Volcanic eruption columns typically have unsteady source conditions, where mass and heat fluxes from the vent evolve or fluctuate on time scales from seconds to hours. However, integral plume models ...routinely assume source conditions that are statistically stationary, and the degree to which source unsteadiness influences the mechanics of column rise and air entrainment has not been established with quantitative predictions. We address this knowledge gap by examining eruptions with varying unsteady character at Sabancaya Volcano, Peru. Using a novel tracking algorithm based on spectral clustering, we track the spatiotemporal evolution of coherent turbulent structures in columns using ground‐based, thermal infrared imagery. For turbulent structures tracked in time and space, we calculate the power law decay exponent of excess temperature with height. In general, the starting pulses of transient events are characterized by power law exponents matching theoretical predictions for an instantaneous point release of buoyancy (i.e., a thermal), which evolve with sustained emissions to values consistent with steady plumes. Our results support previous findings from field evidence and laboratory experiments that entrainment and gravitational stability in unsteady volcanic columns are inadequately captured by time‐averaging or constant entrainment coefficients. We propose a quantitative definition for column source unsteadiness which captures the timing and magnitude of source fluctuations on time scales that influence entrainment mechanics, and which provisionally predicts our observed differences in power law behavior. We argue for systematic experimental and numerical studies of the relationship between source unsteadiness and entrainment to implement unsteady entrainment parameterizations for integral plume models.
Plain Language Summary
Volcanic eruptions are routinely simulated as sustained, jet‐like flows of gas and ash. However, most eruptions in nature are unsteady at the source vent, meaning the flow rate and heat content of erupted material varies substantially over time scales ranging from seconds to hours. This variation impacts mixing of eruption plumes with the background atmosphere (a process called entrainment), ultimately affecting how high plumes rise and where they disperse hazardous ash. To better understand how unsteady conditions influence eruption behavior and hazard, we analyzEd infrared camera imagery of eruption plumes at Amancaya Volcano, Peru. By developing a new algorithm which tracks individual turbulent eddies in the rising plume, we measure how the heat content in the plumes evolve with entrainment of atmosphere. Our measurements show the plume mixing process evolving between theoretical predictions for sustained, jet‐like flows and single, brief pulses, as a result of unsteady, evolving conditions at the plume source. We use our measurements to propose a mathematical framework for quantifying unsteadiness in volcanic plumes, enabling future experiments and computer simulations that include unsteady effects. Ultimately, this will lead to improved forecasts of ash dispersal and resulting hazards for unsteady eruptions.
Key Points
Unsupervised machine learning algorithm tracks evolving plume structures in thermal imagery at Sabancaya Volcano
Temperature evolution in both space and time reflects unsteady transitions between steady plume and discrete thermal regimes
We propose a quantitative unsteadiness metric for the prediction of entrainment regimes as a function of eruption source unsteadiness
Variations in the spectral content of volcano seismicity related to changes in volcanic activity are commonly identified manually in spectrograms. However, long time series of monitoring data at ...volcano observatories require tools to facilitate automated and rapid processing. Techniques such as self-organizing maps (SOM) and principal component analysis (PCA) can help to quickly and automatically identify important patterns related to impending eruptions. For the first time, we evaluate the performance of SOM and PCA on synthetic volcano seismic spectra constructed from observations during two well-studied eruptions at Klauea Volcano, Hawai'i, that include features observed in many volcanic settings. In particular, our objective is to test which of the techniques can best retrieve a set of three spectral patterns that we used to compose a synthetic spectrogram. We find that, without a priori knowledge of the given set of patterns, neither SOM nor PCA can directly recover the spectra. We thus test hierarchical clustering, a commonly used method, to investigate whether clustering in the space of the principal components and on the SOM, respectively, can retrieve the known patterns. Our clustering method applied to the SOM fails to detect the correct number and shape of the known input spectra. In contrast, clustering of the data reconstructed by the first three PCA modes reproduces these patterns and their occurrence in time more consistently. This result suggests that PCA in combination with hierarchical clustering is a powerful practical tool for automated identification of characteristic patterns in volcano seismic spectra. Our results indicate that, in contrast to PCA, common clustering algorithms may not be ideal to group patterns on the SOM and that it is crucial to evaluate the performance of these tools on a control dataset prior to their application to real data.
