Dike Propagation in Volcanic Edifices

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Tectonophysics

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Dike propagation in volcanic ediﬁces: Overview and possible developments

V. Acocella a,⁎, M. Neri b,1

a Dip. Scienze Geologiche Roma Tre. Largo S.L. Murialdo 1, 00146, Roma, Italy b Istituto Nazionale di Geoﬁsica e Vulcanologia, Piazza Roma 2, 95123, Catania, Italy

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Article history: Eruptions are fed by dikes; therefore, better knowledge of dike propagation is necessary to improve our Received 27 December 2007 understanding of how magma is transferred and extruded at volcanoes. This study presents an overview of Received in revised form 16 September 2008 dike patterns and the factors controlling dike propagation within volcanic edifices. Largely based on Accepted 2 October 2008 published data, three main types of dikes (regional, circumferential and radial) are illustrated and discussed. Available online xxxx Dike pattern data from 25 volcanic edifices in different settings are compared to derive semi-quantitative relationships between the topography (relief, shape, height, and presence of sector collapses) of the volcano, Keywords: fi Dikes tectonic setting (presence of a regional stress eld), and mean composition (SiO2 content). The overview Volcanoes demonstrates how dike propagation in a volcano is not a random process; rather, it depends from the Topography following factors (listed in order of importance): the presence of relief, the shape of the edifice and regional Tectonic setting tectonic control. We find that taller volcanoes develop longer radial dikes, whose (mainly lateral) Eruptions propagation is independent of the composition of magma or the aspect ratio of the edifice. Future research, starting from these preliminary evaluations, should be devoted to identifying dike propagation paths and likely locations of vent formation at specific volcanoes, to better aid hazards assessment. © 2008 Published by Elsevier B.V.

1. Introduction orientation of the principal stresses, with the dike orthogonal to the least compressive stress in the crust (e.g. Nakamura, 1977; Rubin and Understanding magma ascent and extrusion at volcanoes is a crucial Pollard, 1988). This relation is best demonstrated in absence of step to minimizing hazards associated with volcanic unrest. Eruptions are prominent relief, as in flat rift zones along divergent plate boundaries often fed by dikes, as observed at numerous active volcanoes worldwide, (Iceland, Afar). In such locations dike propagation may be heavily for example, Afar (Sigmundsson, 2006), Cerro Negro (Nicaragua; La influenced by stiffness contrasts within the host rock (Gudmundsson, Femina et al., 2004), Miyakejima (Japan; Ueda et al., 2005), Iwate (Japan; 2006, and references therein). Sato and Hamaguchi, 2006), Kilauea (Hawaii; Desmarais and Segall, The presence of a volcanic edifice, with some relief, complicates 2007), Montserrat (Lesser Antilles; Mattioli et al., 1998), Piton de la this simple dependence on the regional tectonic setting, introducing Fournaise (Reunion Island; Cayol and Cornet, 1998), Nyiragongo (Congo; significant deviations from expected patterns. Loading by the edifice Komorowski et al., 2002), Etna (Italy; Bousquet and Lanzafame, 2001), focuses the stresses above the center of a magma chamber, promoting and Stromboli (Italy; Acocella et al., 2006a). In many of these episodes, the development of a central vent system (Pinel and Jaupart, 2003). In dikes ruptured the surface close to urban areas, feeding eruptive vents addition, dikes and/or fissure eruptions at many volcanic edifices and sometimes even causing landslides and tsunamis (Komorowski et al., show characteristic radial and/or circumferential patterns (e.g. 2002; Billi et al., 2003; Behncke et al., 2005; Calvari et al., 2005). These Chadwick and Howard, 1991; Takada, 1997), suggesting control by a and other examples illustrate that to improve our understanding of local stress field imposed by a pressurized magma reservoir and/or the magma transport and eruption, and associated consequences, it is load of the edifice. In particular, the latter effect becomes predominant fundamental to advance knowledge of dike propagation. with increasing volcano height (McGuire and Pullen, 1989). The The mechanisms of dike propagation in the crust have been the location and orientation of the dikes may be also controlled by the subject of many theoretical studies in the past several decades (e.g., shape of the edifice (Fiske and Jackson, 1972), or the presence of scarps Anderson, 1936; Ode, 1957; Pollard, 1973; Pollard and Muller, 1976; along the volcano slopes, commonly produced by sector collapses (e.g. Delaney et al., 1986). The orientation of a dike is controlled by the McGuire and Pullen, 1989; Tibaldi, 2003; Walter et al., 2005a). Therefore, while dike propagation in areas without prominent relief is usually controlled by regional tectonism, the propagation of dikes in volcanic edifices seems to depend upon the shape and topography of ⁎ Corresponding author. Tel.: +39 06 57338043; fax: +39 06 57338201. fi E-mail addresses: [email protected] (V. Acocella), [email protected] (M. Neri). the edi ce, as well as the stress conditions within shallow magma 1 Tel.: +39 095 7165858; fax: +39 095 435801. reservoirs.

