Tomomorphometry of the Somma-Vesuvius volcano (Italy)

Transcription

1 GEOPHYSICAL RESEARCH LETTERS, VOL. 33,, doi: /2006gl027116, 2006 Tomomorphometry of the Somma-Vesuvius volcano (Italy) Guido Ventura 1 and Giuseppe Vilardo 2 Received 6 June 2006; revised 25 July 2006; accepted 31 July 2006; published 7 September [1] A tomomorphometric analysis of the Somma-Vesuvius topography is presented. This consists in extracting horizontal cross sections at different altitudes, and in determining some morphometric parameters: radius of the circle with a surface area equal to the cross section, circularity, ratio between the major and minor axis of the best fitting ellipse, orientation a of the ellipse major axis, and the x-y centroid. The Somma includes three portions: the apron zone, the flanks, and the summit caldera boundary. Between 225 m and 525 m, a is Between 600 m and 775 m, a is These are the preferred strike of the eruptive fissures affecting the northwestern Somma flanks, the faults affecting the whole edifice, the nodal planes from local earthquakes. The Somma activity developed along a NE-SW structural discontinuity, whereas the post-caldera activity concentrated along a NW-SE striking structure. Somma activity migrates from SE to NW.. Citation: Ventura, G., and G. Vilardo (2006), Tomomorphometry of the Somma-Vesuvius volcano (Italy), Geophys. Res. Lett., 33,, doi: / 2006GL Introduction [2] The topography and morphological features of volcanoes result from the complex interplay between endogenous and exogenous processes. Studies on this topic mainly focus on [Thouret, 1999]: (a) the analysis of growth and dismantling rates of volcanoes [e.g., Karátson, 1996], (b) the sector collapses, their inception mechanisms and deposits, (c) the transport and deposition mechanisms of volcanogenic flows and the hydrological effects of volcanic eruptions [Fisher, 1995; Pierson, 1995], and (d) the morphological features of lava flows and slopes of basaltic volcanoes [Moore and Mark, 1992; Rowland and Garbeil, 2000]. With few exceptions [e.g., Székely and Karàtson, 2004; Norini et al., 2004], little interest has been devoted to the quantitative analysis of the topography [Thouret and Chester, 2005]. This analysis is generally used to infer the spatial and/or linear distribution of slope, aspect, valleys, ridges, etc., whereas the variation of the topographic features with altitude is surprisingly poorly considered, even if it may provide useful information on (a) the growth modes of volcanoes, i.e. the long-term activity and erosional characteristics of oceanic basaltic volcanoes [Rowland and Garbeil, 2000], and (b) the possible role played by tectonic structures. 1 Department of Seismology and Tectonophysics, Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy. 2 Osservatorio Vesuviano, Istituto Nazionale di Geofisica e Vulcanologia, Naples, Italy. Copyright 2006 by the American Geophysical Union /06/2006GL [3] Here, we apply a tomomorphometric technique to quantitatively analyze the topographic features of the Somma-Vesuvius volcano (Italy) using the elevation contours at 25m-intervals. The adopted procedure, which, up to now, has been applied to large sectors of the Earth surface with the aim to reconstruct the plate motions [Collet et al., 2000], consists in extracting horizontal cross sections with different altitude from a topographic map, and in analyzing these sections in terms of geometric parameters. The results of the tomorphometric analysis of Somma-Vesuvius are discussed in light of the available geomorphological, volcanological and geophysical data and allow us to characterize the growth modes of the volcano and to put constraints on its structural evolution. The analytical approach used here can be extended to virtually all the volcanoes of the Earth and other planets. 