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1 Years to weeks of seismic unrest and magmatic intrusions 2 precede monogenetic eruptions 3 Helena Albert1, Fidel Costa2, and Joan Martí3 4 1 Central Geophysical Observatory, Spanish Geographic Institute (IGN), 28014, Madrid, Spain 5 2 Earth Observatory of Singapore, Nanyang Technological University, 639798, Singapore, Singapore 6 3 Institute of Earth Sciences Jaume Almera, CSIC, 08028, Barcelona, Spain 7 ABSTRACT 8 9 Seismic, deformation, and gas activity (unrest) typically precedes volcanic eruptions. Tracking the changes of this activity with monitoring data is increasingly possible to successfully 10 forecast eruptions from stratovolcanoes. However, this is not the case for monogenetic 11 volcanoes. Eruptions from these volcanoes tend to be small but are particularly difficult to 12 anticipate since they occur at unexpected locations (e.g. Paricutin, 1943), and there is very 13 limited instrumental monitoring data. Many monogenetic volcanic fields occur in highly touristy 14 or populated areas (e.g. Canary Islands, Auckland City, Mexico City, Izu-Tobu volcanic field) 15 and thus even a small eruption can have a major economic and societal impact. We have 16 gathered the available instrumental data for unrest and combined it with new historical factual 17 accounts of seismicity. We find that there is a commonality in the seismic activity preceding 18 these eruptions, with clusters at around one or two years, two or three months, and one or two 19 weeks. The petrological and geochemical characteristics of these eruptions show that multiple 20 magma batches interacted in a subvolcanic reservoir, and multiple intrusions occurred on a 21 similar time scales to the seismicity. We propose a general model where early dike intrusions in 22 the crust do not erupt and create small plumbing systems (e.g. stalled intrusions), but they 23 probably are instrumental in creating a thermal and rheological pathway for later dikes to be able 24 to reach the surface. These observations provide a conceptual framework for better anticipating 25 monogenetic eruptions and should lead to improved strategies for mitigation of their associated 26 hazards and risks. 27 SEISMICITY ASSOCIATED WITH MONOGENETIC ERUPTIONS 28 One of the main problems to quantify the probability of eruption in a monogenetic 29 volcanic field is the lack of data. Monogenetic fields can be active during millions of year, but 30 the magmatic processes and unrest times are very short compared with the inactivity periods of a 31 given volcanic field. Many of these eruptions occurred before there was any instrumental 32 monitoring data, and our knowledge is currently based on a few factual accounts of historical 33 eruptions (Baker, 1946; Romero, 1991; De la Cruz-Reyna et al., 2011; Sánchez, 2014). We have 34 done an exhaustive revision and compilation of the unrest activity (mainly seismicity) of all the 35 historical monogenetic eruptions for which we have had access (ten eruptions in total; Table 1). 36 This includes five events in the Canary Islands, two in the Michoacan-Guanajuato (Mexico), and 37 one event for those of the Higashi-Izu (Japan), the Owen Stanley Range (Papua), and Iceland 38 (Table 1). Instrumental monitoring data are sparse and available in four cases, but only for the 39 eruption of El Hierro 2011 the data are of high quality (López et al., 2012). For the rest eruptions 40 we did a revision of the factual accounts available in historical documents (see supplemental 41 material for details). Some historical documents are very detailed and give the number of seismic 42 events and the effects on people, furniture and buildings, and sometimes an intensity value (e.g. 43 Mercalli scale). Other reports are not detailed enough to discern between intensities or establish a 44 detailed time series of the number of seismic events. It is important to note that the lack of data 45 in some periods for some eruptions could be due to the lack of historical reports, not to the lack 46 of earthquakes. The details of our analysis of seismic activity for each eruption can be found in 47 supplemental material. 48 We compared the time frames and intensity of seismic activity between different events 49 using a normalization of times and number of events (Fig 1). There are some common features 50 shown by several eruptions. We see that in some cases there are seismic crises interspersed by 51 calm periods that occur between a year to a few months prior to eruption. Many of the 52 considered monogenetic volcanoes display an exponential behaviour of the seismic activity from 53 two or three months before the eruption. These seismic crises might reflect the repetitive 54 intrusions of magma in the crust, and thus probably correspond to mid crustal stalled intrusions 55 (Moran et al., 2011). These intrusions would stall and start to create a small plumbing system. 56 The depth at which these intrusions stall is difficult to constrain but some seismic and 57 petrological data suggest 5-15 km (Klügel et al., 1997; Johnson et al., 2008; Domínguez Cerdeña 58 et al., 2014) below the volcano. Virtually all the eruptions we have studied show a sharp increase 59 in the seismic activity about two weeks to two days before eruption. This short time might be 60 related to magma migration towards the surface (Johnson et al., 2008). 