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This study aims at producing an overview of the factors controlling parallel to the regional extensional structures (Fig. 1a). The regional the propagation of dikes within volcanic edifices. Because eruptive dikes within volcanic edifices in rift zones are usually subvertical to fissures and rift zones on volcanic edifices are dike-fed, we include steeply dipping, several meters thick (Gudmundsson, 1998); at times, data from such activity in our analysis. Largely based on published as Hawaii, dikes may constitute more of 50% of the rift zone (Walker, data, the types of dikes and eruptive fissures in volcanic edifices are 1987, and references therein). The development of regionally- discussed to derive general, semi-quantitative insights into dike controlled rift zones is also common to arc stratovolcanoes in behavior as a function of the edifice shape, topography, structural compressional settings, such as Reventador, Ecuador (Tibaldi, 2005); setting, and volcano composition. Izu-Oshima (Ida, 1995; Fig. 1b); Iwate and Chokai, Japan (Nakano and Tsuchiya, 1992; Ida, 1995; Sato and Hamaguchi, 2006); and Batur, 2. Overview of dike patterns and propagation Indonesia (Newhall and Dzurisin, 1988, and references therein). The main difference between regional dike patterns in volcanic edifices in 2.1. Type of dikes within a volcanic edifice and related conditions of extensional and compressional settings lies in the fact that the former formation may often propagate beyond the base of the volcano, reaching considerable distances along the rift. This is the case for regional dikes The most common patterns of dikes (or eruptive fissures) in formed in volcanoes along the divergent plate boundaries in Iceland volcanic regions can be largely categorized as belonging to one of and the Afro-Arabian Rift (Gudmundsson, 1995; Ebinger and Casey, three classes: regional, circumferential and radial. While regional 2001; Sigmundsson, 2006). dikes result from the influence of a far-field (i.e. regional) stress, Independent from their tectonic setting, regional dikes within circumferential and radial dikes result from a near-field (or local) volcanic edifices may undergo vertical (Sato and Hamaguchi, 2006)or stress imposed by the presence of a pressurized magma reservoir and/ lateral (Wright et al., 2006; Paquet et al., 2007, and references therein) or the load of the volcano. The main features of each of these con- propagation. Lateral propagation usually occurs when the magma figurations are summarized below. reaches the level of neutral buoyancy within the host rock (Rubin and Pollard,1987; Morita et al., 2006), which, for most magmas, occurs when 2.1.1. Regional dikes the host rock density is between 2300 and 2700 kg/m3 (Pinel and Regional dikes within volcanic edifices are aligned with the Jaupart, 2000). The load of a volcanic edifice enhances lateral pro- regional tectonic stress field, that is, perpendicular to the least pagation of regional dikes beneath the volcano (Pinel and Jaupart, 2004), compressive stress. Several volcanoes are characterized by a dike which may partly explain the occurrence of regional, laterally complex consistent with such a far-field stress. Most of these are propagating dikes in central volcanoes and within related magmatic located along the axis of extensional rift zones, in oceanic (Askja and segments along the axis of oceanic (Gudmundsson, 1995)and Krafla, in Iceland; Gudmundsson and Nilsen, 2006, and references continental (Ebinger and Casey, 2001; Wright et al., 2006; Sigmundsson, therein) and continental settings (Erta Ale, Ale Bagu, Dabbahu and 2006) rifts. Gabho, in Afar, Nyiragongo and Nyamuragira, in Congo, Tongariro, in New Zealand, early stage of Etna, in Italy; Barberi and Varet, 1970; 2.1.2. Circumferential dikes Nairn et al., 1998; Komorowski et al., 2002; Corsaro et al., 2002; Circumferential dikes form arcuate patterns concentric to the Wright et al., 2006). Common features are the development of focused volcanic edifice, especially at summit craters and calderas, where the and clear rift zones, usually departing from the volcano summit and dikes may be also associated with pre-existing faults or fractures.

Fig. 1. a) Focused regional dike-fed ﬁssures along the rift zones of Erta Ale volcano, Afar (after Acocella, 2006). b) Dispersed regional dike-fed ﬁssures and vents at Izu-Oshima, Japan (after Ida, 1995).