2. Geological Setting [4] The Somma-Vesuvius volcano consists of an older edifice (Somma) dissected by a summit, E-W elongated caldera and a younger cone (Vesuvius, 1281 m a.s.l.), which grew inside the caldera (Figure 1a) [Santacroce, 1987; Santacroce et al., 2003]. The Somma edifice grew up above the 37 ka old deposits of Campi Flegrei [Di Vito et al., 1998], forming a cone-shaped volcano [Cioni et al., 1999]. The Somma products comprise lava flows and scoriae deposits. The Somma caldera originated from summit collapses related to four plinian eruptions which occurred between 18.3 ka [Pomici di Base eruption] and AD 79 [Pompeii eruption]. Five sub-plinian eruptions occurred between 16.1 ka and AD 1631 [Andronico et al., 1996]. The pyroclastics of these eruptions cover the northern and eastern flanks of the Somma edifice with thickness ranging from about 70 m on the lower slopes to virtually 0 m on the higher slopes. Effusive activity in alternation with low energy explosive phases took place between AD 1637 and 1944, though if minor effusions are recognized between AD 79 and AD 472 [Arnò et al., 1987] (Figure 1a). Lava flows of the period fill the caldera depression and extensively cover the southern and western flanks of the volcano. The present-day Vesuvius crater was originated during the last eruption in AD 1944 [Santacroce, 1987]. Minor vents of the post-ad 1631 activity crop out inside the caldera and on the western and southern flanks of the volcano. The 1794 and 1861 eruptive cones are aligned along E-W striking fissures (Figure 1a). On the Somma slopes, parasitic vents developed before 16.1 ka are buried [Andronico et al., 1996]. These vents align along fissures striking NW-SE and NE-SW (Figure 1a). [5] Mesostructural data indicate that Somma-Vesuvius is affected by three major fault systems (Figure 1b): two main systems striking NW-SE and NE-SW, which are those acting at a regional scale, and a second-order E-W system 1of5

2 VENTURA AND VILARDO: TOMOMORPHOMETRY SOMMA-VESUVIUS VOLCANO Figure 1. (a) Location and simplified geological map of the Somma-Vesuvius volcano superimposed on a shaded relief image of the Digital Terrain Model (UTM WGS84 projection, distance in meters). Contour lines are 100 m spaced. Geological data are from Santacroce [1987] and Santacroce et al. [2003]. Structural data are from Bianco et al. [1998] and Bruno and Rapolla [1999]. (b) Rose diagram of the strike of 76 fault planes from mesostructural data (data from Bianco et al. [1998]). (c) Rose diagram of the strike of 98 dikes affecting the south facing wall of the Somma caldera (data from Marinoni [2001]). (d) Rose diagram of the strike of 60 nodal planes from 30 local earthquakes occurred between 1995 and 1998 (data from Bianco et al. [1998]). [Bianco et al., 1998; Bruno and Rapolla, 1999]. These faults move in response to a NNE-SSW extension. The E-W normal faults control the geometry of the Somma caldera. Dikes affecting the south facing wall of the Somma caldera show a NE-SW preferred strike (Figure 1c) [Marinoni, 2001]. Nodal planes of focal mechanisms from local seismic events show a NW-SE preferred strike (Figure 1d) [Ventura and Vilardo, 1999], and seismic anisotropy studies also suggest the occurrence of NW-SE aligned cracks within the volcano [Bianco et al., 1998]. The NE-SW and NW-SE striking structures affecting the Somma-Vesuvius volcano control the distributions of the drainage network of the Somma edifice, which is characterized by incisions with NW-SE and NE-SW preferred orientations [Ventura et al., 2005]. These incisions mainly affect the northern and eastern flanks of the Somma edifice, where the pyroclastics concentrate, with only a few on the western and southern flanks of the volcano, which are mantled by the historical lava flows. 