This paper examines phreatic eruptions which are driven by inputs of magma and magmatic gas. We synthesize data from several significant phreatic systems, including two in Costa Rica (Turrialba and ...Poás) which are currently highly active and hazardous. We define two endmember types of phreatic eruptions, the first (type 1) in which a deeper hydrothermal system fed by magmatic gases is sealed and produces overpressure sufficient to drive explosive eruptions, and the second (type 2) where magmatic gases are supplied via open-vent degassing to a near-surface hydrothermal system, vaporizing liquid water which drives the phreatic eruptions. The surficial source of type 2 eruptions is characteristic, while the source depth of type 1 eruptions is commonly greater. Hence, type 1 eruptions tend to be more energetic than type 2 eruptions. The first type of eruption we term “phreato-vulcanian”, and the second we term “phreato-surtseyan”. Some systems (e.g., Ruapehu, Poás) can produce both type 1 and type 2 eruptions, and all systems can undergo sealing at various timescales. We examine a number of precursory signals which appear to be important in understanding and forecasting phreatic eruptions; these include very long period events, banded tremor, and gas ratios, in particular H
2
S/SO
2
and CO
2
/SO
2
. We propose that if these datasets are carefully integrated during a monitoring program, it may be possible to accurately forecast phreatic eruptions.
This paper examines the behavior of volcanoes that erupt quickly with paroxysmal explosive eruptions, and other volcanoes that erupt over extended periods without such paroxysmal activity. “Fast” ...activity typically occurs over the course of months to years, including precursory unrest, the paroxysmal eruption itself, and post-paroxysmal activity. “Slow” activity comprises extended restlessness over the course of decades, and eruptions are typically small and sometimes uncommon. I review activity at eight volcanoes with fast and slow activity, highlighting the main events, and commonalities in behavior among the different systems. In terms of forecasting, volcanoes with fast unrest typically have short 1–3 month precursory periods prior to the climactic eruption, while volcanoes with slow unrest commonly have an extended period of considerable uncertainty regarding the presence or absence of new magma, as well as unanticipated accelerations in activity. Volcanoes with fast behavior are associated with magmas having elevated volatile contents (up to ~7 wt. % H2O), rapid magma ascent rates, and rapid declines in activity after the climactic eruption. These volcanoes also exhibit well-defined magma plumbing systems containing mobile volatile-rich magma, with the plumbing system often sealed between the top of the shallow magma reservoir and the surface prior to the climactic eruption. Volcanoes with slow behavior have complex plumbing systems comprising cracks, fractures, dykes, and sills and magmas that are crystal-rich, partly degassed, and rheologically sluggish. These volcanoes experience a progressive opening of their systems as magma intrudes and fractures country rock, allowing degassing to occur. The degree to which a system is opened is determined by the rate at which new magma is emplaced at shallow levels. As such, magma emplacement rates which are fast, intermediate, or slow should produce unrest on similar timescales. Slower rates of emplacement enhance the opening process due to a cumulatively high number of fractures and increased fracture density which develop during the extended period of unrest. Many systems both fast and slow receive inputs of more mafic magma which can drive activity seen at the surface. A series of recently developed tools is examined and discussed in order to provide an improved means of forecasting activity at both types of volcanoes. These include assessment of early phreatic activity and associated gases, Vp/Vs ratios of magma by seismic tomography, and estimates of magma volume from precursory seismicity. What is required now are protocols which integrate these approaches in a manner which is useful for accurate forecasting.
Oceanic intraplate volcanoes grow by accumulation of erupted material as well as by coeval or discrete magmatic intrusions. Dykes and other intrusive bodies within volcanic edifices are comparatively ...well studied, but intrusive processes deep beneath the volcanoes remain elusive. Although there is geological evidence for deep magmatic intrusions contributing to volcano growth through uplift, this has rarely been demonstrated by real-time monitoring. Here we use geophysical and petrological data from El Hierro, Canary Islands, to show that intrusions from the mantle and subhorizontal transport of magma within the oceanic crust result in rapid endogenous island growth. Seismicity and ground deformation associated with a submarine eruption in 2011–2012 reveal deep subhorizontal intrusive sheets (sills), which have caused island-scale uplift of tens of centimetres. The pre-eruptive intrusions migrated 15–20 km laterally within the lower oceanic crust, opening pathways that were subsequently used by the erupted magmas to ascend from the mantle to the surface. During six post-eruptive episodes between 2012 and 2014, further sill intrusions into the lower crust and upper mantle have caused magma to migrate up to 20 km laterally, resulting in magma accumulation exceeding that of the pre-eruptive phase. A comparison of geobarometric data for the 2011–2012 El Hierro eruption with data for other Atlantic intraplate volcanoes shows similar bimodal pressure distributions, suggesting that eruptive phases are commonly accompanied by deep intrusions of sills and lateral magma transport. These processes add significant material to the oceanic crust, cause uplift, and are thus fundamentally important for the growth and evolution of volcanic islands. We suggest that the development of such a magma accumulation zone in the lower oceanic crust begins early during volcano evolution, and is a consequence of increasing size and complexity of the mantle reservoir system, and potentially the lithospheric stresses imposed by increasing edifice load.
