Download H.Albert et al.

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Years to weeks of seismic unrest and magmatic intrusions
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precede monogenetic eruptions
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Helena Albert1, Fidel Costa2, and Joan Martí3
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Central Geophysical Observatory, Spanish Geographic Institute (IGN), 28014, Madrid, Spain
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Earth Observatory of Singapore, Nanyang Technological University, 639798, Singapore, Singapore
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Institute of Earth Sciences Jaume Almera, CSIC, 08028, Barcelona, Spain
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ABSTRACT
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Seismic, deformation, and gas activity (unrest) typically precedes volcanic eruptions.
Tracking the changes of this activity with monitoring data is increasingly possible to successfully
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forecast eruptions from stratovolcanoes. However, this is not the case for monogenetic
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volcanoes. Eruptions from these volcanoes tend to be small but are particularly difficult to
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anticipate since they occur at unexpected locations (e.g. Paricutin, 1943), and there is very
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limited instrumental monitoring data. Many monogenetic volcanic fields occur in highly touristy
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or populated areas (e.g. Canary Islands, Auckland City, Mexico City, Izu-Tobu volcanic field)
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and thus even a small eruption can have a major economic and societal impact. We have
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gathered the available instrumental data for unrest and combined it with new historical factual
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accounts of seismicity. We find that there is a commonality in the seismic activity preceding
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these eruptions, with clusters at around one or two years, two or three months, and one or two
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weeks. The petrological and geochemical characteristics of these eruptions show that multiple
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magma batches interacted in a subvolcanic reservoir, and multiple intrusions occurred on a
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similar time scales to the seismicity. We propose a general model where early dike intrusions in
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the crust do not erupt and create small plumbing systems (e.g. stalled intrusions), but they
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probably are instrumental in creating a thermal and rheological pathway for later dikes to be able
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to reach the surface. These observations provide a conceptual framework for better anticipating
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monogenetic eruptions and should lead to improved strategies for mitigation of their associated
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hazards and risks.
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SEISMICITY ASSOCIATED WITH MONOGENETIC ERUPTIONS
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One of the main problems to quantify the probability of eruption in a monogenetic
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volcanic field is the lack of data. Monogenetic fields can be active during millions of year, but
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the magmatic processes and unrest times are very short compared with the inactivity periods of a
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given volcanic field. Many of these eruptions occurred before there was any instrumental
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monitoring data, and our knowledge is currently based on a few factual accounts of historical
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eruptions (Baker, 1946; Romero, 1991; De la Cruz-Reyna et al., 2011; Sánchez, 2014). We have
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done an exhaustive revision and compilation of the unrest activity (mainly seismicity) of all the
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historical monogenetic eruptions for which we have had access (ten eruptions in total; Table 1).
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This includes five events in the Canary Islands, two in the Michoacan-Guanajuato (Mexico), and
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one event for those of the Higashi-Izu (Japan), the Owen Stanley Range (Papua), and Iceland
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(Table 1). Instrumental monitoring data are sparse and available in four cases, but only for the
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eruption of El Hierro 2011 the data are of high quality (López et al., 2012). For the rest eruptions
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we did a revision of the factual accounts available in historical documents (see supplemental
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material for details). Some historical documents are very detailed and give the number of seismic
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events and the effects on people, furniture and buildings, and sometimes an intensity value (e.g.
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Mercalli scale). Other reports are not detailed enough to discern between intensities or establish a
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detailed time series of the number of seismic events. It is important to note that the lack of data
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in some periods for some eruptions could be due to the lack of historical reports, not to the lack
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of earthquakes. The details of our analysis of seismic activity for each eruption can be found in
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supplemental material.
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We compared the time frames and intensity of seismic activity between different events
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using a normalization of times and number of events (Fig 1). There are some common features
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shown by several eruptions. We see that in some cases there are seismic crises interspersed by
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calm periods that occur between a year to a few months prior to eruption. Many of the
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considered monogenetic volcanoes display an exponential behaviour of the seismic activity from
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two or three months before the eruption. These seismic crises might reflect the repetitive
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intrusions of magma in the crust, and thus probably correspond to mid crustal stalled intrusions
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(Moran et al., 2011). These intrusions would stall and start to create a small plumbing system.