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Circumferential dikes have been mostly reported along the sides of even though associated to a regional stress field, resembles an end- several calderas, including Medicine Lake (California; Donnelly-Nolan, member type of highly-clustered radial configuration of dikes (see 1990) Kilauea (Hawaii; Neal and Lockwood, 2003), Suswa (Kenya; also Section 3.1). The anisotropic distribution of radial dikes may Skilling, 1993, and references therein) and Galàpagos Islands (Ecua- result, in general, from the influence of a regional stress field which dor; Chadwick and Howard, 1991). In particular, the best-known ex- orients most dikes perpendicular to the least compressive stress amples of circumferential dikes at active volcanoes are found around (Nakamura, 1977). Anisotropic distributions of radial dikes are also the calderas of shield volcanoes in the western Galápagos archipelago, found at volcanoes in intraplate settings, in absence of a dominant especially Cerro Azul, Darwin, Fernandina, and Wolf (Fig. 2; Chadwick regional stress field. In such locations, the development of rift zones and Howard,1991). Their development may be related to overpressure may be controlled by the stability of the flank of the volcano and/or within a shallow diapir-shaped magma reservoir, which also promotes the presence of nearby volcanoes (e.g. Fiske and Jackson, 1972; radial dike formation on the lower flanks of the volcanoes (Chadwick Walter et al., 2006). Examples include Kilauea and Mauna Loa, and Dieterich, 1995). An additional factor for the development of Hawai'i (Decker, 1987), Fogo, Capo Verde (Day et al., 1999), and Piton circumferential dikes around summit calderas may be the topographic the La Fournaise (Carter et al., 2007, and references therein). In the expression of the caldera, which may re-orient the least compressive Canarian archipelago (Marinoni and Gudmundsson, 2000; Acosta stress radially (Munro and Rowland, 1996); a similar reorientation of et al., 2003, and references therein; Walter, 2003), radial dikes cluster the stress trajectories has been observed at volcanoes characterized by in rift zones that form triple-arms spaced 120° apart and focused on significant scarps, like those related to sector collapses (Fiske and the summit conduit. This configuration is interpreted to result from Jackson, 1972; McGuire and Pullen, 1989; Acocella and Tibaldi, 2005). the shallow emplacement of magma below the volcano summit and/ At extinct caldera complexes there is widespread evidence for the or the growth of nearby spreading volcanoes, promoting connecting presence of circumferential dikes lying at a deeper level within the rift zones (Walter, 2003, and references therein). volcanic edifice (Yoshida, 1984; Troll et al., 2000; Kennedy and Styx, Two mechanisms favour the development of radial dikes in a 2007), even though the nature of some of these is debated (Driscoll volcano. The first is the distribution of the gravitational stresses due to et al., 2006, and references therein). These circumferential dikes the load of the edifice. This controls the trajectories of the maximum usually form above the periphery of a shallow magma chamber, compressive stress, which becomes subparallel to the slope of the resulting from an increase (cone sheets) or decrease (ring dikes) in the volcano (Dieterich, 1988), while the minimum compressive stress is magmatic pressure (Gudmundsson, 2006, and references therein). tangential (Fig. 4a; Acocella and Tibaldi, 2005, and references therein). The larger and taller the volcano, therefore, the stronger is the 2.1.3. Radial dikes maximum local stress (McGuire and Pullen, 1989). Locally, the pro- This group of dikes has a characteristic radial distribution with pagation path of radial dikes may be controlled by the presence of regard to the axis of the volcanic edifice. The radial pattern may be significant scarps or other topographic irregularities on the volcano isotropic, with a similar frequency of dikes in every direction, or, flanks. In this case, the trajectory of the maximum compressive stress more often, anisotropic, with clustering along preferred orientations. at the scarp margin varies, becoming subparallel to the scarp; the Examples of the first group include Fernandina, Galápagos (Chadwick minimum compressive stress becomes perpendicular to the direction and Dieterich, 1995); Summer Coon volcano, Colorado (Poland et al., of the scarp (Fig. 4b). As a result, dikes tend to propagate parallel to the 2004, 2008); Kliuchevskoi, Kamchatka (Takada, 1997); as well as major scarps on volcanoes, as at Stromboli and Etna, Italy (McGuire extra-terrestrial volcanoes (Krassilnikov and Head, 2003). Examples and Pullen, 1989; Ferrari et al., 1991; Tibaldi, 2003; Neri et al., 2004; from the second group are more numerous, including Vesuvio, Etna, Acocella and Tibaldi, 2005; Rust et al., 2005; Walter et al., 2005b; Neri and Stromboli, Italy (Fig. 3; Acocella and Neri, 2003; Tibaldi, 2003; and Acocella, 2006; Neri et al., 2007). Similarly, elongated volcanic Acocella et al., 2006b); Spanish Peaks, Colorado (Ode, 1957); Hekla, edifices will be characterized by a maximum compressive stress Iceland (Gudmundsson et al., 1992, and references therein); Fuji and oriented parallel to the major axis of the edifice, these conditions will Sakurajima, Japan (Takada,1997, and references therein; Takada et al., result in the development of dikes oriented parallel to the elongation 2007); and Erta Ale, Ethiopia (Acocella, 2006).Theriftzonepassing of the edifice (Fig. 4c; Fiske and Jackson, 1972). Notable examples of through the summit of some of these volcanoes (Hekla, Erta Ale), rift zones parallel to the major elongation of the edifice include Mauna Loa and Kilauea volcanoes, Hawai'i (Decker, 1987), even though the shape of both volcanoes may be controlled by lateral instability. The second mechanism controlling the development of radial dikes is radially-oriented maximum compressive stress due to pressurization in a subsurface magma reservoir (Knopf, 1936; Ode, 1957). In this case, radial dikes form to accommodate the enlargement of the circumfer- ence of the edifice with relief, due to volcano inflation (Acocella et al., 2001, and references therein). Radial dikes may propagate vertically or laterally from the volcano's central conduit along the slope of the volcano. The mechanism of lateral propagation of dikes from the summit of a volcanic edifice remains poorly understood. Observational evidence suggests that propagation direction may depend in part on the closure/opening of the central conduit (Fig. 5; Acocella et al., 2006b). When the central conduit is closed or solidified, magma is emplaced in the edifice by means of vertically propagating dikes; in the upper part of the edifice, along the frozen conduit, these may follow gravitational stresses, becoming radial. Conversely, the lateral propagation of dikes is widespread, but not exclusive (Acocella and Neri, 2003; Lanzafame et al., 2003), in volcanoes characterized by an open summit conduit. Here, magma in the upper part of the conduit degasses and, Fig. 2. Circumferential and radial eruptive fissures (solid lines) at Fernandina volcano, becoming denser, intrudes laterally, propagating downslope. Notable Galápagos. examples of volcanoes with such a behaviour include Tenerife

