The 2010 eruption is the largest explosive event at Merapi volcano since 1872. The high energy of the initial 26 October explosions cannot be explained by simple gravitational collapse, and the paroxysmal explosions were preceded by the growth of a lava dome not large enough to ensure significant pressurization of the system. We sampled pumice from these explosive phases and determined the preexplosive depths of the pumices by combining textural analyses with glass water content measurements. Our results indicate that the magma expelled was tapped from depths of several kilometers. Such depths are much greater than those involved in the pre‐2010 effusive activity. We propose that the water‐rich magma liberated enough gas to sustain the explosivity. Our results imply that the explosive potential of volcanoes having dome‐forming, effusive activity is linked to the depth from which fresh magma can be evacuated in a single explosion, regardless of the evacuated volume.

We analyzed pumice from the February 11, 2010 Vulcanian explosion that immediately followed a large dome collapse at Soufrière Hills volcano. We obtained pre-explosive values of porosity, pressure, and depth by combining textural analyses and glass water content determinations. Our data suggest that the February 2010 explosion evacuated the upper 3 km of the conduit from the dense magma (≤10 vol.% porosity) it contained. The low porosity distribution in the volcanic conduit implies that the magma rising from the reservoir had time to extensively degas during emplacement. We use a conduit flow model to characterize the effects of permeability on ascent conditions. Model input parameters were fitted so as to match our pre-explosive porosity data, which yielded first-order constraints on conduit radius, mass flux, outgassing efficiency, and permeability. This parametric study suggests that efficient lateral gas escape is necessary to explain the low pre-explosive porosities. Steady-state solutions fitting the observed range of dome extrusion rate in the month preceding the February 11 event suggest permeabilities <10-13 m 2 deeper than 500 m, which are values typical of crack-supported permeability. Conversely, solutions with parameters consistent with bubble-supported permeability imply transient flow conditions prior to the February 11 event. The transient conditions imply that our data represent a snapshot of the porosity distribution within the conduit that does not preclude the temporary presence of much higher porosities in the conduit. Such unsteady conduit flow conditions are consistent with the irregular but active dome growth in the month prior to the February 11 event.

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The recurrent explosive eruptions of Calbuco (Andean Southern Volcanic Zone (SVZ)) threat a rapidly expanding touristic and economic region of Chile. Providing tighter constraints on its magmatic system is therefore important for better monitoring its activity. Calbuco is also distinguished by hornblende-bearing assemblages that contrast with the anhydrous parageneses of most Central SVZ volcanoes. Here we build on previous work to propose a detailed petrological model of the magmatic system beneath Calbuco. Geochemical data acquired on a hundred samples collected in the four units of the volcano show no secular compositional change indicating a steady magmatic system since ~ 300 ka. A tholeiitic Al 2 O 3 -rich (20 wt. %) basalt (Mg# = 0.59) is the parent magma of a differentiation trend straddling the tholeiitic/calc-alkaline fields and displaying a narrow compositional Daly gap. Amphibole crystallization was enabled by the higher H 2 O content of the basalt (3-3.5 wt. % H 2 O at 50 wt. % SiO 2 ) compared to neighboring volcanoes. This characteristic is inherited from the primary mantle melt and possibly results from a lower degree of partial melting induced by the mantle wedge thermal structure. Although macrocrysts are not all in chemical equilibrium with their host rocks and were thus presumably unlocked from the zoned crystal mush and transported in the carrier melt, the bulk-rock trend follows both experimental liquid lines of descent and the chemical trend of calculated melts in equilibrium with amphibole (AEMs). These contradictory observations can be reconciled if minerals are transported in near cotectic proportions. The AEMs overlap the Daly gap revealing that the missing liquid compositions were present in the storage region. Geothermobarometers all indicate that the chemical diversity from basalt to dacite was acquired at a shallow depth (210-460 MPa). We suggest that differentiation from the primary magma to the parental basalt took place either in the same storage region or at the MOHO.

&lt;p&gt;Magmatic arcs are usually considered to be major sites of new continental crust formation. However, the detailed differentiation processes that produce the characteristic calc-alkaline trends are still controversial. More particularly, the depth of differentiation in the arc crustal column and possible changes during the lifespan of a volcano are current subject of discussion.&lt;/p&gt;&lt;p&gt;The Central Southern Volcanic Zone (CSVZ) in Chile is characterized by a thin crust (~ 35 km; Hickey-Vargas et al., 2016) and by the presence of a major dextral transpressional crustal scaled structure (Liqui&amp;#241;e-Ofqui Fault Zone), two features that favor a rapid ascent of magmas from the mantle wedge to the surface. Recent petrological data acquired on volcanoes of the CSZV further indicate that most of the differentiation takes place at about 0.2 GPa, a depth corresponding to a major intracrustal discontinuity. However, for Villarrica stratovolcano (VR; 39.3&amp;#176;S, 71.6&amp;#176;W), estimates suggest two depths of differentiation, respectively at 0.8 and 0.2 GPa (Morgado et al. 2015, 2017).&lt;/p&gt;&lt;p&gt;VR is one of the most active volcanoes in the Andean Cordilleras. Since the mid 80&amp;#8217;s, it has been constantly degasing through an open conduit filled by a summit lava lake. Several Holocene, monogenetic small eruptive centers (SECs) surround VR which forms together with Quetrupill&amp;#225;n and Lanin stratovolcanoes a NW-SE oriented chain. It gives thus a perfect opportunity to study how the mentioned features influence the differentiation processes, their corresponding depth and the observed differentiation trends. VR is mainly composed of basaltic andesites and basaltic lavas and pyroclasts with less andesitic lavas and minor dacitic &amp;#8211; rhyodacitic domes, while rocks from Quetrupill&amp;#225;n and Lanin are compositionally more evolved (e.g. Hickey-Vargas et al., 1989).&lt;/p&gt;&lt;p&gt;Here we present mineral compositions (plagioclase, olivine, clinopyroxene) and whole-rock (lavas, pyroclasts) geochemical data for different units of VR as well as for some nearby SECs (Los Nevados, Chaillup&amp;#233;n, San Jorge). The WR data combined with published analyses define a single differentiation trend extending from ~50 &amp;#8211; 71 wt.% SiO&lt;sub&gt;2&lt;/sub&gt;, with a compositional &amp;#8220;Daly&amp;#8221; gap between 58 &amp;#8211; 62 wt.% SiO&lt;sub&gt;2&lt;/sub&gt;. Moreover, a few VR samples have high Mg# up to 62 (SiO&lt;sub&gt;2&lt;/sub&gt; 50.3-52.6, MgO 7.98 wt.%) and a tholeiitic affinity (e.g. AFM, K&lt;sub&gt;2&lt;/sub&gt;O/Yb vs. Ta/Yb). The most mafic, tholeiitic basalts found in the area where produced by the proximate San Jorge SEC (Mg# 69, SiO&lt;sub&gt;2&lt;/sub&gt; 50.6, MgO 9.5 wt.%) and interpreted by McGee et al. (2019) as reflecting a deep, melt-exhausted region of the mantle wedge. Major- and trace elements data together with supportive mass balance modelling and thermodynamic simulations with rhyolite-MELTS imply fractional crystallization as a major differentiation process.&lt;/p&gt;