Magnetic anisotropy plays a central role in many petrofabric and paleomagnetic studies. Anisotropy is typically represented by a second-order symmetric tensor that reflects the combined contributions from mineral populations with differing grain sizes, orientation distributions and particle scale anisotropies. Thus, the quality of geologically significant information obtained from magnetic anisotropy data depends on our ability to disentangle the complexity of these coexisting fabrics. In this study, we present a least-squares technique that can be employed in combination with additional geological or other supporting evidence to separate measured anisotropy tensors into independent contributions with distinct physical meaning. The analysis is readily adaptable and widely applicable to interpreting composite hybrid magnetic anisotropies, like those which arise from tectonic forces. Here, we revisit published deposition experiments and anisotropy of anhysteretic remanence (AARM) measurements to demonstrate the usefulness of the tensor decomposition approach. Remanence anisotropy measurements are decomposed into idealized tensorial sub-components originating from the preferred alignment of particles parallel to the magnetic field (field-aligned fabric) and within the bedding plane (sedimentary fabric). The least-squares decomposition isolates the field-aligned fabric by subtracting the sedimentary-compaction fabric. After subtraction of the sedimentary fabric, quantitative paleofield strength and direction can be directly inferred from the field-aligned subfabric.

Seventeen pyroclastic density currents (PDCs) were produced before, during and after the Plinian phase of the ad 79 eruption of Vesuvius. Their deposits were correlated using the proportions of components, together with the recognition of distinctive intercalated regionally traceable fall marker layers, revealing sectoral and distance-dependent variations. During an extensive field analysis, 27 lithostratigraphic units were detected and mapped and the lateral and vertical variations of 15 lithofacies were documented, described and interpreted. The total volume of PDC units is 1.25 km(3). We consider that the early PDCs were generated by partial collapses from the sustained Plinian eruption column, whereas the subsequent post-Plinian PDCs were generated by more sustained pyroclastic fountaining. New facies mapping revealed lateral migration and extended travel distances for some of the currents. Most of the lobed PDCs remained relatively uniform, whereas the radial PDCs exhibited fluctuating waxing and waning pulses and changed gradationally down-current. Re-evaluation of the timing of caldera collapse and the transition from a magmatic to a phreatomagmatic eruption style suggests that substrate fracturing during incremental caldera subsidence may have allowed the gradual ingress of groundwater into the erupting magma. These results provide a new chronology of the eruption, revealing that the post-Plinian collapse phase lasted c. 12 h.

Caldera-forming eruptions sequences produce a wide range of volcanogenic sedimentary deposits, from primary mesobreccia (blocks > 1m) that form from the collapse of caldera wall scarps during the eruption, to co-ignimbrite ash plumes (particles < 2mm) deposited during the waning of outflowing pyroclastic density currents, and even secondary aqueously reworked sediments deposited within caldera lakes. Understanding the temporal and special distribution of these deposits is vital to both locate and reconstruct the collapse sequence of these catastrophic events. However, research of caldera volcanoes is greatly hindered at modern examples due to subsequent burial of their own deposits. Ordovician deposits (452 Ma) of the Borrowdale Volcanic Group (BVG) in the English Lake District are dominated by large-scale, caldera-forming deposits that have been tectonically uplifted and dissected by glacial erosion. Each of the BVG calderas have the potential to provide a wealth of knowledge regarding how calderas erupt, collapse, and continue to shape geological processes when their explosive activity ceases. However, due to intense faulting, alteration, and impersistent exposure across the Lake District, in-depth research of these calderas has been largely prevented using standard field-mapping techniques. This leaves most of the understanding for the overall nested caldera complex to be inferred. This investigation utilizes whole-rock geochemistry as the primary method to establish correlations between potential large-scale, caldera-forming pyroclastic deposits across large distances. Once the extent of each deposit was determined, they were individually traced back to their source vents. Detailed fieldwork was then conducted to identify characteristic caldera-forming features, such as rapid thickness changes over volcanotectonic faults, and extensive intercalated mesobreccia deposits. Geochemical analysis of immobile elements, including Nb, Th, Y, and Zr, of potential caldera-forming pyroclastic deposits has allowed for several notable correlations to be established, or disproven. Firstly, proximal outflow sheets from Langdale Caldera have been successfully correlated to pyroclastic sheets over 15 km from the caldera margin and suggest a multi-phase collapse sequence. Secondly, significant differences in the geochemistry of deposits associated with the Lincomb Tarns Formation (> 500 km2 ignimbrite) indicates the presence numerous pyroclastic sheets, originating from two separate caldera volcanoes within the Helvellyn and Ambleside areas. Finally, geochemical variations within the Haweswater ‘caldera’ infill deposit prove that sudden thickness changes, previously associated with syn-collapse volcanotectonic faulting, are the result of separate eruption events that have since between adjacently faulted.