Please cite this article as: Acocella, V., Neri, M., Dike propagation in volcanic ediﬁces: Overview and possible developments, Tectonophysics (2008), doi:10.1016/j.tecto.2008.10.002 4 laect hsatcea:Aoel,V,Nr,M,Dk rpgto nvlai edi volcanic in propagation Dike M., Neri, V., Acocella, as: article this cite Please 20) doi: (2008), 10.1016/j.tecto.2008.10.002 RIL NPRESS IN ARTICLE .Aoel,M ei/Tcoohsc x 20)xxx (2008) xxx Tectonophysics / Neri M. Acocella, V. ﬁ e:Oeve n osbedvlpet,Tectonophysics developments, possible and Overview ces: – xxx

Fig. 3. Examples of radial eruptive fissures. a) Etna, period 1900–2005; inset b) shows fissure orientation; inset c) shows the three main rift zones. d) Vesuvio, period 1631–1944; inset e shows fissure orientation. ARTICLE IN PRESS

V. Acocella, M. Neri / Tectonophysics xxx (2008) xxx–xxx 5

Fig. 4. Most common shapes for volcanic ediﬁces with relief (2D, map view) and related local stress conditions (σHMAX =maximum horizontal stress; σHmin =minimum horizontal stress). (a) cone, (b) cone with sector collapse, (c) ridge. The maximum horizontal stress is radial in a cone, slightly diverging in a cone with sector collapse and elongated in a ridge.

(Soriano et al., 2008), Etna (Acocella and Neri, 2003), Stromboli (Acocella et al., 2006a) and Vesuvio (between 1631 and 1944; Acocella et al., 2006b). Estimates from Etna and Vesuvio suggest that the mean along-strike top surface of a laterally propagating dike has a moderate downslope dip, on the order of 10°–15° (Acocella et al., 2006c, and references therein). This mechanism of emplacement of radial dikes is usually limited to the upper part of the edifice, below its slope. Therefore, the specific conditions controlling it (i.e. opening/closure of the conduit) need not to be the same as those of the radial dikes propagating at the base of the volcanic edifice, as for example observed at Summer Coon Volcano (Poland et al., 2008).

2.2. What controls dike propagation in different volcanic ediﬁces

The features discussed above establish general guidelines for dike propagation in volcanic edifices. The relations between these features (e.g., topography, regional vs. local stress field) and other factors (e.g., shape of the volcano, composition of magma) in the context of dike propagation has not yet been investigated in detail. Here, we aim to minimizing this gap, with a semi-quantitative analysis of the relations between various factors related to dike emplacement. We consider several features, listed in Table 1, related to dike emplacement at 25 active volcanic centers. Those features include: 1) height (H1) to the base of the edifice, including any submerged portion. 2) Aspect ratio (A) of the edifice (where A=height/width). 3) Eccentricity (E) of the edifice (where E =minimum elongation/maximum elongation).

4) Mean SiO2 content of the erupted magmas, which is expected to approximate, to a first order, average magma viscosity. It is possible that dikes have far from average compositions, but this possibility has not been taken into account in this study. 5) Maximum length (L) reached by the dikes or, more commonly, the eruptive fissures in a volcano; in both cases, it is possible that the real length of the dike may be larger than that of its visible part, so this length has to be considered as minimum value. 6) Difference in height (Hd) between the highest and lowest part of the longest fissure or Fig. 5. Dikes mostly propagate laterally (a) when the summit conduit is open, and the outcropping dike; this value refers to the subaerial part of the dike magma has already degassed, and vertically (b) when the conduit is closed. or fissure and thus may significantly differ from H1. 7) Frequency of

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Table 1 Mean features which may be related to the dike patterns of selected volcanoes