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The depth at which these intrusions stall is difficult to constrain but some seismic and
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petrological data suggest 5-15 km (Klügel et al., 1997; Johnson et al., 2008; Domínguez Cerdeña
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et al., 2014) below the volcano. Virtually all the eruptions we have studied show a sharp increase
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in the seismic activity about two weeks to two days before eruption. This short time might be
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related to magma migration towards the surface (Johnson et al., 2008).
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PETROLOGICAL EVIDENCE FOR MAGMA INTERACTIONS AND THEIR TIME
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SCALES
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The temporal analyses of seismic activity that we report below already provide a
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framework for anticipating the monitoring data that can be expected for monogenetic volcanoes.
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However, it is necessary to have a conceptual model of the processes that are occurring for
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making knowledge-based forecast. Given the large number of variables and parameters that play
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a role for a magma being able to erupt or not it is important to have deeper knowledge of what is
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occurring at depth. This allows adapting our interpretations of monitoring data for each case
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when the local situations are different from one volcanic field to another.
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A set of complementary data that is readily available for many monogenetic volcanoes and some
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historical eruptions are the petrological and geochemical characteristic of the erupted rocks. The
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eruptions we have studied (Table 1) and many others for which there is no knowledge of unrest
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data show that the erupted magmas were affected by open-system processes involving multiple
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magmas (Klügel et al., 2000; Johnson et al., 2008; Rowe et al., 2011; Martí et al., 2013; Longpré
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et al., 2014; Albert et al., 2015). Mixing between mafic magmas has been reported in seven of
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the ten considered cases (Table 1). In the cases of Jorullo and Paricutin, in addition to the mixing
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between similar magmas, upper crustal assimilation has also been identified, and implies stalling
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magma batches at crustal levels. Thus, these petrological studies also show that the monogenetic
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eruptions are not driven simply by dikes that travel from the mantle to the surface, but support a
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more complex scenario of magma interactions and assimilation in subvolcanic reservoirs
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(Johnson et al., 2008; Rowe et al., 2011; Martí et al., 2013; Longpré et al., 2014; Albert et al.,
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2015). Thus, the seismic and petrological studies imply that there is an incubation period of the
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early intrusions at mid crustal depths before eruption.
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Studies of the zoning pattern of crystals from stratovolcanoes have been used to provide a good
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framework for the interpretation of unrest data in stratovolcanoes like Mt. Etna (Kahl et al.,
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2011), Vesuvius (Morgan et al., 2006) and Mt.St Helens (Saunders et al., 2012). A few studies
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have also been done in monogenetic volcanoes. For example, the zoning patterns of olivine
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crystals of the Siete Fuentes, Fasnia and Arafo eruptions (Albert et al., 2015) shows that there
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were several magma mixing events that occurred about a year, two months and two weeks before
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eruption. Similar time frames from crystals have been found in the other studied eruptions of San
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Juan, Jorullo and El Hierro. The time frames of crystals and seismicity are very similar (Fig 1).
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This means that we can associate the changes in seismicity with intrusion that might erupt or not,
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but it typically seems to involve at least two distinct periods of intrusion before eruption. Some
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monogenetic eruptions carry mantle xenoliths which may be interpreted as direct magma transfer
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to the surface (Bolós et al., 2015), but detailed petrological studies have shown stalling of the
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magmas at shallower depths and thus the existence of subvolcanic reservoirs and multiple
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intrusions (Klügel et al., 1997; Klügel et al., 2001).