•Lateral magma movement and accumulation within the deep oceanic crust is common.•Deep magma intrusions cause rapid volcano uplift and modify the oceanic crust.•Pressures obtained from petrological studies are consistent with seismic data.•Endogenous volcano growth is fundamentally important for ocean island evolution.
Volcanic volatile emissions provide information about volcanic unrest but are difficult to detect with satellites. Volcanic degassing affects plants by elevating local CO2 and H2O concentrations, ...which may increase photosynthesis. Satellites can detect plant health, or a reaction to photosynthesis, through a Normalized Difference Vegetation Index (NDVI). This can act as a potential proxy for detecting changes in volcanic volatile emissions from space. We tested this method by analyzing 185 Landsat 5 and 8 images of the Tern Lake thermal area (TLTA) in northeast Yellowstone caldera from 1984 to 2022. We compared the NDVI values of the thermal area with those of similar nearby forests that were unaffected by hydrothermal activity to determine how hydrothermal activity impacted the vegetation. From 1984 to 2000, plant health in the TLTA steadily increased relative to the background forests, suggesting that vegetation in the TLTA was fertilized by volcanic CO2 and/or magmatic water. Hydrothermal activity began to stress plants in 2002, and by 2006, large swathes of trees were dying in the hydrothermal area. Throughout most of the 1990s, the least healthy plants were located in the area which became the epicenter of hydrothermal activity in 2000. These findings suggest that plant‐focused measurements are sensitive to minor levels of volcanic unrest which may not be detected by other remote sensing methods, such as infrared temperature measurements. This method could be a safe and effective new tool for detecting changes in volatile emissions in volcanic environments which are dangerous or difficult to access.
Plain Language Summary
Scientists often measure volatile emissions from volcanoes to understand how the magma underneath the volcano is behaving to anticipate potential volcanic hazards. These emissions are difficult and often hazardous to measure on the ground; therefore, measuring them with satellites would facilitate consistent and safe detection of changes in volcanic activity. It is nearly impossible to directly measure volcanic carbon dioxide and H2O from space, so we need another method. We use plants as a proxy because they use carbon dioxide and water for photosynthesis, so if plants receive these extra gases from the volcano, it should improve plant health. We detected signals of this increased health from 1984 to 2001 in Yellowstone National Park, USA. One area of the forest that was exposed to volcanic gases was healthier than nearly identical, nearby forests growing without the influence of the volcano. This supports the idea that extra volcanic carbon dioxide and water promote tree health. As volcanic activity intensifies, it can harm plants. We detected increases in plant stress caused by increases in soil temperature and sulfur emissions before they were detectable by other types of satellites. Combining these two contrasting effects represents a promising new path for additional monitoring of active volcanoes.
Key Points
Plant responses to elevated CO2 may be a viable proxy for volcanic CO2
We have detected variations in diffuse volatile emissions in Yellowstone using plant responses
Plants responded to changes in volatile emissions several years before hydrothermal activity was previously thought to have started
Mantle source regions feeding hotspot volcanoes likely contain recycled subducted material. Anomalous sulphur (S) isotope signatures in hotspot lavas have tied ancient surface S to this deep ...geological cycle, but their potential modification by shallow magmatic processes has generally been overlooked. Here we present S isotope measurements in magmatic sulphides, silicate melt inclusions and matrix glasses from the recent eruption of a hotspot volcano at El Hierro, Canary Islands, which show that degassing induces strongly negative δ
S fractionation in both silicate and sulphide melts. Our results reflect the complex interplay among redox conditions, S speciation and degassing. The isotopic fractionation is mass dependent (Δ
S = 0‰), thus lacking evidence for the recycled Archaean crust signal recently identified at other hotspot volcanoes. However, the source has an enriched signature (δ
S ~ + 3‰), which supports the presence of younger
S-rich recycled oceanic material in the Canary Island mantle plume.
