Description: A submarine eruption started off the south coast of El Hierro, Canary Islands, on 10 October 2011 and continues at the time of this writing (February 2012). In the first days of the event, peculiar eruption products were found floating on the sea surface, drifting for long distances from the eruption site. These specimens, which have in the meantime been termed "restingolites" (after the close-by village of La Restinga), appeared as black volcanic "bombs" that exhibit cores of white and porous pumice-like material. Since their brief appearance, the nature and origin of these "floating stones" has been vigorously debated among researchers, with important implications for the interpretation of the hazard potential of the ongoing eruption. The "restingolites" have been proposed to be either (i) juvenile high-silica magma (e.g. rhyolite), (ii) remelted magmatic material (trachyte), (iii) altered volcanic rock, or (iv) reheated hyaloclastites or zeolite from the submarine slopes of El Hierro. Here, we provide evidence that supports yet a different conclusion. We have analysed the textures and compositions of representative "restingolites" and compared the results to previous work on similar rocks found in the Canary Islands. Based on their high-silica content, the lack of igneous trace element signatures, the presence of remnant quartz crystals, jasper fragments and carbonate as well as wollastonite (derived from thermal overprint of carbonate) and their relatively high oxygen isotope values, we conclude that "restingolites" are in fact xenoliths from pre-island sedimentary layers that were picked up and heated by the ascending magma, causing them to partially melt and vesiculate. As they are closely resembling pumice in appearance, but are xenolithic in origin, we refer to these rocks as "xeno-pumice". The El Hierro xeno-pumices hence represent messengers from depth that help us to understand the interaction between ascending magma and crustal lithologies beneath the Canary Islands as well as in similar Atlantic islands that rest on sediment-covered ocean crust (e.g. Cape Verdes, Azores). The occurrence of "restingolites" indicates that crustal recycling is a relevant process in ocean islands, too, but does not herald the arrival of potentially explosive high-silica magma in the active plumbing system beneath El Hierro.

Description: We use clinopyroxene-liquid thermobarometry, aided by petrography and mineral major element chemistry, to reconstruct the magma plumbing system of the late Miocene, largely mafic Teno shield-volcano on the island of Tenerife. Outer rims of clinopyroxene and olivine phenocrysts show patterns best explained by decompression-induced crystallization upon rapid ascent of magmas from depth. The last equilibrium crystallization of clinopyroxene occurred in the uppermost mantle, from ∼20 to 45 km depth. We propose that flexural stresses or, alternatively, thermomechanical contrasts create a magma trap that largely confines magma storage to an interval roughly coinciding with the Moho at ∼15 km and the base of the long-term elastic lithosphere at ∼40 km below sea level. Evidence for shallow magma storage is restricted to the occurrence of a thick vitric tuff of trachytic composition emplaced before the Teno shield-volcano suffered large-scale flank collapses. The scenario developed in this study may help shed light on some unresolved issues of magma supply to intraplate oceanic volcanoes characterized by relatively low magma fluxes, such as those of the Canary, Madeira and Cape Verde archipelagoes, as well as Hawaiian volcanoes in their postshield stage. The data presented also support the importance of progressive magmatic underplating in the Canary Islands.

Description: Understanding magmatic systems feeding volcanoes is critical for accurate interpretation of monitoring data and, ultimately, eruption forecasting. Following 3 months of precursory unrest, the first historical eruption at El Hierro, Canary Islands, took place ~2 km offshore from October 2011 to March 2012. Our detailed petrological analysis of lava samples reveals that at least two distinct magmas initially supplied from reservoirs in the mantle underwent hybridization at 15–25 km depth, i.e., also largely within the upper mantle beneath El Hierro. Diffusion chronometry applied to zoned olivine crystals indicates that magma mixing began during the period of preeruptive seismicity and continued for weeks after the eruption onset. Our data also capture a magma stagnation level at 10–15 km depth in the lower crust, consistent with lateral propagation of an intrusion over substantial distances before rapid magma transit to the seafloor. The remarkable spatial and temporal correlation of petrological and geophysical data at El Hierro suggests that the observed seismicity records magma mixing and forceful intrusion as well as subsequent reservoir dynamics. These results demonstrate that eruptions at El Hierro are controlled principally by deep-seated processes, with little influence from shallow crustal levels, and have important implications for monitoring of renewed unrest at long-dormant volcanoes.