3. Data and Analysis [6] The vector height data used in this study are extracted from the 1:5,000 Technical Map of the Naples Province [Sistema Informativo Territoriale della Provincia di Napoli, 2001]. Our interest is to analyze in a quantitative way the large-scale topographic features of Somma-Vesuvius. In order to remove the highest topographic frequencies, and because the extracted parameters (see below) can be estimated only for closed contour lines (polygons), our final dataset consists of 50 horizontal cross-sections spaced at 25 m height interval between 50 and 1275 m of altitude. The parameters calculated for each cross-section are: (1) the perimeter P, (2) the surface area A, (3) the equivalent radius r, which is the radius of the circle with a surface area equal to the cross section, (4) the circularity C, which is defined as C = (4pA)/P2 (C = 1 for a circle) (5) the ratio R between the major and minor axis of the best fitting ellipse, determined following Gander et al. [1994], (6) the orientation a (in degrees from the North in a clockwise trend) of the major axis of the best fitting ellipse, and (7) the x-y centroid. These parameters are determined using the AutoCADlt2000 software by Autodesk Inc. 4. Results [7] Results of the tomomorphometric analysis are summarized in Figure 2. In a r vs. altitude plot (Figure 2a), different trends defined by a continuous variation of r with altitude may be recognized: a trend A, from 50 m to 875 m, includes the Somma-Vesuvius apron and the flanks of the volcano; the trend B, which extends from 900 m to 1275 m, and the trend C, from 975 m to 1100 m, include the outer flanks of the Vesuvius cone and the inner crater, respectively; the trend D identifies the top of the Somma edifice and includes the northern caldera rim. The Colle Umberto tholoid is defined by the two separate points at 850 m and 875 m of altitude in Figure 2a. In a C vs. altitude plot 2 of 5

3 VENTURA AND VILARDO: TOMOMORPHOMETRY SOMMA-VESUVIUS VOLCANO Figure 2. (a) r vs. altitude plot. Letters highlight the trends depicted in the text. Legend refers to the a d plots. The shaded relief from a DTM is coloured according to the trends identified in the plot. (b) C and number of valleys vs. altitude. Number of valleys is calculated by counting the valleys from the drainage network of Ventura et al. [2005] crossing sections with a constant altitude. Brackets underline the different portions of the Somma edifice discussed in the text. (c) R vs. altitude plot. (d) a vs. altitude plot. Brackets underline the cross sections characterized by nearly constant a-values with altitude. (e) Location, in a UTM-WGS84 coordinate system, of the centroids of cross sections with altitude from 50 m to 1275 m. The fields of Somma edifice, caldera apical zone and Vesuvius cone are based on the topographic structures identified in Figure 2a. (f) Rose diagram summarizing the results of the Fry s analysis applied to the volcanic vents reported in Figure 1a. (Figure 2b), C decreases from 50 m to 225 m and increases from 250 m to 750 m. From 750 m to 900 m, C decreases. Most of the cross-sections of the top of Somma show C values less than 0.22, whereas the Vesuvius cone is characterized by C values generally larger than 0.8. In a R vs. altitude plot (Figure 2b), the Somma-Vesuvius flanks, the Vesuvius cone, and the Colle Umberto tholoid show R values generally less than 1.5, whereas the top of Somma is characterized by values larger than 2 between 900 m and 1075 m. The Somma-Vesuvius cross sections with altitude between 225 m to 525 m are characterized by nearly constant a-values between 50 and 60 (Figure 2d). Cross sections with altitude between 600 m to 775 m show also nearly constant a-values between 130 and 135. The cross sections of the top of Somma with altitude between 900 m and 1050 m are characterized by a values between 100 and 105. Figure 2e reports the map of the x-y centroids of the cross sections. In this map, the three structures of the Somma edifice, Somma caldera, and Vesuvius cone may be easily recognized. 