Four historic caldera‐forming events were studied to understand the relationship of magma withdrawal processes and caldera subsidence mechanisms. Two calderas are silicic (Katmai in 1912 and Pinatubo ...in 1991), and two are basaltic (Fernandina in 1968 and Miyakejima in 2000). All events have sufficient geophysical, geologic, and petrologic data with which to examine and model magma withdrawal and caldera collapse. The data reveal that the magmas erupted at Katmai and Pinatubo were in a bubbly state in the reservoir immediately before and during caldera collapse. The bubbly magma allowed for its efficient extraction from the reservoir, causing significant underpressures to develop rapidly, particularly in the case of Katmai where the erupted rhyolite was voluminous and nearly aphyric and has very low viscosity. The rapidly developing underpressures at Katmai and Pinatubo caused sudden en masse caldera collapse halfway through the climactic eruptions, thereby liberating large amounts of seismic energy. At Fernandina and Miyakejima, by contrast, caldera collapse was initiated early and continued for an extended period of time from weeks to months, consisting of a series of discrete subsidence events manifested by large earthquakes at Fernandina and by very long period (VLP) signals at Miyakejima. Systematic changes in earthquake magnitudes and quiescent intervals at both volcanoes reveal changes in friction, as collapse took place during extended time intervals.
Understanding the behavior of Cu, Zn, and Pb in magmatic–hydrothermal systems is essential in developing models for the genesis of hydrothermal ore deposits in subduction environments. A commonly ...held view is that the metals originate in magmas of intermediate to felsic composition from which they partition directly into exsolving aqueous fluids. In this paper, we build on the results of an earlier study in which we showed that Fe, Cu, Co and Ni at Merapi volcano, Indonesia, were transferred from a mafic melt to an immiscible sulfide melt, and then to a magmatic volatile phase which carried them to the surface. Here we examine the pathways taken by the volatile phase and the behavior of Cu, Zn and Pb in the upper part of the magmatic system beneath Merapi volcano. Based on the composition of a suite of silicate melt inclusions and thermodynamic modeling, we show that the mafic melt did not hybridize with the resident felsic melt but instead transferred metals to the latter by exsolving a magmatic volatile phase which dissolved the Cu-rich sulfide melt and then percolated upwards through the felsic melt, enriching it in Cu and Pb. It is this fluid which transported the metals to the surface and potentially also to subsurface environments of ore formation.
► We analyzed major elements and metals in silicate melt inclusions from Merapi's lavas and scorias. ► We modeled assimilation and fractional crystallization (AFC) of Merapi deep mafic and shallow felsic melts. ► AFC cannot account for the atypical enrichment of Cu and Pb in Merapi shallow melt. ► We thus proposed a new model to explain how this enrichment occurred. ► In this model, a hydrothermal fluid percolated upward and partitioned its Cu and Pb into the shallow magma.
We present new experiments on replenishment of rhyolite magma chambers by rhyolite magma using corn syrup‐water solutions. We emphasize small density contrasts and show that buoyancy is the key ...controlling factor for whether injections will rise to the top (if buoyant) or pond at the base (if denser). During emplacement, we observe little or no mixing of the injected liquid with the reservoir liquid, as predicted by the fact that our injections have low Reynolds numbers (<10, typically). At later stages, the low‐buoyancy (≤1 kg m−3) injected liquid, which has accumulated at the top of the reservoir, undergoes mixing with the reservoir liquid, which may originate by the gravitational destabilization of a thin layer of denser resident liquid trapped above the injected liquid layer. The presence of a basal crystal mush, modeled by acrylic beads in a corn syrup‐water solution matrix is also considered. Slightly buoyant injections entrain a small fraction of mush particles to the top of the overlying liquid layer. Entrainment efficiency increases dramatically for high‐buoyancy injections. We hypothesize that the injected liquid can entrain a maximum quantity of mush particles, which corresponds to the amount required for the injected liquid/mush particle suspension to attain neutral buoyancy in the resident liquid. Hence for silicic systems, a replenishing melt can entrain up to 12.5% crystals during its ascent through the mush. Our results have implications for rhyolites bearing crystals with disequilibrium features, as they may represent mush crystals remobilized by a replenishing silicic magma.
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