•We develop a benchmarking dataset for volcano seismic pattern recognition.•Hierarchical clustering on SOM patterns does not reproduce the known input patterns.•Hierarchical clustering applied to PCA results captures the known input patterns.•Clustering on PCA results promising for automated pattern recognition in spectra
Permeability, the ease of fluid flow through porous rocks and soils, is a fundamental but often poorly quantified component in the analysis of regional‐scale water fluxes. Permeability is difficult ...to quantify because it varies over more than 13 orders of magnitude and is heterogeneous and dependent on flow direction. Indeed, at the regional scale, maps of permeability only exist for soil to depths of 1–2 m. Here we use an extensive compilation of results from hydrogeologic models to show that regional‐scale (>5 km) permeability of consolidated and unconsolidated geologic units below soil horizons (hydrolithologies) can be characterized in a statistically meaningful way. The representative permeabilities of these hydrolithologies are used to map the distribution of near‐surface (on the order of 100 m depth) permeability globally and over North America. The distribution of each hydrolithology is generally scale independent. The near‐surface mean permeability is of the order of ∼5 × 10−14 m2. The results provide the first global picture of near‐surface permeability and will be of particular value for evaluating global water resources and modeling the influence of climate‐surface‐subsurface interactions on global climate change.
Supercontinent assembly and breakup can influence the rate and global extent to which insulated and relatively warm subcontinental mantle is mixed globally, potentially introducing lateral ...oceanic‐continental mantle temperature variations that regulate volcanic and weathering controls on Earth's long‐term carbon cycle for a few hundred million years. We propose that the relatively warm and unchanging climate of the Nuna supercontinental epoch (1.8–1.3 Ga) is characteristic of thorough mantle thermal mixing. By contrast, the extreme cooling‐warming climate variability of the Neoproterozoic Rodinia episode (1–0.63 Ga) and the more modest but similar climate change during the Mesozoic Pangea cycle (0.3–0.05 Ga) are characteristic features of the effects of subcontinental mantle thermal isolation with differing longevity. A tectonically modulated carbon cycle model coupled to a one‐dimensional energy balance climate model predicts the qualitative form of Mesozoic climate evolution expressed in tropical sea‐surface temperature and ice sheet proxy data. Applied to the Neoproterozoic, this supercontinental control can drive Earth into, as well as out of, a continuous or intermittently panglacial climate, consistent with aspects of proxy data for the Cryogenian‐Ediacaran period. The timing and magnitude of this cooling‐warming climate variability depends, however, on the detailed character of mantle thermal mixing, which is incompletely constrained. We show also that the predominant modes of chemical weathering and a tectonically paced abiotic methane production at mid‐ocean ridges can modulate the intensity of this climate change. For the Nuna epoch, the model predicts a relatively warm and ice‐free climate related to mantle dynamics potentially consistent with the intense anorogenic magmatism of this period.