5. Discussion and Conclusions [8] The data summarized in Figures 2a and 2e show that the Somma-Vesuvius volcano results from the superimposition of three main topographic/morphological structures: the Somma edifice, the caldera apical zone, and the Vesuvius cone. Results from Figure 2b allow us to recognize, within the Somma edifice, a lower portion (altitude < 225 m) characterized by a decrease of C with altitude, a middle portion (225 m < altitude < 750 m) characterized by an increase of C, and an upper portion (750 m < altitude < 900 m) in which C decreases. The lower portion represents the apron zone of the volcano, where the transition between the alluvial plain and the volcano flanks occurs (Figure 1a). The middle portion includes the flanks of the volcano covered by the 3of5

4 VENTURA AND VILARDO: TOMOMORPHOMETRY SOMMA-VESUVIUS VOLCANO pyroclastics of the plinian and sub-plinian eruptions, while the upper portion includes the steeper flanks of the volcano, where the Somma lavas outcrop. The progressive decrease of C with altitude in the apron zone could be due to an increase in the complexity of the topography moving from the alluvial plain to lower slopes of the volcano, where numerous, small incisions affect the pyroclastics [Ventura et al., 2005], as also suggested by the increase in the number of valleys with altitude (Figure 2b). The increase of C with altitude recognized in the middle portion is related to the progressive simplification of the drainage network with altitude. In this portion, the number of valleys decreases with altitude (Figure 2b), and the small incisions coalesce in the larger valleys that characterize the higher portion of the Somma. The decrease of C with altitude characterizing the higher portions of the Somma may explained taking into account that this sector of the volcano represents the transition between the Somma upper slopes and the caldera rim zone, which is characterized by the lowermost values of C (Figure 2b). As a matter of fact, the cross-sections of the caldera rim zone show an elliptical shape with R values usually larger than 2 (Figure 2c). Results from Figures 2c and 2d show that the Somma cross-sections between the apron zone and 525 m of altitude are slightly elongated (Figure 2c), with the major axis of the best fitting ellipses striking from the North, i.e. NE-SW to ENE-WSW. This is also the strike of the eruptive fracture that affects the northeastern flank of Somma and extends from the caldera rim to the lower slopes of the edifice (Figure 1a). In addition, the preferred strike of the dikes cutting the south-facing walls of the Somma caldera is also NE-SW (Figure 1c). The major axis of the best fitting ellipses of the cross-sections between 500 m and 775 m of altitude follows a nearly constant NW-SE strike. This is the strike of the eruptive fracture outcropping in the northwestern sectors of the Somma edifice (Figure 1a), and that of the main fault system affecting the volcano (Figure 1b). In addition, it worth nothing that the focal mechanism of the most energetic (Md = 3.6) earthquake which occurred at Vesuvius in the last 25 years (October 9, 1999) is also consistent with a NW-SE striking, normal faulting [Del Pezzo et al., 2004] and results from seismic anisotropy evidence the occurrence of NW-SE aligned cracks within the volcano [Bianco et al., 1998]. The above observations suggests that, despite the cone-shaped morphology of the Somma edifice (Figure 2c), the early stages of the Somma activity were characterized by magma emissions along a NE- SW to ENE-WNW structural discontinuity, whereas the later stages developed along a NW-SE striking structure, which, on the basis of seismic data, is that acting at the present time. To verify with an independent method if the tectonic structures play a role in controlling the volcanism, as suggested above, we apply the Fry s spatial analysis of Vearncombe and Vearncombe [2002] to the Somma-Vesuvius vents reported in Figure 1a with the aim to evidence significant alignments of vents. This analysis consists of placing in turn every vent to a common origin and plotting all other