While most ocean-island volcanism is effusive, recent evidence has demonstrated that intraplate ocean island volcanoes can exhibit protracted explosive histories, with catastrophic eruption styles and hazardous behaviour more typically associated with volcanoes in continental and plate-margin settings. Tenerife is the largest explosive ocean-island volcano on Earth, with a prolonged (-2 Ma) post-erosional history of caldera-forming, plinian and ignimbrite eruptions of evolved composition. The 0.7-1.8 Ma succession with 20 newly defined formations is reported for southern Tenerife, Canary Islands. In the last 2 Myr, the Las Can similar to adas volcano has produced >42 pumice-fall eruptions, 21 with extensive ignimbrites, and 12 inferred caldera collapse events. Pyroclastic density currents have repeatedly travelled more than15 km from source to the ocean, filling valleys and burying extensive interfluves. A robust whole-rock chemistry dataset, selected mineral chemistry, coupled with new 40Ar/39Ar ages of units through the pyroclastic stratigraphy, allow recognition of magmatic trends within the system on the order of 100 ky. The catastrophic explosive eruptions form three, 0.2-0.5 Myr-duration clusters (the Ucanca, Guajara and Diego-Hernandez) that do not appear to correspond simply with geochemical cycles, or to cycles of increasing eruption size or explosivity as has been previously proposed. During the clusters, large eruption frequencies averaged 1 every 20-45 kyrs. The eruption clusters were separated by hiatuses of-240-260 kyr, recorded by soils and unconformities, and may reflect marked changes in geographic dispersals following giant landslide breaches in Las Can similar to adas caldera wall. Two concurrent evolutionary magmatic trends are distinguished: one producing crystal-rich magmas, the other formed the cooler crystal-poor magmas: both spanned over a million years until 0.66 Ma, when the former ceased.

The impact of hazardous pyroclastic density currents (PDCs) increases with runout distance, which is strongly influenced by the mass flux. This article shows that the mass flux of a PDC may derive not only from vent discharge during the eruption, but also from partly hot, temporary stores (accumulations) of aerated pyroclastic material perched high on the volcano. The unforeseen PDC at Fuego volcano (Guatemala) on 3 June 2018 happened c.1.5 hr after the eruption climax. It overran the village of San Miguel Los Lotes causing an estimated 400+ fatalities. Analysis of the facies architecture of the deposit combined with video footage shows that a pulsatory block‐and‐ash flow flowed down the Las Lajas valley and rapidly waxed, the runout briefly increasing to 12.2 km as it filled and then spilled out of river channels, entered a second valley where it devastated the village and became increasingly erosive, prior to waning. Paleomagnetic analysis shows that the PDC contained only 6% very hot (>590°C) clasts, 39% moderately hot (∼200°C–500°C) clasts, and 51% cool (<200°C) clasts. This reveals that the block‐and‐ash flow mostly derived from collapse of loose and partly hot pyroclastic deposits, stored high on the volcano, gradually accumulated during the last 2–3 years. Progressive collapse of unstable deposits supplied the block‐and‐ash flow, causing a bulk‐up process, waxing flow, channel overspill and unexpected runout. The study demonstrates that deposit‐derived pyroclastic currents from perched temporary tephra stores pose a particular hazard that is easy to overlook and requires a new, different approach to hazard assessment and monitoring.