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THE MAGMA PLUMBING SYSTEM AND PROCESSES LEADING TO
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MONOGENTIC ERUPTIONS
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An intriguing aspect of the petrological and geochemical data for these eruptions is that
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open system and mixing seem are prevalent and imply that magmas coming from depth
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commonly intercepts a shallower reservoir. Although this might can be expected in areas with
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high volcanic fluxes such as Iceland, it is not to be expected that for example in the Canaries
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there are numerous melt lenses at shallow depth and ‘waiting to erupt’. The commonality of
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open-system processes for many of these events may rather be indicative that the early seismicity
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associated with intrusions are stalled intrusions. In other words, it seems that magmas coming
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from depth in dikes are rarely able to go straight to the surface, but stall at some intermediate
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depth (Fig 2a and b). There are many parameters that control whether mafic dikes from the
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mantle will be able to reach the surface, including magma buoyancy, thermal survival, tectonic
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stress or pre-existing crustal discontinuities (Rubin 1995; Valentine and Gregg, 2008; Le Corvec
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et al., 2013; Rivalta et al., 2015). Our dataset doesn’t allow us to identify a precise control on
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each of the eruptions, but the existence of a shallow plumbing system suggest that dikes
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separated from their sources and travelled as small batches. Once at shallow depths, magmas can
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cool, probably degas and evolve toward more differentiated compositions. Repetitive intrusions
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of small magma batches on the same location are probably able to modify the thermal and
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rheological state of the crust through which they pass and reside, and thus when renewed dike
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intrusions from depth occur they find an increasingly easier path, and lower energy required to
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reach the surface and erupt (Fig. 2c) (Strong and Wolff, 2003). Our conceptual model fits well
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with our new compilation of seismic unrest and petrology of the erupted magmas and thus
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should lead to more informed and process based anticipating of monogenetic eruption. More
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experiments and numerical models of dike migration should be able to test the importance of
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repetitive intrusions in allowing mafic magmas from monogenetic eruption to reach the surface.
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ACKNOWLEDGMENTS
We are grateful for the help and discussions about seismicity and deformation with C. del
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Fresno and L. García. T. Girona is thanked for discussion about dikes. We also thank the Teide
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National Park for giving us permission to access the studied sites. H. Albert was funded by the
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Spanish Geographic Institute (IGN). This research was partially funded by the Earth Observatory
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of Singapore (EOS) “Magma Plumbing Project”.
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FIGURE CAPTIONS
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Figure 1. Pre-eruptive unrest in historical monogenetic volcanoes and calculated mixing times.
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The eruption is represented by a black line at time = 0. Red dashed lines indicate 90 and 15 days
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prior to the eruption. Grey shadow area corresponds to the 90 days before the eruption. A:
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Comparison between factual account (triangles) and monitored data (dots) of the unrest period of
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multiple eruptions. The plots represent the days with felt earthquakes. Generally the trend of the
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activity changes around two or three months and one or two weeks before the eruption for the
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studied monogenetic eruptions. The inset shows a zoom view of the grey shadow area. Siete
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Fuentes, Fasnia and Arafo (light blue triangles); Chinyero (garnet triangles); San Juan (dark blue
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triangles); Teneguía (purple triangles); El Hierro (orange circles); Jorullo (yellow triangles);
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Paricutín (green dots); Ito-oki (grey dots). Details of the seismicity are given in the
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supplementary data. B: Calculated mixing times from some of the considered eruptions. Times
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from San Juan eruption are not exact (dashed lines).
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Figure 2. Possible plumbing system configuration and evolution of events that may occur below
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monogenetic volcanoes. This is schematic and not to scale. The depth of the subvolcanic system
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may vary from system to system, but evidences suggest 5-15 km. The depth of the magma source
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is also variable but is at least 20 km. A: Intrusion of magma about one or two years prior to the
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eruption. Stalling of magma at 5-15 km due to the loss of buoyancy or freezing of dykes. Mixing
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processes registered by the crystals. Crust assimilation in some cases. Seismic activity felt by the
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population in some cases. B. Renew magma intrusion. Progressive opening of the path between
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deep and shallow reservoirs. Mixing processes registered by the crystals. Crust assimilation in
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some cases. The seismic activity is commonly felt by the population. C: Continued intrusion of
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mafic magma leads to easier transfer from deep to shallow reservoirs and this allows the magma
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to finally erupt. Mixing processes (and crust assimilation in some cases) registered by the
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crystals. Seismicity felt by the population.