vents relative to this origin. The resulting plot is analyzed by construction of a rose diagram recording the relationships in each radial sector. This diagram shows the preferred directions of spatial continuity that may correspond to geological lineaments. Results of this analysis are shown in Figure 2f. The rose diagram shows an absolute maximum oriented NW-SE and a second-order maximum at NE-SW. This result indirectly confirms the above reported considerations and led us to conclude that the Somma-Vesuvius volcanism was mainly controlled by NW- SE and NE-SW striking structures, which, following Bianco et al. [1998], are those acting at a larger, regional scale. In light of the results from the tomomorphometric analysis reported here (Figure 2d), these latter structures mainly acted during the early stages of evolution of the Somma activity, i.e., pre-18 ka. Following the structural model of Acocella and Funiciello [2006] for the Tyrrhenian margin of the Apennine chain, the subvertical attitude of the NE-SW structures enhances the uprising of magma to the surface, whereas the listric geometry of the NW-SE faults favours the storage of magma at shallow depth. We conclude that the early (pre-18 ka) effusive to lowenergy activity of Somma-Vesuvius was characterized by the uprising of magma along the subvertical NE-SW structures. The successive (post-18 ka), mainly sub-plinian and plinian activity was characterized by the formation of shallow magma chambers controlled by the NW-SE structures. [9] Data reported in Figure 2e show that the centroids of the Somma cross sections move from SE to NW as the altitude increases. This suggests a migration of the Somma activity from SE to NW, possibly along the above recognized NW-SE structural discontinuity. The cross-sections of the caldera rim between 900 m and 1050 m are characterized by a values between 100 and 105, i.e. E-W (Figure 2d). The centroids of the caldera cross sections also roughly align along a N100 E trend (Figure 2e). The Somma-Vesuvius elliptic caldera is E-W elongated and the orientation of cross sections of the Somma apical zone, which includes the caldera rim, reflects the elongation of the caldera depression. [10] As concerns the Vesuvius cone, our data indicate that it represents a nearly ideal circular structure (C generally larger than 0.8 in Figure 2b). However, Figure 2e shows that the centroids align along a NW-SE direction with a migration from NE to SW as the altitude increases. As matter of fact, the present-day Vesuvius cone consists of two coalescing cones: the cone of the 1906 eruption, which is located northeast of the present-day crater, and whose crater rim is covered by the products of the 1944 activity, and the 1944 cone with crater (Figure 1a) [Santacroce, 1987]. Then, the observed migration of centroids from NE to SW (Figure 2e) could reflect the superimposition of the 1944 cone on the older, 1906 cone. [11] The results presented here indicate that the tomomorphometric analysis of volcanoes can give useful information on their large-scale morphostructural evolution. This analysis can be applied to other volcanoes of the Earth and other planets provided that (1) the highest topographic frequencies, which generally mirror the morphology of lavas, pyroclastic- and debris-flow deposits, or parasitic vents, can be removed, and (2) the contour lines are closed (polygons). [12] Acknowledgment. This study was financially supported by grant from Civil Protection Department of Italy, National Group of Volcanology. References Acocella, V., and R. Funiciello (2006), Transverse systems along the extensional Tyrrhenian margin of central Italy and their influence on volcanism, Tectonics, 25, TC2003, doi: /2005tc Andronico, D., R. Cioni, and R. Sulpizio (1996), General stratigraphy of the past 19,000 yrs at Somma-Vesuvius, paper presented at Vesuvius 4of5

