Style Of Volcanism Research Articles

Large volcanoes on Venus: Morphology, morphometry, and stratigraphy

We studied a revised and updated population of 93 large volcanoes on Venus using a combined topographic and morphologic analyses, which permits characterization of their general shapes and determination of their diameters. Our study has allowed the establishment of two distinctive types of the volcano shapes (complanate and cone-shaped) and stratigraphic ages of the volcanoes. The complanate volcanoes are significantly larger, the mean diameter (D) is ~500 km and have lower aspect ratio ((h/D)*1000 = ~2.4). The cone-shaped volcanoes are systematically smaller (D = ~150–300 km) and have higher aspect ratio, ~6. There is a pronounced gap in the size-frequency distribution (SFD) of the central volcanoes on Venus that separates the older and smaller edifices of shield plains (mean diameter of entire population is ~9 km) from the younger large volcanoes (mean diameter of entire population is ~280 km). Such a bimodality in the SFD suggests the existence of two different global volcanic environments during the formation of the older, and younger, populations of central volcanoes. The stratigraphy of the large volcanoes indicates that they represent the later stages of volcanic activity on Venus that characterized the end of the Guinevereian (the smaller cone-shaped volcanoes in association with the upper unit of regional plains, rp2) and the entire Atlian periods (the complanate and larger cone-shaped volcanoes in association with the younger lobate plains, pl). Transitional volcanoes that show both units are rare. The older rp2-volcanoes comprise only ~7 % of the total area of unit rp2. This suggests that the mode of formation of rp2 was significantly shifted toward the emplacement of areally distributed lava flows. In contrast, the pl-volcanoes make up ~40 % of lobate plains, which implies that the formation of volcanoes of the central type was an important component of volcanic style of lobate plains. The spatial association of most of the pl-volcanoes with the dome-shaped rises suggests that this change was related to the further focusing of central, large edifice volcanism over a few sites, perhaps representing a change in the nature of mantle source regions (e.g., more plume-like).

Oligocene to Miocene magmatic and hydrothermal evolution in the Maricunga belt (Región de Copiapó, Chile): A new look to a long known metallogenic belt

The Maricunga metallogenic belt, herein defined to encompass the region from 25° S to 28° S along the Cordillera Frontal of northern Chile, is known for hosting a series of gold-copper and lesser copper-molybdenum porphyries and gold-silver epithermal systems that are hosted in a segment of the late Oligocene-Miocene Andean calc-alkaline magmatic arc. Based on an up-to-date geochronological database, four episodes of magmatism and related hydrothermal deposits are proposed to characterize intensity and style of volcanism and hypabyssal intrusive rocks over time. The late Oligocene episode (26-21 Ma) is characterized by stratovolcanoes, dome complexes, pyroclastic sequences, and hypabyssal intrusives of andesitic to dacitic composition that host gold-rich porphyry systems such as Caspiche, Maricunga, and La Pepa. The early Miocene episode (21–17 Ma) has similar rock types but is volumetrically reduced, hosting epithermal and porphyry deposits like La Coipa and Caserones, respectively. A reinvigoration of volcanism occurred during the third stage between 17 and 11 Ma, giving rise to large volcanic complexes and intra-to extra-caldera ignimbritic deposits that host several gold-rich porphyry deposits such as Lobo-Marte, Luciano, and Cerro Casale; as well as epithermal deposits such as Fenix. Some of the 17 to 11 Ma and younger igneous rock belong to the high-K calc-alkaline series, showing a higher alkali content compared to previous periods. The last magmatic episode, from 11 to 5 Ma, coincides with the shallowing of the subduction angle and is characterized by an eastward migration of volcanism, and more heterogeneous rocks including trachytic compositions. During this period mineralization was emplaced at the Salares Norte epithermal deposit and El Volcan porphyry Au–Cu deposit.Porphyry and epithermal systems formed throughout the evolution of the Maricunga belt. Overprint of advanced argillic alteration on porphyry style alteration is widespread albeit commonly devoid of significant addition of epithermal Au–Ag mineralization. The Au-rich nature of the majority of the porphyry systems is attributed to the shallow crustal depths of emplacement compared to Cu- and Mo-rich porphyry systems. Conversely, high-sulfidation epithermal Au–Ag deposits such as La Coipa and Nueva Esperanza are not known to be telescoped on coeval porphyry gold deposits.The location of the mineralized centers was favored by crustal weaknesses expressed by intersection of oblique- and parallel-to arc structures, the former inherited from the construction of the continent before to the formation of the Andean range. The arc oblique structures also segment the Maricunga belt into domains with unique magmatic and hydrothermal styles.