Volcano H1 AESiO2 L Hd RT RR References (km) (wt.%) (km) (km) Hekla (Iceland) 1.5 0.08 0.37 48±2 9 0.8 1 1 10, 27 Kraﬂa (Iceland) 0.6 0.042 0.5 48±3 12 0.2 1 1 27 Erta Ale (Ethiopia) 0.8 0.042 0.70 49±1 4.5 0.3 1 1 1, 12, 24 Nyamuragira (Congo) 1 0.071 0.85 45±2 12 0.9 1 0.56 5 Nyiragongo (Congo) 2 0.11 0.70 40±8 20 1.8 1 0.625 21, 32 Etna (Italy) 3.3 0.12 0.83 50±3 16 2.6 1 0.37 17, 26 Vesuvio (Italy) 1.2 0.082 0.85 57±5 4 1 1 0.57 25 Stromboli (Italy) 2.9 0.16 0.76 52±1 3 0.9 0.77 0.39 33 Kliuchevskoi (Russia) 4.7 0.26 0.92 (54) 15 3.7 1 0.18 9, 10 Terevaka (Chile) 3.5 0.031 0.73 51±3 8 0.4 0.96 0 28 Lonquimay (Chile) 2.8 0.076 0.53 58±5 11 1 1 0.77 34 Villarrica (Chile) 2.8 0.078 0.75 53±5 11 1.4 1 0.75 22, 34 Fuji (Japan) 3.7 0.3 0.95 58±8 13 2.7 1 0.45 10, 23, 31 Sakurajima (Japan) 1.2 0.1 0.73 62±5 4 1 1 0.40 10 Izu-Oshima (Japan) 1.5 0.047 0.6 4 0.7 1 0.83 6, 10 Miyake (Japan) 1.3 0.087 0.82 53±3 4.8 0.8 1 0.42 14, 15 Chokai (Japan) 2.1 0.129 0.69 58±5 10 1.6 1 0.69 20, 34 Fernandina (Galapagos) 4.4 0.041 0.79 48±2 14 1.3 0.6 0 4, 7 Wolf (Galapagos) 4.6 0.089 0.78 48±2 16 1.5 0.66 0 4, 7 Darwin (Galapagos) 4.3 0.056 0.94 48±2 15 1.1 0.59 0 4, 16 Fogo (Capo Verde) 6.7 0.108 0.92 42±2 16 2.7 0.9 0 13, 18 Piton (Reunion) 6.5 0.109 0.74 48±3 12 2.1 1 0 8, 10, 30 Kilauea (Hawaii) 5.7 0.015 0.37 48±2 17⁎ 1.1 1 0 2, 3, 10, 19 Mauna Loa (Hawaii) 8.5 0.067 0.59 49±2 22⁎ 1.8 1 0 2, 3, 10 Tenerife (main; Spain) 7.1 0.075 0.69 50±7 28 2.7 1 0 11, 29

H1=total height to the base of the edifice (including any portion below sea level); A=aspect ratio of edifice (where A=height/width); E=eccentricity of edifice (where E=minimum elongation/maximum elongation); L=maximum length reached by the dikes or eruptive fissures; Hd=difference in height between the highest and lowest subaerial parts of the longest fissure or outcropping dike; RT=frequency of the radial vs. circumferential dikes or fissures (where RT=number of radial dikes /number of radial+circumferential dikes); RR=frequency of regional dikes vs. radial+circumferential dikes (where RR=number of regional dikes/number of regional+radial+circumferential dikes). (data from: (1) Barberi et al., 1980; (2) Holcomb, 1987; (3) Lockwood and Lipman, 1987; (4) Chadwick and Howard, 1991; (5) Hayashi et al., 1992; (6) Ida, 1995; (7) Munro and Rowland 1996; (8) Albarède et al., 1997; (9) Ozerov et al., 1997;(10)Takada, 1997;(11)Ablay et al., 1998;(12)Barrat et al., 1998;(13)Day et al., 1999, and references therein; (14) Amma-Miyasaka and Nakagawa, 2002;(15)Geshi et al., 2002;(16)Naumann et al., 2002;(17)Acocella and Neri, 2003;(18)Doucelance et al., 2003;(19)Thornber et al., 2003; (20) Kondo et al., 2004;(21)Platz et al., 2004; (22) Witter et al., 2004; (23) Yamamoto et al., 2005; (24) Acocella, 2006; (25) Acocella et al., 2006b; (26) Allard et al., 2006, and references therein; (27) Gudmundsson and Nilsen, 2006; (28) Vezzoli and Acocella, 2006; (29) Carracedo et al., 2007; (30) Carter et al., 2007, and references therein; (31) Takada et al., 2007; (32) Tedesco et al., 2007, and references therein; (33) Corazzato et al., 2008 and (34) Nakamura, 1977). Asterisk refers to values of length of dikes or fissures in portions of the volcano least affected by flank slip.