5 VENTURA AND VILARDO: TOMOMORPHOMETRY SOMMA-VESUVIUS VOLCANO Decade Volcano Workshop, Int. Assoc. of Volcanol. and Chem. of the Earth s Inter., Naples, Italy, Sept. Arnò, V., C. Principe, M. Rosi, R. Santacroce, A. Sbrana, and M. F. Sheridan (1987), Eruptive history, in Somma-Vesuvius, edited by R. Santacroce, Quad. Ric. Sci., 114, Bianco, F., M. Castellano, G. Milano, G. Ventura, and G. Vilardo (1998), The Somma-Vesuvius stress field induced by regional tectonics: Evidences from seismological and mesostructural data, J. Volcanol. Geotherm. Res., 82, Bruno, P. P., and A. Rapolla (1999), Study of the sub-surface structure of Somma-Vesuvius (Italy) by seismic reflection data, J. Volcanol. Geotherm. Res., 92, Cioni, R., R. Santacroce, and A. Sbrana (1999), Pyroclastic deposits as a guide for reconstructing the multi-stage evolution of the Somma-Vesuvius Caldera, Bull. Volcanol., 61(4), Collet, B., J. F. Parrot, and H. Taud (2000), Orientation of absolute African plate motion revealed by tomomorphometric analysis of the Ethiopian dome, Geology, 28(12), Del Pezzo, E., F. Bianco, and G. Saccorotti (2004), Seismic source dynamics at Vesuvius volcano, Italy, J. Volcanol. Geotherm. Res., 133, Di Vito, M. A., R. Sulpizio, G. Zanchetta, and G. Calderoni (1998), The geology of the south western slopes of Somma-Vesuvius, Italy, as inferred by borehole stratigraphies and cores, Acta Vulcanol., 10(2), Fisher, R. (1995), Decoupling of pyroclastic currents Hazard assessments, J. Volcanol. Geotherm. Res., 66(1 4), Gander, W., G. H. Golub, and R. Strebel (1994), Least-squares fitting of circles and ellipses, BIT, 34, Karátson, D. (1996), Rates and factors of stratovolcano degradation in a continental climate: A complex morphometric analysis of 19 Neogene/ Pleistocene crater remnants in the Carpathians, J. Volcanol. Geotherm. Res., 73, Marinoni, L. (2001), Crustal extension from exposed sheet intrusions: Review and method proposal, J. Volcanol. Geotherm. Res., 107, Moore, J. G., and R. K. Mark (1992), Morphology of the island of Hawaii, GSA Today, 2, Norini, G., G. Groppelli, L. Capra, and E. De Beni (2004), Morphological analysis of Nevado de Toluca volcano (Mexico): New insights into the structure and evolution of an andesitic to dacitic stratovolcano, Geomorphology, 62(1 2), Pierson, T. C. (1995), Flow characteristics of large eruption-triggered debris flows at Snow-Cald volcano, J. Volcanol. Geotherm. Res., 66(1 4), Rowland, S. K., and H. Garbeil (2000), Slopes of oceanic basalt volcanoes, in Remote Sensing of Active Volcanism, Geophys. Monogr. Ser., vol. 116, edited by P. J. Mouginis-Mark, J. A. Crisp, and J. H. Fink, pp , AGU, Washington, D. C. Santacroce, R. (1987), Somma-Vesuvius, Quad. Ric. Sci., 114, Santacroce, R., et al. (2003), The geological map of Vesuvius, 1:15000, Soc. Elaborazioni Cartogr., Florence, Italy. Sistema Informativo Territoriale della Provincia di Napoli (2001), Carta Tecnica Provinciale in scala 1:5.000, CTP5000, Naples, Italy. Székely, B., and D. Karàtson (2004), DEM-based morphometry as a tool for reconstructing primary volcanic landforms: Examples from the Börzsöny Mountains, Hungary, Geomorphology, 63, Thouret, J. C. (1999), Volcanic geomorphology An overview, Earth Sci. Rev., 47, Thouret, J. C., and D. K. Chester (2005), Volcanic landforms, processes and hazards, Z. Geomorphol., 140, Vearncombe, S., and J. R. Vearncombe (2002), Tectonic controls on kimberlite location, southern Africa, J. Struct. Geol., 24, Ventura, G., and G. Vilardo (1999), Seismic-based estimate of hydraulic parameters at Vesuvius Volcano, Geophys. Res. Lett., 26(7), Ventura, G., G. Vilardo, G. Bronzino, M. Gabriele, R. Nappi, and C. Terranova (2005), Geomorphological map of the Somma-Vesuvius volcanic complex (Italy), J. Maps, 1, G. Ventura, Department of Seismology and Tectonophysics, Via di Vigna Murata 605, Istituto Nazionale di Geofisica e Vulcanologia, I Rome, Italy. (ventura@ingv.it) G. Vilardo, Osservatorio Vesuviano, Istituto Nazionale di Geofisica e Vulcanologia, I Naples, Italy. 5of5