Indications of volcanic style of the Silurian Nová Ves Volcano (Hemrovy Rocks)

Studied locality of Hemrovy Rocks belongs to Nová Ves Volcanic Center, one of the Silurian volcanic centers of Prague Basin of Teplá-Barrandien area (lower Wenlock – lower Ludlow age). The aim of this work is to describe details of the volcanic rock structures and to contribute to the interpretation of the Silurian volcanic style. The macroscopic and microscopic structures of basaltic rocks in and close to the abandoned Kační Quarry in the southern part of Prague and at the adjacent locality – ridge of Hemrovy Rocks were studied. At these sites, volcaniclastics dominate over solid volcanic rocks and sediments. The volcanic rocks are represented by basalts, which form thin lava flows overlying both the volcaniclastics and sedimentary rocks. Massive fine-grained basalts pass to pillow lava facies, autobreccia or hyaloclastite breccia in-situ. These brecciated structures evolve into peperites or to unconsolidated volcaniclastic rocks of a previous eruption. Coarse-grained volcaniclastics with oversized subrounded to angular clasts of vesiculated lava or sediments within glassy matrix, sporadically with sediment admixture, are unsorted and thick-bedded. They were probably formed by gravity-driven mass flow. Basalt in the Kační Quarry probably represents a synvolcanic intrusion, as documented by its hyaloclastite rim and presence of clusters of resedimented volcaniclastics in the adjacent fine-grained sediments. Matrix of the volcaniclastics is mostly unsorted, formed mostly by quenched fragments of altered palagonitized and chloritized basaltic glass, locally scoriaceous, and fragments of chilled basaltic lava. Originally highly porous volcaniclastics, resp. hyaloclastites were secondarily cemented by calcite and silica. Secondary chlorite, calcite and silica also fill vesicles and cavities in fragments of glass and lava clasts. Dense vesicularity of lava fragments in volcaniclastics indicates an effective explosive interaction of lava with water. It is probable that much of the wet volcaniclastic material repeatedly slumped back down into the volcano’s crater to be re-ejected by subsequent phreato-magmatic explosions. Hydroclastic fragmentation was iniciated by repeated subaquatic eruptions in shallow subphotic marine zone. Unsorted crinoidal packstones mostly with trilobites, trepostomate bryozoan and volcaniclastic matrix were deposited in the vicinity of the Nová Ves Volcanic Center.

Geology of the Imdr Regio area of Venus

ABSTRACT We present a 1:5,000,000 geological map of the Imdr Regio area of Venus. Geological mapping was conducted using synthetic aperture radar (SAR) images, altimetry and stereo-derived topography data from NASA's Magellan mission. The map covers an area of approximately 7.9 × 106 km2 and exhibits a variety of tectonic structures and units of volcanic origin related to the evolution of Imdr Regio and surrounding plains. We have differentiated primary structures related to the emplacement of the different units from tectonic structures that deform them. These structures are also organized between those that are regional in extent and those that are related to the evolution of local large tectono-volcanic structures. The units in the map area represent different geologic processes (e.g. volcanism) that took place during the evolution of the large topographic rise. Geologic mapping illustrates a complex evolution with different styles of deformation and volcanism in this part of the planet.

Dynamics of the December 2020 Ash‐Poor Plume Formed by Lava‐Water Interaction at the Summit of Kīlauea Volcano, Hawaiʻi

AbstractOn 20 December 2020, after more than 2 years of quiescence at Kīlauea Volcano, Hawaiʻi, renewed volcanic activity in the summit crater caused boiling of the water lake over a period of ∼90 min. The resulting water‐rich, electrified plume rose to 11–13 km above sea level, which is among the highest plumes on record for Kīlauea. Although conventional models would infer a high mass flux from explosive magma‐water interaction, the plume was not associated with an infrasound signal indicative of “explosive” activity, nor did it produce a measurable ash‐fall deposit. We use multisensor data to characterize lava‐water interaction and plume generation during this opening phase of the 2020–21 eruption. Satellite, weather radar, and eyewitness observations revealed that the plume was rich in water vapor and hydrometeors but transported less ash than expected from its maximum height. Volcanic lightning flashes detected by ground‐based cameras were confined to freezing altitudes of the upper cloud, suggesting that the ice formation drove the electrification of this plume. The low acoustic energy from lava‐water interaction points to a weakly explosive style of hydrovolcanism. Heat transfer calculations show that the lava to water heat flux was sufficient to boil the lake within 90 min. Limited mixing of lava and water inhibited major steam explosions and fine fragmentation. Results from one‐dimensional plume modeling suggest that the models may underpredict plume height due to overestimation of crosswind air‐entrainment. Our findings shed light on an unusual style of volcanism in which weakly explosive lava‐water interaction generated an outsized plume.