the radial vs. circumferential dikes or fissures in a volcano, designated by the relief of the volcanic edifice, prevails at shallow depths, RT (where RT=number of radial dikes/number of radial+circumferential inhibiting the upward propagation of circumferential dikes and/or dikes). 8) Frequency of regional dikes vs. radial+circumferential dikes rotating these towards the surface. in a volcano, designated RR (where RR=number of regional dikes/ There is a poor correlation between the degree of regional control number of regional+radial+circumferential dikes). The 25 active on dike or fissure emplacement (RR) and the eccentricity of the base of volcanoes have been selected based on availability of data (from the volcanic edifice (Fig. 6c). In fact, strongly elongated volcanic previously published studies), variability of edifice type (e.g., calderas, edifices are not always controlled by regional tectonics. However, shield volcanoes, composite volcanoes), and tectonic setting (e.g., neglecting Kilauea and Mauna Loa (arrows in Fig. 6c), whose rift zone regional extension, regional compression, hot spots). The general shapes are significantly influenced by the slip of the SE flank of Hawaii relations between these features are summarized in Fig. 6. and the topography of existing volcanoes (Fiske and Jackson, 1972; The distribution of the RR values for the selected volcanoes shows a Walter et al., 2006, and references therein), a more significant cluster at RR=0, where radial or circumferential systems are dominant correlation is obtained (dashed line in Fig. 6c). This relationship is and regional dikes are absent (Fig. 6a). These volcanoes are ocean island strengthened by the fact that, except for hot spot volcanoes, the shields related to hot spot activity and away from plate boundaries with edifices are usually elongated normal to the regional minimum com- strong regional stress fields. Another at RR∼0.5 represents stratovol- pressive stress. Therefore, for volcanoes that are isolated and without canoes with similar proportions of local- and regional-controlled dikes major flank slip, the data suggest that there may be a general or fissures. A third cluster of data, approaching RR=1, corresponds to correlation between the elongation of a volcano and the pattern of shield volcanoes along the axis of oceanic rifts, characterized by a strong regionally-controlled dikes. regional control on dike emplacement. The histogram shown in Fig. 6a There is also an inverse correlation between the regional tectonic therefore indicates that the regional setting influences the dike pattern control on dike emplacement (RR) and the maximum height of a of a volcano with different degrees of intensity. volcano above its base (H1; Fig. 6d). Volcanoes located in hot spot The distribution of RT values for the selected volcanoes suggests settings have not been considered in this diagram, as lacking of any that volcanic edifices are largely dominated, at surface, by dikes or regional control. The inverse correlation suggests that taller volcanoes fissures with a radial attitude (RT=1; Fig. 6b). A minor number of are characterized by a distribution of radial dikes or fissures which volcanoes have a significant component of circumferential dikes or tends to be more isotropic, implying that the control of a regional fissures, but RT values are never lower than 0.5. Fig. 6b thus confirms stress field fades with the size of the volcano. In general, volcanoes that circumferential dikes on the surface of active volcanoes are taller than 3 km do not show significant evidence of regional tectonic unusual, in contrast to observations of dike patterns below the surface control on dike emplacement. Similar conclusions have been (see Section 2.1.2). The discrepancy between surface and depth dike suggested for Etna, where it has been proposed that the regional orientations suggests that a radial most compressive stress, induced tectonic control on dike propagation in the uppermost part of the

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Fig. 6. Relations between factors controlling dike emplacement at selected volcanoes (see Table 1). a) Distribution of RR (number of regional dikes/number of regional+radial and circumferential dikes); b) Distribution of RT (number of radial dikes/number or radial+circumferential dikes); c) Eccentricity E of volcanoes vs. RR; d) RR of volcanoes vs. their height

H1; e) H1 of volcanoes vs. the maximum length of their dikes or fissures L; f) differential height of the fissure Hd vs. L; g) Mean SiO2 value of the magmatic products of a volcano vs L; h) Aspect ratio A (thickness/width) of a volcano vs. L;i)A vs. Hd. edifice is replaced by a local, topographic stress field (McGuire and results are suggested by Fig. 6f, which indicates a direct proportion- Pullen, 1989). ality between L and the differential vertical height of the fissure or The development of radial dikes was analysed as a function of H1, dike. The behaviour may be associated with dikes propagating

Hd, A and SiO2. There is a direct proportion between the maximum laterally downslope, from the summit of an open conduit. In fact, it length of an eruptive ﬁssure or dike (L) and the total height of the is expected that the shallower the dike separates from the central volcano (Fig. 6e). The association of longer ﬁssures or dikes with taller conduit, the higher its potential energy and propagation force will be; volcanoes suggests topographic control on dike propagation. Similar therefore the dike will propagate laterally a greater distance. There is