61 PETROLOGICAL EVIDENCE FOR MAGMA INTERACTIONS AND THEIR TIME 62 SCALES 63 The temporal analyses of seismic activity that we report below already provide a 64 framework for anticipating the monitoring data that can be expected for monogenetic volcanoes. 65 However, it is necessary to have a conceptual model of the processes that are occurring for 66 making knowledge-based forecast. Given the large number of variables and parameters that play 67 a role for a magma being able to erupt or not it is important to have deeper knowledge of what is 68 occurring at depth. This allows adapting our interpretations of monitoring data for each case 69 when the local situations are different from one volcanic field to another. 70 A set of complementary data that is readily available for many monogenetic volcanoes and some 71 historical eruptions are the petrological and geochemical characteristic of the erupted rocks. The 72 eruptions we have studied (Table 1) and many others for which there is no knowledge of unrest 73 data show that the erupted magmas were affected by open-system processes involving multiple 74 magmas (Klügel et al., 2000; Johnson et al., 2008; Rowe et al., 2011; Martí et al., 2013; Longpré 75 et al., 2014; Albert et al., 2015). Mixing between mafic magmas has been reported in seven of 76 the ten considered cases (Table 1). In the cases of Jorullo and Paricutin, in addition to the mixing 77 between similar magmas, upper crustal assimilation has also been identified, and implies stalling 78 magma batches at crustal levels. Thus, these petrological studies also show that the monogenetic 79 eruptions are not driven simply by dikes that travel from the mantle to the surface, but support a 80 more complex scenario of magma interactions and assimilation in subvolcanic reservoirs 81 (Johnson et al., 2008; Rowe et al., 2011; Martí et al., 2013; Longpré et al., 2014; Albert et al., 82 2015). Thus, the seismic and petrological studies imply that there is an incubation period of the 83 early intrusions at mid crustal depths before eruption. 84 Studies of the zoning pattern of crystals from stratovolcanoes have been used to provide a good 85 framework for the interpretation of unrest data in stratovolcanoes like Mt. Etna (Kahl et al., 86 2011), Vesuvius (Morgan et al., 2006) and Mt.St Helens (Saunders et al., 2012). A few studies 87 have also been done in monogenetic volcanoes. For example, the zoning patterns of olivine 88 crystals of the Siete Fuentes, Fasnia and Arafo eruptions (Albert et al., 2015) shows that there 89 were several magma mixing events that occurred about a year, two months and two weeks before 90 eruption. Similar time frames from crystals have been found in the other studied eruptions of San 91 Juan, Jorullo and El Hierro. The time frames of crystals and seismicity are very similar (Fig 1). 92 This means that we can associate the changes in seismicity with intrusion that might erupt or not, 93 but it typically seems to involve at least two distinct periods of intrusion before eruption. Some 94 monogenetic eruptions carry mantle xenoliths which may be interpreted as direct magma transfer 95 to the surface (Bolós et al., 2015), but detailed petrological studies have shown stalling of the 96 magmas at shallower depths and thus the existence of subvolcanic reservoirs and multiple 97 intrusions (Klügel et al., 1997; Klügel et al., 2001). 98 THE MAGMA PLUMBING SYSTEM AND PROCESSES LEADING TO 99 MONOGENTIC ERUPTIONS 100 An intriguing aspect of the petrological and geochemical data for these eruptions is that 101 open system and mixing seem are prevalent and imply that magmas coming from depth 102 commonly intercepts a shallower reservoir. Although this might can be expected in areas with 103 high volcanic fluxes such as Iceland, it is not to be expected that for example in the Canaries 104 there are numerous melt lenses at shallow depth and ‘waiting to erupt’. The commonality of 105 open-system processes for many of these events may rather be indicative that the early seismicity 106 associated with intrusions are stalled intrusions. In other words, it seems that magmas coming 107 from depth in dikes are rarely able to go straight to the surface, but stall at some intermediate 108 depth (Fig 2a and b). There are many parameters that control whether mafic dikes from the 109 mantle will be able to reach the surface, including magma buoyancy, thermal survival, tectonic 110 stress or pre-existing crustal discontinuities (Rubin 1995; Valentine and Gregg, 2008; Le Corvec 111 et al., 2013; Rivalta et al., 2015). Our dataset doesn’t allow us to identify a precise control on 112 each of the eruptions, but the existence of a shallow plumbing system suggest that dikes 113 separated from their sources and travelled as small batches. Once at shallow depths, magmas can 114 cool, probably degas and evolve toward more differentiated compositions. Repetitive intrusions 115 of small magma batches on the same location are probably able to modify the thermal and 116 rheological state of the crust through which they pass and reside, and thus when renewed dike 117 intrusions from depth occur they find an increasingly easier path, and lower energy required to 118 reach the surface and erupt (Fig. 2c) (Strong and Wolff, 2003). Our conceptual model fits well 119 with our new compilation of seismic unrest and petrology of the erupted magmas and thus 120 should lead to more informed and process based anticipating of monogenetic eruption. More 121 experiments and numerical models of dike migration should be able to test the importance of 122 repetitive intrusions in allowing mafic magmas from monogenetic eruption to reach the surface. 