‘The making of magma’ discusses the formation of magma. Volcanism is driven by prodigious amounts of heat from deep inside the Earth that partly melts rock to form magma, which accumulates and evolves deep underground, and may drive powerful eruptions that expel ash and lava on to the surface, creating a volcano. This process can be studied by inferences based upon volcano behaviour, seismic information, chemical tracers in volcanic materials, experimentation, and other clues. From enormous, diffuse networks deep in the Earth, the magma rises and gathers together in the crust as a complex system of interconnected fractures, reservoirs, and conduits. These magma reservoirs can be detected by seismic imaging.

‘How do volcanoes explode?’ examines how volcanoes explode, looking at the anatomy of an explosive eruption. It studies pumice-forming rhyolitic eruptions. One of the first things a volcanologist may do on arrival at a volcano is to figure out how the volcano has behaved in the past: what sort of eruptions, how large, and how frequent. Basaltic explosivity can be compared with rhyolitic and andesitic explosivity. Phoenix plumes rise from pyroclastic currents and contain mostly fine ash.

Super-eruptions are amongst the most extreme events to affect Earth's surface, but too few examples are known to assess their global role in crustal processes and environmental impact. We demonstrate a robust approach to recognize them at one of the best-preserved intraplate large igneous provinces, leading to the discovery of two new super-eruptions. Each generated huge and unusually hot pyroclastic density currents that sterilized extensive tracts of Idaho and Nevada in the United States. The ca. 8.99 Ma McMullen Creek eruption was magnitude 8.6, larger than the last two major eruptions at Yellowstone (Wyoming). Its volume exceeds 1700 km(3), covering >= 12,000 km(2). The ca. 8.72 Ma Grey's Landing eruption was even larger, at magnitude of 8.8 and volume of >= 2800 km(3). It covers >= 23,000 km(2) and is the largest and hottest documented eruption from the Yellowstone hotspot. The discoveries show the effectiveness of distinguishing and tracing vast deposit sheets by combining trace-element chemistry and mineral compositions with field and paleomagnetic characterization. This approach should lead to more discoveries and size estimates, here and at other provinces. It has increased the number of known super-eruptions from the Yellowstone hotspot, shows that the temporal framework of the magmatic province needs revision, and suggests that the hotspot may be waning.

‘Volcanoes beyond Earth’ highlights volcanoes on other planets. There are many more volcanoes on Venus than there are on Earth, and many remain active. In the absence of plate tectonics and the kind of tectonic forces that raise Earth-style mountain belts, and of streams, rivers, and shorelines, it is volcanism and volcanic products that dominate the surface of this planet. Fossil volcanism occurs in the Moon, Mercury, and Mars; Io, the hypervolcanic moon of Jupiter; and the ice volcanoes of the Solar System. There is potential for volcanism on exoplanets within distant planetary systems.

‘Volcanoes, climate, and the biosphere’ explores how volcanism has perturbed both climate and the complement of living organisms on Earth, both locally and globally. Volcanic outbursts, depending on their nature and scale, may cause global warming or global cooling. In the historical record, even geologically modest eruptions have had dramatic repercussions. Volcanoes can affect local weather. It is possible that climate change can, in turn, affect volcanism.

‘Volcanoes and water’ assesses how water affects the behaviour of volcanoes. When rising hot magma encounters groundwater, as in a water-saturated sandstone, the pore water in the sandstone next to the magma flashes to steam, and the force of the expanding steam within the rock tears the sand grains apart from each other, and streams them away as it escapes. Peperite, ‘phreatic’ (steam) eruptions, phreatomagmatic eruptions, submarine eruptions, emergent Surtseyan eruptions, and ‘phreatoplinian’ eruptions are thought to be the most violent on Earth. Glacial volcanoes, and catastrophic watery ‘lahars’ accompany many eruptions.

‘Hidden volcanoes’ highlights ancient volcanoes, which offer crucial perspectives that would not be clear from the study of modern volcanoes alone. Exhumed (uplifted and dissected) volcanoes provide vital glimpses deep inside a volcano, including how they work. They are windows showing what really happens when hot magma interacts with bedrock, ice-sheets, lakes, and deep oceans. Ancient volcanoes also reveal how volcanic processes vary with time. They have enabled us to discover new and awesome styles of eruption. Examples are described, from the highest mountain in Wales, Yr Wyddfa, or Snowdon, which was an explosive caldera volcano that emerged from shallow seas. Also described are ‘Large igneous provinces’, diamond-bearing volcanoes, and the most ancient volcanism on Earth.