Description: Cosigüina volcano, in northwestern Nicaragua, erupted violently on 20–24 January 1835, producing pumice, scoria, ash fall deposits, and pyroclastic flows with a bulk tephra volume of ~6 km 3 . New geochemical data are presented for bulk-rocks, matrix glasses, melt inclusions and minerals from the 1835 deposits and a pre-1835 basaltic andesite tephra, with the aim of shedding light on the magmatic processes and associated timescales that led to the eruption. Our results reveal that the 1835 eruption was fed by a compositionally and thermally zoned magma reservoir situated ~4 km ( P H2O ~100 MPa) beneath the volcano. Small volumes of crystal-poor dacite (〈10 wt % phenocrysts, 63·8–64·8 wt % SiO 2 , ~950°C) and silicic andesite (〈10 wt % phenocrysts, 62·2 wt % SiO 2 , 960–1010°C) were erupted first, followed by relatively crystal-rich andesite (15–30 wt % phenocrysts, 57·4–58·8 wt % SiO 2 , 960–1010°C), which accounts for ~90% of the erupted magma. The pre-1835 basaltic andesite (~20 wt % phenocrysts, 52·4 wt % SiO 2 , 1110–1170°C) represents a mafic end-member for Cosigüina. The major and trace element compositions of the bulk-rocks, melt inclusions and matrix glasses suggest that (1) the pre-1835 basaltic andesite is a plausible parent for the 1835 magmas, (2) the 1835 andesite bulk-rocks do not represent true melts, but instead mixtures of silicic andesite liquid and a component of accumulated crystals dominated by plagioclase, and (3) the silicic andesite and dacite formed from the andesite magma through liquid extraction followed by fractional crystallization. Observed bimodal to trimodal crystal populations are consistent with a multi-stage, polybaric differentiation process, with calcic plagioclase (An 75–90 , An 90–95 ) and magnesian clinopyroxene (Mg# = 67–75), plus olivine and magnetite, forming from mafic andesite, basaltic andesite and basalt in the lower crust. The calcic plagioclase exhibits sieve textures, which may be the result of H 2 O-undersaturated decompression during magma ascent to the upper crust; An 50–65 plagioclase lacking a sieve texture, orthopyroxene (Mg# = 61 and 63–72), clinopyroxene (Mg# = 67), magnetite and apatite crystallized from andesite to dacite liquids in the shallow magma reservoir. An 75–90 plagioclase comprising entire phenocrysts or cores with An 50–65 rims in the 1835 magmas is cognate from earlier stages of differentiation and shows evidence of extensive diffusion of Mg when compared with similar An 75–95 crystals hosted in the pre-1835 basaltic andesite. Using plagioclase–melt Mg partitioning and modelling of the Mg diffusion process, we constrain the residence time of these crystals in the silicic liquids to more than 100 years and less than 2000 years, with detailed analysis of three crystals yielding ~400 years. We propose that magma reservoir zonation occurred on timescales of 10 2 –10 3 years at Cosigüina. The occurrence of H 2 O-rich fluid inclusions in all 1835 samples and volatile element systematics in melt inclusions imply that the magmas were saturated with a vapour phase (H 2 O, S, ± CO 2 ) during much of their evolution in the upper crust. Accumulation of free gas at the top of the magma reservoir may have led to overpressurization of the system, triggering the eruption. Catastrophic release of this exsolved vapour and syn-eruptive devolatilization of the melt injected several teragrams of S into the atmosphere. Our data, coupled with independent evidence from ice cores and tree rings, indicate that the Cosigüina eruption had a sizeable atmospheric impact comparable with or larger than that of the 1991 Pinatubo eruption. Stratigraphic evidence shows that Cosigüina has produced 〉15 compositionally zoned explosive eruptions in the past, suggesting that similar future eruptions are likely. The products of the 1835 eruption of Cosigüina share many features with compositionally zoned eruptive sequences elsewhere, such as the climactic eruption of Mount Mazama, the ad 79 ‘Pompei’ eruption of Vesuvius and the 1912 eruption of Novarupta–Katmai.

Description: A submarine eruption started off the south coast of El Hierro, Canary Islands, on 10 October 2011 and continues at the time of this writing (February 2012). In the first days of the event, peculiar eruption products were found floating on the sea surface, drifting for long distances from the eruption site. These specimens, which have in the meantime been termed “restingolites” (after the close-by village of La Restinga), appeared as black volcanic “bombs” that exhibit cores of white and porous pumice-like material. Since their brief appearance, the nature and origin of these “floating stones” has been vigorously debated among researchers, with important implications for the interpretation of the hazard potential of the ongoing eruption. The “restingolites” have been proposed to be either (i) juvenile high-silica magma (e.g. rhyolite), (ii) remelted magmatic material (trachyte),(iii) altered volcanic rock, or (iv) reheated hyaloclastites or zeolite from the submarine slopes of El Hierro. Here, we provide evidence that supports yet a different conclusion. We have analysed the textures and compositions of representative “restingolites” and compared the results to previous work on similar rocks found in the Canary Islands. Based on their high-silica content, the lack of igneous trace element signatures, the presence of remnant quartz crystals, jasper fragments and carbonate as well as wollastonite (derived from thermal overprint of carbonate) and their relatively high oxygen isotope values, we conclude that “restingolites” are in fact xenoliths from pre-island sedimentary layers that were picked up and heated by the ascending magma, causing them to partially melt and vesiculate. As they are closely resem- bling pumice in appearance, but are xenolithic in origin, we refer to these rocks as “xeno-pumice”. The El Hierro xeno- pumices hence represent messengers from depth that help us to understand the interaction between ascending magma and crustal lithologies beneath the Canary Islands as well as in similar Atlantic islands that rest on sediment-covered ocean crust (e.g. Cape Verdes, Azores). The occurrence of “restingolites” indicates that crustal recycling is a relevant process in ocean islands, too, but does not herald the arrival of potentially explosive high-silica magma in the active plumbing system beneath El Hierro. results of our textural, mineralogical, elemental and isotopic analysis lead us to conclude that the early floating stones of El Hierro are vesiculated crustal xenoliths that originate from the substantial layer of sub-volcanic pre-island sedimentary rocks (layer 1 of the oceanic crust) that is present underneath the Canary archipelago.

Description: The Swedish and the German Science Foundations (VR and DFG), the ERC grant EVOKES and the Center for Natural Disaster Science (CNDS) Sweden

Description: Published

Description: 97-110

Description: N/A or not JCR

Description: open