123 124 125 ACKNOWLEDGMENTS We are grateful for the help and discussions about seismicity and deformation with C. del 126 Fresno and L. García. T. Girona is thanked for discussion about dikes. We also thank the Teide 127 National Park for giving us permission to access the studied sites. H. Albert was funded by the 128 Spanish Geographic Institute (IGN). This research was partially funded by the Earth Observatory 129 of Singapore (EOS) “Magma Plumbing Project”. 130 131 REFERENCES CITED 132 133 134 Albert, H., Costa, F., and Martí, J., 2015, Timing of magmatic processes and unrest associated with mafic historical monogenetic eruptions in Tenerife Island: Journal of Petrology, accepted after major revisions. 135 136 137 Araña, V., and Ibarrola, E., 1973, Rhyolitic pumice in the basaltic pyroclasts from the 1971 eruption of Teneguía volcano, Canary Islands: Lithos, v. 6, no. 3, p. 273–278, doi: 10.1016/0024-4937(73)90088-1. 138 139 140 Baker, G., 1946, Preliminary note on volcanic eruptions in the Goropu Mountains, Southeastern Papua, during the period December, 1943, to August, 1944: The Journal of Geology, v. 54, no. 1, p. 19–31. 141 142 143 Bolós, X., Martí, J., Becerril, L., Planagumà, L., Grosse, P., and Barde-Cabusson, S., 2015, Volcano-structural analysis of La Garrotxa Volcanic Field (NE Iberia): Implications for the plumbing system: Tectonophysics, v. 642, p. 58–70, doi: 10.1016/j.tecto.2014.12.013. 144 145 Carreón Nieto, M.C., 2002, Un castigo divino: el volcán de Jorullo: Tzintzun, Revista de Estudios Históricos, v. 35, p. 37-64. 146 147 De la Cruz-Reyna, S., and Yokoyama, I., 2011, A geophysical characterization of monogenetic volcanism: Geofisica Internacional, v. 50, p. 465–484. 148 149 150 151 Le Corvec, N., Bebbington, M.S., Lindsay, J.M., and McGee, L.E., 2013, Age, distance, and geochemical evolution within a monogenetic volcanic field: Analyzing patterns in the Auckland Volcanic Field eruption sequence: Geochemistry, Geophysics, Geosystems, v. 14, no. 9, p. 3648–3665, doi: 10.1002/ggge.20223. 152 153 154 Domínguez Cerdeña, I., del Fresno, C., and Gomis Moreno, A., 2014, Seismicity patterns prior to the 2011 El Hierro eruption: Bulletin of the Seismological Society of America, v. 104, no. 1, p. 567–575, doi: 10.1785/0120130200. 155 156 157 158 Higgins, M.D., and Roberge, J., 2007, Three magmatic components in the 1973 eruption of Eldfell volcano, Iceland: Evidence from plagioclase crystal size distribution (CSD) and geochemistry: Journal of Volcanology and Geothermal Research, v. 161, no. 3, p. 247–260, doi: 10.1016/j.jvolgeores.2006.12.002. 159 IGN, Seismic catalogue: http://www.ign.es. 160 161 162 163 164 Johnson, E.R., Wallace, P.J., Cashman, K. 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Pre-eruptive unrest in historical monogenetic volcanoes and calculated mixing times. 233 The eruption is represented by a black line at time = 0. Red dashed lines indicate 90 and 15 days 234 prior to the eruption. Grey shadow area corresponds to the 90 days before the eruption. A: 235 Comparison between factual account (triangles) and monitored data (dots) of the unrest period of 236 multiple eruptions. The plots represent the days with felt earthquakes. Generally the trend of the 237 activity changes around two or three months and one or two weeks before the eruption for the 238 studied monogenetic eruptions. The inset shows a zoom view of the grey shadow area. Siete 239 Fuentes, Fasnia and Arafo (light blue triangles); Chinyero (garnet triangles); San Juan (dark blue 240 triangles); Teneguía (purple triangles); El Hierro (orange circles); Jorullo (yellow triangles); 241 Paricutín (green dots); Ito-oki (grey dots). Details of the seismicity are given in the 242 supplementary data. B: Calculated mixing times from some of the considered eruptions. Times 243 from San Juan eruption are not exact (dashed lines). 244 245 Figure 2. Possible plumbing system configuration and evolution of events that may occur below 246 monogenetic volcanoes. This is schematic and not to scale. The depth of the subvolcanic system 247 may vary from system to system, but evidences suggest 5-15 km. The depth of the magma source 248 is also variable but is at least 20 km. A: Intrusion of magma about one or two years prior to the 249 eruption. Stalling of magma at 5-15 km due to the loss of buoyancy or freezing of dykes. Mixing 250 processes registered by the crystals. Crust assimilation in some cases. Seismic activity felt by the 251 population in some cases. B. Renew magma intrusion. Progressive opening of the path between 252 deep and shallow reservoirs. Mixing processes registered by the crystals. Crust assimilation in 253 some cases. The seismic activity is commonly felt by the population. C: Continued intrusion of 254 mafic magma leads to easier transfer from deep to shallow reservoirs and this allows the magma 255 to finally erupt. Mixing processes (and crust assimilation in some cases) registered by the 256 crystals. Seismicity felt by the population.