‘What have volcanoes done for us?’ outlines how humans have benefited from volcanoes. Volcanism has helped make the planet as it is today, by contributing to the chemical composition of the atmosphere and oceans. Volcanoes provide rich, fertile environments for wildlife and agriculture. Ultimately, volcanoes have been essential for the evolution of the biosphere, the emergence of humanity and human civilization, and for the flowering of culture. However, volcanoes across the world are increasingly being exploited for tourism, quarrying, deforestation, and urbanization. They also pose a hazard, one that increases as burgeoning human populations encroach ever closer. The chapter then looks at methods of monitoring and mitigating volcanic hazards.

‘Making and breaking volcanoes’ addresses how volcanoes are constructed and denuded and explains the shape of volcanoes and their internal architecture, including the differences between scoria cones, tuff rings, maars, and dome fields, shield volcanoes, and stratocones. Some volcanoes (‘monogenetic’ volcanoes) erupt just once, whereas others (‘polygenetic’ volcanoes) may continue erupting intermittently for millions of years. When sufficient magma is rapidly expelled from the shallow reservoirs beneath the volcano the overlying ground is left unsupported and collapses, creating a large topographic basin known as a caldera. As the caldera founders, its steep sides, formed so abruptly, are unstable and collapse inwards as a series of landslides. Tall volcanoes tend to collapse sideways in giant landslides, then grow and collapse again. Rain and meltwater also wears away volcanoes, forming lahars and floods, and choking drainage systems.

‘Lava’ focuses on lava. Within a kilometre or two of the ground surface, the rise of hot magma is accelerated by the growth of gas bubbles which increase the magma’s buoyancy. If the gases can readily escape from the magma, the magma may ascend more slowly without being ripped apart, and it eventually emerges from the volcano as a lava flow. There are many different types of lava flows: pahoehoe, a’a, block-lavas, inflated lavas, and lava domes, which decrepitate to form hazardous ‘block-and-ash flows’.

Volcanoes: A Very Short Introduction explains how volcanoes work and how volcanologists forensically decipher the processes involved. Volcanic eruptions are amongst the most dramatic expressions of the powerful tectonic forces at work in the Earth beneath our feet. However, volcanism encompasses much more than just volcanoes themselves. On a planetary scale, it is an indispensable heat release mechanism; shaping landscapes, releasing gases into the atmosphere, and allowing the conditions for life on Earth. Thus, this VSI also considers how volcanoes interact with other physical processes on the Earth, with life, and with human society.

Upwelling plumes from the deep mantle have an impact on the Earth's surface for tens to hundreds of millions of years. During the lifetime of a mantle plume, periodic fluctuations in its composition and temperature have the potential to generate changes in the nature and volume of surface volcanism. We constrain the spatial and temporal scale of compositional changes in a plume using high-resolution Pb isotopes, which identify chemical pulses emerging from the Canary Islands hotspot over the last 15 million years (Myr). Surface volcanism spanning similar to 400 km along the island chain changes composition systematically and synchronously, representing a replenishment of the plume head by a distinct mantle flavour on timescales of 3-5 Myr. These low-frequency compositional changes are also recorded by individual volcanoes, and comprise a sequence of closely-spaced isotopic trajectories. Each trajectory is maintained for similar to 1 Myr and is preceded and followed by similar to 0.3 Myr transitions to magmas with distinct isotope ratios. Relatively sharp transitions between periods of sustained isotopic stability require discrete yet coherent heterogeneities rising at speeds of similar to 100-200 km Myr(-1) and extending for similar to 150 km vertically in the conduit. The long-term synchronous changes require larger scale isotopic domains extending similar to 600 km vertically through in the plume stem. These observations demonstrate that plumes can chemically "pulse" over short and long-timescales reflecting the characteristics and recycling history of the deep mantle. (C) 2020 Elsevier B.V. All rights reserved.