8 V. Acocella, M. Neri / Tectonophysics xxx (2008) xxx–xxx clear proof of such lateral propagation of dikes only at ∼30% of the in Fig. 7. Below, we consider each factor independently, as end-member; considered volcanoes (Table 1), so further evidence is needed to however, corresponding examples are not always found in nature, given generalize such behaviour. the complex interactions between the factors or the speciﬁcity of some The correlation between the maximum length of an eruptive conditions. Examples given (italic in Fig. 7) are supposed to provide the

fissure or dike (L) and the SiO2 content of the related magma (Fig. 6g) best available approximation, even though the dike patterns at those is poor, indicating that the composition and/or viscosity of the magma volcanoes may result from more than a single mechanism. does not significantly limit dike propagation. This is in agreement with The most important first-order control on dike propagation within recent data from Stromboli, showing that the petrochemical features an edifice is topographic relief. Topography always introduces a local, of the magma, including viscosity, have a very low influence on the gravitational stress field which is superimposed over other local (e.g., geometry of dike propagation (Corazzato et al., 2008). induced by a shallow magma reservoir) or regional stress field. The There is also no correlation between the maximum length of an general consequence of significant topography is the development of a eruptive fissure or dike (L) and the aspect ratio of the volcanic edifice radial (more or less isotropic) dike pattern. The lack of topographic (A; Fig. 6h). Apparently, the dip of the slope of the volcano, related to relief may lead to circumferential and/or regional dike patterns. the aspect ratio of the edifice, does not influence dike propagation. Edifices without significant relief may develop subparallel dike Finally, there is a direct proportionality between the aspect ratio of swarms consistent with the regional tectonic trend, as exemplified at the volcanic edifice (A) and the differential vertical height of the Krafla, Iceland. In absence of a regional stress field,thedikepatternmaybe eruptive fissure (Hd; Fig. 6i). The result implies that steeper volcanoes controlled by the presence of topographic irregularities, such as calderas; are also associated with eruptive fissures with larger difference in circumferential dike systems may form at the periphery of the caldera. To height; therefore, even though the length of the dike is not controlled our knowledge, there is not any particular example of active volcano with by the steepness of the volcano (Fig. 6h), the drop in altitude of the exposed circumferential dikes related to the lack of relief and regional dike may depend from the dip of the slope of the volcano. stress field. Another possible control on dike propagation is that induced by a shallow magma reservoir; circumferential dikes are still mostly 3. Discussion expected to form. Underpressure conditions within the reservoir will generate ring dikes, whereas overpressure conditions will generate cone 3.1. General features of dike propagation sheets (Anderson, 1936; Phillips, 1974). Again, we are not aware of any clear dike pattern resulting from a magma chamber and the absence of The overview above indicates that the propagation path followed by topographic relief and a regional stress field in an active volcano. There- dikes within a volcanic edifice is influenced by several factors. More than fore, these conditions, even though feasible, remain largely theoretical. one of these factors may act simultaneously, producing complex dike Edifices with topographic relief are characterized by a radial most patterns, and the role of each factor on dike propagation is summarized compressive stress and will be dominated by radial dikes. Any departure

Fig. 7. Main features, seen as end-members, controlling dike patterns at volcanoes. Dike patterns at given volcanoes result from the features listed in the upper part of the ﬁgure, in order of importance. Right side shows schematic map view of typical dike patterns (with relief=grey; without relief=white).