Key Points
Supercontinent assembly modulates volcanic sources and weathering sinks for
CO2 through effects on mantle convective thermal mixing
Punctuated mixing during Pangea/Rodinia epochs can induce climate cooling‐warming consistent with Mesozoic and Cryogenian‐Ediacaran proxies
Thorough thermal mixing during Precambrian Nuna assembly and breakup can maintain an ice‐free climate consistent with geological data
It is generally assumed that continents, acting as thermal insulation above the convecting mantle, inhibit the Earth's internal heat loss. We present theory, numerical simulations, and laboratory ...experiments to test the validity of this intuitive and commonly used assumption. A scaling theory is developed to predict heat flow from a convecting mantle partially covered by stable continental lithosphere. The theory predicts that parameter regimes exist for which increased continental insulation has no effect on mantle heat flow and can even enhance it. Partial insulation leads to increased internal mantle temperature and decreased viscosity. This, in turn, allows for the more rapid overturn of oceanic lithosphere and increased oceanic heat flux. Depending on the ratio of continental to oceanic surface area, global mantle heat flow can remain constant or even increase as a result. Theoretical scaling analyses are consistent with results from numerical simulations and laboratory experiments. Applying our results to the Earth we find, in contrast to conventional understanding, that continental insulation does not generally reduce global heat flow. Such insulation can have a negligible effect or even enhance mantle cooling, depending on the magnitude of the temperature dependence of mantle viscosity. The theory also suggests a potential constraint on continental surface area. Increased surface area enhances the subduction rate of oceanic lithosphere. If continents are produced in subduction settings this could enhance continental growth up to a critical point where increased insulation causes convective stress levels to drop to values approaching the lithospheric yield stress. This condition makes weak plate margins difficult to maintain which, in turn, lowers subduction rates and limits the further growth of continents. The theory is used to predict the critical point as a function of mantle heat flow. For the Earth's rate of mantle heat loss, the predicted continental surface area is in accord with the observed value.
•Experiments and theory are combined to test the stability of turbulent volcanic jets.•Vent geometry and inertial particles strongly affect entrainment in volcanic jets.•Volcanic jets from ...ring-fracture vents are less stable than jets from circular vents.•Super eruption ignimbrite volume is high due to instabilities induced by vent geometry.
Turbulent volcanic jets are produced by highly-energetic explosive eruptions and may form buoyant plumes that rise many tens of kilometres into the atmosphere to form umbrella clouds or collapse to generate ground-hugging pyroclastic flows. Ash injected into the atmosphere can be transported for many hundreds of kilometres with the potential to affect climate, disrupt global air travel and cause respiratory health problems. Pyroclastic flows, by contrast, are potentially catastrophic to populations and infrastructure close to the volcano. Key to which of these two behaviours will occur is the extent to which the mechanical entrainment and mixing of ambient air into the jet by large (entraining) eddies forming the jet edge changes the density of the air–ash mixture: low entrainment rates lead to pyroclastic flows and high entrainment rates give rise to buoyant plumes. Recent experiments on particle-laden (multi-phase) volcanic jets from flared and straight-sided circular openings suggest that the likelihood for buoyant plumes will depend strongly on the shape and internal geometry of the vent region. This newly recognised sensitivity of the fate of volcanic jets to the structure of the vent is a consequence of a complex dynamic coupling between the jet and entrained solid particles, an effect that has generally been overlooked in previous studies. Building on this work, here we use an extensive series of experiments on multi-phase turbulent jets from analogue linear fissures and annular ring fractures to explore whether the restrictive vent geometry during cataclysmic caldera-forming (CCF) eruptions will ultimately lead a relatively greater frequency of pyroclastic flows than eruptions from circular vents on stratovolcanoes. Our results, understood through scaling analyses and a one-dimensional theoretical model, show that entrainment is enhanced where particle motions contribute angular momentum to entraining eddies. However, because the size of the entraining eddies scales approximately with vent width, the extent of entrainment is reduced as the vent width becomes small in comparison to its length. Consequently, our work shows that for specified mass eruption rates, the high length-to-width ratio vents typical of CCF events are more likely to produce pyroclastic flows. We suggest that the enigmatic trend in the geological record for the largest CCF eruptions to produce pyroclastic flows is an expected consequence of their being erupted through continuous or piece-wise continuous caldera ring fractures.
The heat production budget of a planet exerts a first order control on its thermal evolution, tectonics, and likelihood for habitability. However, our knowledge of heat producing element ...concentrations for silicate-metal bodies in the solar system—including Earth—is limited. Here we review the chronicle of heat producing elements (HPEs) in the solar system, from the interstellar medium, to their incorporation in the protoplanetary disk and accreting planetesimals, to later collisional or atmospheric-erosion modifications. We summarise the state of knowledge of the HPEs in terrestrial planets and meteorites, and current Earth models from emerging constraints, and assess the effect variations may have on the thermal and tectonic history of terrestrial planets.