Martian meteorite Northwest Africa (NWA) 8114 - a paired stone to NWA 7034 - provides an opportunity to examine the thermal history of a martian regolith and study near-surface processes and ancient environmental conditions near an impact crater on Mars. Our study reports petrographic and alteration textures and focuses on pyroxene and iron oxide grains. Some of the pyroxene clasts show exsolution lamellae, indicating a high temperature magmatic origin and slow cooling. However, transmission electron microscopy reveals that other predominantly pyroxene clasts are porous and have partially re-crystallised to form magnetite and a K-bearing feldspathic glassy material, together with relict pyroxene. This breakdown event was associated with oxidation, with up to 25% Fe3+/Sigma Fe in the relict pyroxene measured using Fe-K XANES. By comparison with previous studies, this breakdown and oxidation of pyroxene is most likely to be a result of impact shock heating, being held at a temperature above 700 degrees C for at least 7 days in an oxidising regolith environment. We report an approximate 40Ar-39Ar maximum age of 1.13-1.25 Ga for an individual, separated, augite clast. The disturbed nature of the spectra precludes precise age determination. In section, this clast is porous and contains iron oxide grains. This shows that it has undergone the high temperature partial breakdown seen in other relict pyroxene clasts, and has up to 25% Fe3+/Sigma Fe. We infer that the age corresponds to the impact shock heating event that led to the high temperature breakdown of many of the pyroxenes, after consolidation of the impact ejecta blanket. High temperatures, above 700 degrees C, may have been maintained for long enough to remobilise and congruently partially melt some of the alkali feldspar clasts to produce the feldspar veins and aureoles that crosscut, and in some cases surround, the oxidised pyroxene. However, the veins could alternatively be the result of a hydrothermal event in the impact regolith. A simple Fourier cooling model suggests that a regolith of at least five metres depth would be sufficient to maintain temperatures associated with the pyroxene breakdown for over seven days. Low temperature hydrous alteration took place forming goethite, identified via XRD, XANES and FTIR. Comparing with previous studies, the goethite is likely to be terrestrial alteration pseudomorphing martian pyrite. (C) 2018 The Authors. Published by Elsevier Ltd. Martian meteorite Northwest Africa (NWA) 8114 - a paired stone to NWA 7034 - provides an opportunity to examine the thermal history of a martian regolith and study near-surface processes and ancient environmental conditions near an impact crater on Mars. Our study reports petrographic and alteration textures and focuses on pyroxene and iron oxide grains. Some of the pyroxene clasts show exsolution lamellae, indicating a high temperature magmatic origin and slow cooling. However, transmission electron microscopy reveals that other predominantly pyroxene clasts are porous and have partially re-crystallised to form magnetite and a K-bearing feldspathic glassy material, together with relict pyroxene. This breakdown event was associated with oxidation, with up to 25% Fe3+/Sigma Fe in the relict pyroxene measured using Fe-K XANES. By comparison with previous studies, this breakdown and oxidation of pyroxene is most likely to be a result of impact shock heating, being held at a temperature above 700 degrees C for at least 7 days in an oxidising regolith environment. We report an approximate 40Ar-39Ar maximum age of 1.13-1.25 Ga for an individual, separated, augite clast. The disturbed nature of the spectra precludes precise age determination. In section, this clast is porous and contains iron oxide grains. This shows that it has undergone the high temperature partial breakdown seen in other relict pyroxene clasts, and has up to 25% Fe3+/Sigma Fe. We infer that the age corresponds to the impact shock heating event that led to the high temperature breakdown of many of the pyroxenes, after consolidation of the impact ejecta blanket. High temperatures, above 700 degrees C, may have been maintained for long enough to remobilise and congruently partially melt some of the alkali feldspar clasts to produce the feldspar veins and aureoles that crosscut, and in some cases surround, the oxidised pyroxene. However, the veins could alternatively be the result of a hydrothermal event in the impact regolith. A simple Fourier cooling model suggests that a regolith of at least five metres depth would be sufficient to maintain temperatures associated with the pyroxene breakdown for over seven days. Low temperature hydrous alteration took place forming goethite, identified via XRD, XANES and FTIR. Comparing with previous studies, the goethite is likely to be terrestrial alteration pseudomorphing martian pyrite. (C) 2018 The Authors. Published by Elsevier