), the width of the channel seems to be the leading parameter, being less subject to changes in boundary conditions. In Fig. 14 we explore the relation between (i) flow width vs. channel width and (ii) flow section area vs. channel width for the two flows. As expected, the difference is evident. Indeed, the 2004 flow unit had the time (5–6 days) and the moderate, relatively regular magma supply conditions (2–4 m 3 s − 1, Harris et al., 2007) to grow in height, while the 1999 flow unit emplaced quickly as a single, vigorous surge overflowing from a crater experiencing a frantic activity (Calvari and Pinkerton, 2002; Harris and Neri, 2002; Behncke et al., 2003). When both parameter ratios plotted in Fig. 14 are very high, a distinctive hung flow unit is present in the 2004 flow, characterized by a narrow channel “suspended” up to 15–20 m above the pre-emplacement topography (e.g. between 500 and 600 m downflow, just uphill from the overflows). We propose that for the 2004 flow a similar hung flow unit is the morphological evidence that a moderate, regular lava supply lasted for several days. 5.4. Detailed morphometry and flow modeling The emplacement of lava flows on Earth and other planets is a very complicated phenomenon which has stimulated a number of works (e.g. Guest et al., 1987; Lipman and Banks, 1987; Hon et al., 2003; Glaze and Baloga, 2006; Harris et al., 2009). Some authors approached the problem of lava flow behavior from an analytical point of view (e.g. Valerio et al., 2008, 2011), while others start from the collection of actual morphometric data to apply simplified models (e.g. Baloga and Glaze, 2008; Glaze et al., 2009). Both approaches provided insightful results which can contribute to our understanding of the evolution of lava flows. A common assumption in these models is that of a steady state system (e.g. for lava supply). Nevertheless, it is clear that the typically unsteady evolution with time of natural flows (e.g. Lautze et al., 2004; Bailey et al., 2006; Favalli et al., 2010a) tends to undermine or constrain the effectiveness of similar models. The continuous, systematic collection of detailed morphometric measurements along lava flow units (such as shown here in Fig. 11) provides the opportunity to improve significantly the calibration and validation of similar computational models. In addition, the enhanced morphological data collection can also be used to better configure the geometric boundary conditions in analytical models (e.g. channel width and depth). The presented technique can therefore provide an improved benchmark or a precise constraint for existing models for lava flow emplacement. Furthermore, the newly available data can inspire, per se, new models and concepts for an enhanced understanding of lava flow behavior. 6. Concluding remarks

Fig. 13. Zoom on the fronts of flow unit A of Fig. 12b. (a) The DEM difference map (2004 DEM–1998 DEM) with the correction (green line) of an evident oversight in the map of the 1999 flow field which overflowed from Bocca Nuova presented by Tarquini and Favalli (2011; black line). The DEM difference map shows that fA1 ceased to move at the green line, which compares well with the SWIR radiance map in (b).

We presented and validated a semi-automatic, GIS-based procedure for the morphometric analysis of single lava flow units. Provided topography at the necessary resolution, this procedure yields a thorough, detailed morphological description of lava flows expressed on a flow axis-reference system. The systematic application of this procedure over a number of different flow units may provide the

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Fig. 14. Ratios between morphometric parameters for the two analyzed flow units. High values of both ratios (gray strips) highlight where a characteristic hang flow unit is present in the 2004 flow. See caption of Fig. 6 for the images at the bottom and related color table.

necessary layout of data to better constrain numerical modeling of a complex phenomenon such as lava flow emplacement. We used this new procedure along with a LIDAR-derived, 2-mresolution topography of Mount Etna, to process two lava flow units: one formed during the October–November 1999 effusive activity from the Bocca Nuova crater; and another which was forming at the beginning of the 2004–2005 flank eruption. The results obtained show that the width of the channel seems to accommodate first‐order trends of the pre-emplacement slope along the flow unit axis, while it is hardly or not affected by high frequency changes in slope. In contrast, flow unit width and flow unit thickness are clearly influenced by small‐scale changes in slope. The different emplacement histories of the two flow units, documented by the existing literature and confirmed by our analyses, are reflected by two contrasting morphologies: the 1999 flow unit is relatively thin (3.8 m on average) and has a low ratio between flow unit width and channel width (2.3 on average); while the 2004 flow unit is considerably thicker (7.2 m on average) and has a higher ratio between flow unit width and channel width (3.2 on average). The application of the presented procedure is promising, and can potentially contribute to a better understanding of lava flow emplacement on Earth and other planets. Acknowledgments The authors wish to thank D. Pyle for the thorough review of the manuscript and L. Wilson for kind, supportive editorial handling. References Allard, P., Behncke, B., D'Amico, S., Neri, M., Gambino, S., 2006. Mount Etna 1993–2005: anatomy of an evolving eruptive cycle. Earth-Science Reviews 78, 85–114, http:// dx.doi.org/10.1016/j.earscirev.2006.04.002. Andronico, D., Lodato, L., 2005. Effusive activity at Mount Etna volcano (Italy) during the 20th century: a contribution to volcanic hazard assessment. Natural Hazards 36, 407–443, http://dx.doi.org/10.1007/s11069-005-1938-2. Applegarth, L.J., Pinkerton, H., James, M.R., Calvari, S., 2010. Morphological complexities and hazards during the emplacement of channel-fed `a`ā lava flow fields: a study of the 2001 lower flow field on Etna. Bulletin of Volcanology 72, 641–656, http:// dx.doi.org/10.1007/s00445-010-0351-1. Bailey, J.E., Harris, A.J.L., Dehn, J., Calvari, S., Rowland, S.K., 2006. The changing morphology of an open lava channel on Mt. Etna. Bulletin of Volcanology 68, 497–515.

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