Explosive volcanic eruptions are one of the most important driver of climate variability. Yet, we still lack a fundamental understanding of how climate change may affect future eruptions. Here, we ...use an ensemble of simulations by 1‐D and 3‐D volcanic plume models spanning a large range of eruption source and atmospheric conditions to assess changes in the dynamics of future eruptive columns. Our results shed new light on differences between the predictions of 1‐D and 3‐D plume models. Furthermore, both models suggest that as a result of ongoing climate change, for tropical eruptions, (i) higher eruption intensities will be required for plumes to reach the upper troposphere/lower stratosphere and (ii) the height of plumes currently reaching the upper troposphere/lower stratosphere or above will increase. We discuss the implications of these results for the climatic impacts of future eruptions. Our simulations can directly inform climate model experiments on climate‐volcano feedback.
Key Points
We compare the impacts of climate change on the dynamics of eruptive columns, as predicted by 1‐D and 3‐D plume models
Both models agree that higher eruption intensities will be required to inject sulfur into the tropical stratosphere
Eruptive column‐climate interactions are key to understand the climatic impacts of future eruptions
Understanding the longevity of volcanic ash‐clouds generated by powerful explosive eruptions is a long standing problem for assessing volcanic hazards and the nature and time scale of volcanic ...forcings on climate change. It is well known that the lateral spreading and longevity of these clouds is influenced by stratospheric winds, particle settling and turbulent diffusion. Observations of the recent 2010 Eyjafjallajökull and 2011 Grimsvötn umbrella clouds, as well as the structure of atmospheric aerosol clouds from the 1991 Mt Pinatubo event, suggest that an additional key process governing the cloud dynamics is the production of internal layering. Here, we use analog experiments on turbulent particle‐laden umbrella clouds to show that this layering occurs where natural convection driven by particle sedimentation and the differential diffusion of primarily heat and fine particles give rise to a large scale instability. Where umbrella clouds are particularly enriched in fine ash, this “particle diffusive convection” strongly influences the cloud longevity. More generally, cloud residence time will depend on fluxes due to both individual settling and diffusive convection. We develop a new sedimentation model that includes both sedimentation processes, and which is found to capture real‐time measurements of the rate of change of particle concentration in the 1982 El Chichon, 1991 Mt Pinatubo and 1992 Mt Spurr ash‐clouds. A key result is that these combined sedimentation processes enhance the fallout of fine particles relative to expectations from individual settling suggesting that particle aggregation is not the only mechanism required to explain volcanic umbrella longevity.
Key PointsNew theory for layer formation in volcanic ash‐cloudsParticle diffusive convection affects the cloud residence timeA new sedimentation model captures real-time measurements of historical events
Although inhibited on established fluid mechanical grounds, extensive magma mixing can play a critical role in creating andesite magmas at volcanic arcs and triggering effusive or explosive ...volcanism. We use analog experiments, scaling theory, and thermodynamic modeling of natural volcanic systems to show that mechanical mixing is enhanced if an intruding magma crystallizes to acquire a yield strength. Mafic magmas intruding highly silicic reservoirs will fragment as crystal‐sized blobs/enclaves to produce textural homogeneity at the outcrop scale and heterogeneity at the scale of individual crystals. Rapid degassing from these enclaves can favor the production of permeable magmatic foams that facilitate gas loss from the reservoir and effusive volcanism. Crystallizing magmas intruding similar composition reservoirs of comparable or smaller effective viscosity will fragment at scales approaching the dike width. Sluggish degassing and consequent volatile retention in the reservoir can enhance volatile exsolution in an erupting conduit and tendency for explosive volcanism.
Key Points
Crystallization of basalts injected into silicic reservoirs facilitates mechanical magma mixing and modulates resulting volcanism
Very low viscosity injections fragment at crystal scales, enhancing mixing and degassing and the likelihood for effusive volcanism
Higher viscosity injections fragment at larger scales, reducing mixing and degassing and increasing the likelihood for explosive volcanism
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