Ltd. Martian meteorite Northwest Africa (NWA) 8114 - a paired stone to NWA 7034 - provides an opportunity to examine the thermal history of a martian regolith and study near-surface processes and ancient environmental conditions near an impact crater on Mars. Our study reports petrographic and alteration textures and focuses on pyroxene and iron oxide grains. Some of the pyroxene clasts show exsolution lamellae, indicating a high temperature magmatic origin and slow cooling. However, transmission electron microscopy reveals that other predominantly pyroxene clasts are porous and have partially re-crystallised to form magnetite and a K-bearing feldspathic glassy material, together with relict pyroxene. This breakdown event was associated with oxidation, with up to 25% Fe3+/Sigma Fe in the relict pyroxene measured using Fe-K XANES. By comparison with previous studies, this breakdown and oxidation of pyroxene is most likely to be a result of impact shock heating, being held at a temperature above 700 degrees C for at least 7 days in an oxidising regolith environment. We report an approximate 40Ar-39Ar maximum age of 1.13-1.25 Ga for an individual, separated, augite clast. The disturbed nature of the spectra precludes precise age determination. In section, this clast is porous and contains iron oxide grains. This shows that it has undergone the high temperature partial breakdown seen in other relict pyroxene clasts, and has up to 25% Fe3+/Sigma Fe. We infer that the age corresponds to the impact shock heating event that led to the high temperature breakdown of many of the pyroxenes, after consolidation of the impact ejecta blanket. High temperatures, above 700 degrees C, may have been maintained for long enough to remobilise and congruently partially melt some of the alkali feldspar clasts to produce the feldspar veins and aureoles that crosscut, and in some cases surround, the oxidised pyroxene. However, the veins could alternatively be the result of a hydrothermal event in the impact regolith. A simple Fourier cooling model suggests that a regolith of at least five metres depth would be sufficient to maintain temperatures associated with the pyroxene breakdown for over seven days. Low temperature hydrous alteration took place forming goethite, identified via XRD, XANES and FTIR. Comparing with previous studies, the goethite is likely to be terrestrial alteration pseudomorphing martian pyrite. (C) 2018 The Authors. Published by Elsevier Ltd. Martian meteorite Northwest Africa (NWA) 8114 - a paired stone to NWA 7034 - provides an opportunity to examine the thermal history of a martian regolith and study near-surface processes and ancient environmental conditions near an impact crater on Mars. Our study reports petrographic and alteration textures and focuses on pyroxene and iron oxide grains. Some of the pyroxene clasts show exsolution lamellae, indicating a high temperature magmatic origin and slow cooling. However, transmission electron microscopy reveals that other predominantly pyroxene clasts are porous and have partially re-crystallised to form magnetite and a K-bearing feldspathic glassy material, together with relict pyroxene. This breakdown event was associated with oxidation, with up to 25% Fe3+/Sigma Fe in the relict pyroxene measured using Fe-K XANES. By comparison with previous studies, this breakdown and oxidation of pyroxene is most likely to be a result of impact shock heating, being held at a temperature above 700 degrees C for at least 7 days in an oxidising regolith environment. We report an approximate 40Ar-39Ar maximum age of 1.13-1.25 Ga for an individual, separated, augite clast. The disturbed nature of the spectra precludes precise age determination. In section, this clast is porous and contains iron oxide grains. This shows that it has undergone the high temperature partial breakdown seen in other relict pyroxene clasts, and has up to 25% Fe3+/Sigma Fe. We infer that the age corresponds to the impact shock heating event that led to the high temperature breakdown of many of the pyroxenes, after consolidation of the impact ejecta blanket. High temperatures, above 700 degrees C, may have been maintained for long enough to remobilise and congruently partially melt some of the alkali feldspar clasts to produce the feldspar veins and aureoles that crosscut, and in some cases surround, the oxidised pyroxene. However, the veins could alternatively be the result of a hydrothermal event in the impact regolith. A simple Fourier cooling model suggests that a regolith of at least five metres depth would be sufficient to maintain temperatures associated with the pyroxene breakdown for over seven days. Low temperature hydrous alteration took place forming goethite, identified via XRD, XANES and FTIR. Comparing with previous studies, the goethite is likely to be terrestrial alteration pseudomorphing martian pyrite. (C) 2018 The Authors. Published by Elsevier Ltd.