V. Acocella, M. Neri / Tectonophysics xxx (2008) xxx–xxx 9 from an isotropic radial pattern and clustering of the dikes along important controlling parameters on dike propagation. Studies that preferred orientations depends upon the shape of the edifice and, to a account for dike emplacement and eruption are essential in any lesser extent, the regional stress field. If a sector collapse scar or other project concerning hazard mitigation at active volcanoes. In fact, major scarp is present, radial dikes will partly focus around and parallel many volcanic eruptions are fed by dikes, and dike-fed vents may the relief, as observed at the Sciara del Fuoco flank collapse at Stromboli develop at significant distances from the summit of a volcano. and the Valle del Bove at Etna (McGuire and Pullen, 1989; Acocella and Therefore, understanding the controls on dike propagation is crucial Tibaldi, 2005). to predict where a vent may develop and what hazards may result. In the case of subcircular or elongated edifices, the dike pattern will On these premises, we suggest that future research on dike depend upon the presence or absence of a regional stress field. Without emplacement should focus on the following points. 1) A better a regional stress field, dikes follow the major axis of the edifice, as at definition of the mechanical conditions for the lateral development Kilauea and Mauna Loa, or develop characteristic triple configurations of dikes originating from the central conduit of a volcanic edifice. at 120°, as at Tenerife. With a regional stress field, elongated volcanoes Such dikes are a common feature in many types of volcano, regardless develop dikes focused along preferred regional orientations (as at Erta of tectonic setting. 2) The evaluation of the potential propagation Ale, Afar, or Hekla, Iceland) or simply dike swarms that parallel the paths of dikes at individual volcanoes. To the latter aim, the nature of most compressive stress, as at Izu-Oshima (Fig. 1). The first situation the boundary conditions of individual volcanic systems need to be can be interpreted as an end-member type represented by a highly defined, including the opening or closure of the central conduit (e.g., asymmetric radial configuration of dikes, focused along a direction. Acocella et al., 2006b), the stress distribution within the volcanic The second situation, where the dikes are more dispersed, regards the edifice (e.g., Dieterich, 1988), also as a function of its topography emplacement of non-radial dike swarms. The development of one (Acocella et al., 2006a), the magmatic pressure in subsurface reser- configuration (focused) or the other (dispersed) may depend from the voirs (e.g. Gudmundsson, 2002, 2003), the presence of central or intensity of the regional stress. At oceanic and continental divergent eccentric reservoirs (e.g. Neri et al., 2005, and references therein), the plate boundaries (exemplified by Erta Ale and Hekla), extensional recent eruptive history (e.g. Behncke and Neri, 2003; Behncke et al., stress focuses strain on the axis of the rift, where volcanoes are usually 2005; Allard et al., 2006), the occurrence of pre-eruptive earthquakes located. Extension is therefore restricted to a narrow axial zone passing (Nostro et al., 1998; Walter et al., 2005b) and the presence of layers through the volcano summit. Conversely, at arc or back-arc settings (for with different stiffness or mechanical properties (Gudmundsson, example, at Izu-Oshima; Takada, 1997), weaker tectonic extension 2006, and references therein). results in more dispersed extensional structures, forming wider rift For example, investigations at Vesuvio, Stromboli and Etna suggest zones (Macdonald, 1998, and references therein). As a consequence, that most of the recent dikes propagate laterally from the central dike swarms may be found throughout the volcanic edifice and not conduit, with a path largely controlled by the topography, as significant restricted to the axial zone of the volcano. scarps (Neri et al., 2005, and references therein; Acocella et al., 2006a,b, Similar to elongated edifices, the dike pattern at subcircular edifices c; Neri et al., 2008). These studies may constitute a starting point to try also depends upon the presence or absence of a regional stress field. to evaluate with more precision the future propagation paths of the Without any regional stress field, the dikes follow a radial path, as at dikes as a function of different boundary conditions in these volcanoes. Fogo (Capo Verde). With a regional stress field, the dike pattern may More in general, the propagation paths of dikes, as well as their likely also depend upon the height of the edifice. Dike propagation at short lengths, may be estimated at a given volcano, allowing for the creation volcanoes may be still partly influenced by a far-field stress, developing of hazard maps with the probability of dike-fed activity in different radial patterns with a clustering along directions parallel to the areas of the volcano. Such an approach may predict dike propagation at regional most compressive stress (for example, Nyamuragira, Nyir- a volcano, considering not only the distribution of the most recent agongo, and Miyakejima). Dike propagation at taller volcanoes is less vents or fissures, but especially understanding why and under which influenced by a regional stress field, displaying a more (though seldom conditions these dikes and fissures developed. fully) isotropic radial pattern (examples include Etna, Fuji and Kliuchevskoi). The boundary that defines the relation between the 4. Conclusions height of a volcano and any anisotropy of the pattern of its radial dikes is not sharp, as several volcanoes (e.g., Vesuvio and Sakurajima), We describe the formation of three types of dikes (regional, circum- display intermediate values of H1 and RR. ferential and radial) in relation to several factors, including volcano The final dike pattern on a volcano may also depend upon the topography (positive or negative relief, shape, height, sector collapses), superimposition of different evolutionary stages of the edifice. In fact, tectonic setting (presence of a regional stress field) and mean com- the development of the main rift zones may be also due to lateral position (SiO2 content). Data from 25 volcanic edifices in different collapses or stress variations (Walter et al., 2005a; Tibaldi et al., 2006; settings indicate that dike propagation depends, in order of importance, Corazzato et al., 2008, and references therein). upon the presence of any relief, the shape of the edifice, and the The dike patterns described above may be more or less presence of a regional stress field. Taller volcanoes develop longer dikes pronounced depending upon the length reached by the dikes. As with radial orientations, with negligible effects of the composition of introduced above, the most important parameter controlling the magma or the aspect ratio of the volcano. Our overview demonstrates extent of dike propagation is probably the height of the edifice. Other that the style of dike emplacement may be predicted at given volcanoes. factors, such as the composition of magma or the aspect ratio of the Future research and hazards assessments should evaluate the possible edifice, have moderate or negligible influence (Fig. 6). The comparison propagation paths of dikes at specific volcanoes, as well as the role of with known active cases (Hawaii, Tenerife, Etna, Vesuvio, Stromboli) the boundary conditions of the system, including the opening or closure suggests that these conditions may significantly apply to laterally of conduit, the regional stress trajectories within an edifice, the magma- propagating dikes. This common process of lateral dike propagation tic pressure, the recent eruptive history, the occurrence of pre-eruptive may be related to the opening of the central conduit, even though a earthquakes, and the presence of layers with different stiffness. more advanced study is needed (Acocella et al., 2006b). Acknowledgements 3.2. Implications for hazard and future research T. Yoshida and V. Zanon kindly provided useful data and informa- The factors above suggest that the emplacement of dikes in a tion. Constructive reviews from M. Poland and A. Tibaldi significantly given volcano may be predicted based on knowledge of the most improved the paper. Partly funded with DPC-INGV funds (LAVA Project).

10 V. Acocella, M. Neri / Tectonophysics xxx (2008) xxx–xxx

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Please cite this article as: Acocella, V., Neri, M., Dike propagation in volcanic ediﬁces: Overview and possible developments, Tectonophysics (2008), doi:10.1016/j.tecto.2008.10.002