El Hierro eruption started on 10 October 2011 after an unrest episode that initiated on 17 July 2011. This is the first eruption in the Canary Islands that has been tracked in real time. Although being submarine and not directly observable, the data recorded allowed its reconstruction and to identify its causes and mechanisms. Seismicity, surface deformation, and petrological data indicate that a batch of basanitic magma coming from a reservoir located at a depth of about 25 km below the El Hierro Island was emplaced at shallower depth creating a new reservoir about 10–12 km above, where magma evolved until the initiation of the eruption. The characteristics of seismicity and surface deformation suggest that the necessary space to accumulate magma at this shallower position, which coincides with the crust/mantle boundary beneath El Hierro, was created in about 2 months by elastic deformation and magma‐driven fracturing of the crust. After this first intrusion episode, part of the magma started to migrate laterally toward the SE for nearly 20 km, always keeping the same depth and following a path apparently controlled by stress barriers created by tectonic and rheological contrasts in the upper lithosphere. This lateral migration of magma ended with a submarine eruption at about 5 km offshore from the southern corner of El Hierro Island. The total seismic energy released during the unrest episode was of 8.1 × 10¹¹ J, and the total uplift previous to the onset of the eruption was of 40 mm. Combining geological, geophysical, and petrological data and numerical modeling, we propose a volcanological model of the causes and mechanisms of El Hierro eruption that shows how the stress distribution in the crust beneath El Hierro, which was influenced by rheological contrasts, tectonic stresses, and gravitational loading, controlled the movement and eruption of magma. We also discuss the implications of this model in terms of eruption forecast in the Canary Islands.

The application of petrography, mineral chemistry, geochemistry, and experimental petrology, including mineral–melt thermodynamic and diffusion modelling, on quenched basanitic magma samples from the recent (2011–2012) submarine eruption of El Hierro (Canary Islands) has permitted the identification of major physico-chemical variations prior to and during magma eruption that correlate in time with monitored geophysical changes. After nearly 3 months of seismic unrest the eruption of El Hierro started on October 10, 2011 and ended by late February 2012. We studied 10 lava balloons and pyroclastic fragments collected floating on the sea surface between October 15 and late January. Based on petrological and geophysical data we distinguish two main eruptive episodes. Magma erupted from the beginning of the eruption until late November 2011 was an evolved basanite (≈5 wt % MgO), changing to more primitive compositions (≈8–9 wt % MgO) with time, thus suggesting extraction from a compositionally zoned magma system. Experimental data and mineral–melt thermodynamic modelling indicate that the erupted magma equilibrated at a pressure of about 400 MPa, which corresponds to a depth of 12–15 km. This depth is consistent with the location of the crust–mantle discontinuity beneath El Hierro and with the hypocentral location of seismicity during the unrest episode. Preliminary modelling of the olivine chemical zoning of crystals erupted in this first episode suggests that the time scale for basanite fractionation and magma replenishment in the shallower reservoir was of the order of a few months. This is within the same time frame as the duration of the unrest episode preceding the eruption. The first eruption episode coincided with intense seismicity mostly located north of the island, first at a depth of 20–25 km and a few days later also at 10–15 km depth, with strong seismic tremor beneath the vent site. An abrupt change in magma composition and crystal content was observed at the end of November 2011. After that, more primitive and less viscous magma erupted contemporaneously with a change in the frequency and intensity of seismic events. During this period, seismicity was mostly north of the island at depths of 10–15 km. At the same time the tremor intensity at the eruption site significantly dropped. This marked the onset of the second eruption episode, which is correlated with an intrusion of fresh, more primitive magma into the shallow magmatic system that raised the temperature of the remaining magma. Experiments reveal that subtle changes in temperature of about 50°C (i.e. 1100–1150°C) were enough to produce large changes in the crystal content (10–60 wt %). This non-linear behaviour between crystal content and temperature had important effects on magma dynamics during transport and cooling. Our results suggest the existence of two interconnected mafic magma reservoirs during the El Hierro eruption, which agrees with the pattern shown by the seismicity. Stress readjustments of the plumbing system, caused by decompression during the eruption, influenced the thermodynamic evolution of the erupting magma and facilitated the intrusion of the deeper magma into the shallow reservoir, thus forcing a change in its rheological characteristics and eruption dynamics.

Mafic microgranular enclaves from polygenetic and monogenetic swarms in the Gredos sector of the Central System batholith (central Spain) were sampled and studied to elucidate field, geochronological, and geochemical relations. Among-enclave variations in terms of major elements demonstrate contamination with the pelitic country rocks, and not with the host granitic magma. Magmatic enclaves from monogenetic swarms in Gredos are interpreted as fragments from the chilled margins generated at the sidewall of ascent conduits. In situ U-Pb zircon age determinations by sensitive high-resolution ion microprobe yielded two sets of ages. One corresponds to the ages of crystallization at 290 ± 3.1 Ma and 300 ± 3.4 Ma of two enclaves from two different monogenetic swarms. These ages are slightly younger than the zircon ages of the host granodiorites (ca. 319–312 Ma). A secondary set of ages present in the two studied samples (at 322 ± 6.7 Ma and 320 ± 8.5 Ma) corresponds to the age of the main Variscan regional migmatization of host Neoproterozoic pelitic metasediments. These older ages are interpreted as xenocrystic cores that were derived from the pelitic host during magma ascent. Together with the geochemical relations, they prove quenching against the regional host in con¬duits and not against the host granodiorite as classically thought. Monogenetic swarms can be seen as high-energy pulses denoting the switching from steady-state to transient flow regimes within magma ascent conduits. By contrast, polygenetic swarms are generated by collapse of accumulated resisters of igneous and metamorphic origin that were previously present in migmatites at the top of subhorizontal, layer-type intrusions.