Architectural and Compositional Diversity of Early Earth Ocean Floor Evidenced by the Paleoarchean Nondweni Greenstone Belt, South Africa

Abstract The nature of the early Archean ocean floor remains a topic of important debate. There are relatively few well-preserved occurrences worldwide where such terrains may be studied in detail because of structural dismemberment, metamorphic overprinting and pervasive early stage hydrothermal alteration to recent weathering. The 3.41-Ga dominantly mafic formations of the Nondweni Greenstone Belt (NGB) covering 270 km2 in the south-eastern Kaapvaal Craton comprise submarine volcanics that exhibit a wide range of textural features, including pillows, chill zones and brecciated flow tops, and various spinifex textures, including the rare platy pyroxene type, cumulate layers, and tuffs. Channelized subaqueous lava lakes that underwent fractionation are capped by thick spinifex-textured units and pillows. Early stage seafloor alteration is regionally variable, ranging from intense to minimal, with preservation of original mineralogy in many areas. Mafic volcanic rocks of the NGB contrast with those of the Barberton Greenstone Belt both in the style of volcanism and in the associated compositional range of komatiitic basalt to basalt with a complete absence of high-Mg komatiites. Olivine-phyric rocks, or derivatives thereof, are largely absent and pyroxene is the main controlling phase with orthopyroxene in the most primitive komatiitic basalts and clinopyroxene in the evolved lava lake sequences. The abundance of orthopyroxene typifies the long-standing silica-enriched character of the Kaapvaal Craton. Three exceptionally well-preserved and well-exposed sequences were studied utilizing hand-drilled samples and deep coring providing unprecedented stratigraphic and textural detail and field controls for more than 400 samples. A unifying feature of the mafic volcanics of the NGB is the range of compositions and ratios of incompatible elements most clearly illustrated by a series of high- and low-Ti compositional lineages reflecting differing sources or degrees of mantle partial melting. Sharp boundaries between high- and low-Ti flow successions indicate sudden changes in the melting regimes or the interaction of flow sequences from different volcanic centres. Th/Nb ranges from 0.1 to 0.2 and reveals crustal contamination of primitive lavas. The primary magma that gave rise to the most primitive komatiitic basalts with 19.5% MgO was derived from partial melting of a mantle plume source in the garnet stability field. Trace element modeling shows that the sequences studied in detail have been modified by fractionation and crustal contamination with the most likely contaminant being the Ancient Gneiss Complex (3.43–3.66 Ga), which is extensively exposed in Eswatini and probably underlies the Paleoarchean terrains in the southern Kaapvaal Craton. The geotectonic setting was likely that of a submerged felsic crustal platform as enclaves within an oceanic plateau.

Pitted-Ground Volcanoes on Mercury

On the planet Mercury, pyroclastic deposits formed by explosive volcanism are developed around rimless volcanic pits that are up to dozens of kilometers in diameters. Some pyroclastic deposits on Mercury, however, host no discernable main eruption centers but feature pitted-ground terrains that each consists of many similar sized and irregularly shaped pits. Individual pits are usually much smaller and shallower than typical volcanoes on Mercury. The origin of these landforms is unknown, but it is indicative of styles of volcanism on Mercury and/or post-volcanic modifications. Here, we investigate the possible origin of these peculiar landforms based on their geological context, morphology, geometry, reflectance spectra, and geophysical background. Reflectance spectra of pyroclastic deposits around such volcanoes are comparable with those erupted from typical volcanic pits on Mercury, suggesting a genetic relation between these pitted-ground terrains with explosive volcanism, and the source magma might have similar compositions. Pitted-ground volcanoes are mainly observed in impact structures, and two cases were formed in high-reflectance smooth plains and channeled lava flows. Most pitted-ground volcanoes are relatively degraded compared with typical volcanoes on Mercury, and some might have been formed in geological recent times judged by both their pristine preservation and crosscutting relationship with impact rays. All pitted-ground volcanoes have unconfined morphology boundaries, and each case is composed by dozens of rimless pits that have similar preservation states and interconnected edges. Such morphological characteristics are unique among volcanic landforms on terrestrial bodies, and they cannot be explained by multiple post-eruption collapses of a main explosive volcano. Pitted-ground volcanoes that are developed in lava flows with the same age have different preservation states, suggesting that the pits were not formed by escape of thermally destabilized volatiles from substrate and subsequent roof collapses. The largest pitted-ground volcano (~3700 km2) is located on the Borealis Planitia, and Bouguer gravity data reveal no larger mass concentration in the subsurface than surrounding terrains, consistent with a paucity of shallow intrusions in the crust of Mercury. Short-term and spatially-clustered explosive eruptions could explain the peculiar morphology and geometry of the pits, suggesting that pits in a given pitted-ground volcano are akin to swarms of monogenetic volcanoes. However, possible magma dynamics for the formation of pitted-ground volcanoes cannot be confirmed until future high-resolution gravity mapping could reveal detailed interior structures beneath these volcanoes. Based on comparative studies with spatially-clustered and similarly aged volcanoes on Earth, we interpret that a combination of pervasive crustal fractures and regional thermal anomaly in the thin mantle of Mercury might have caused such short-term and spatially-clustered explosive eruptions. If this interpretation was true, the heavy degradation state of most pitted-ground volcanoes and the few well-preserved cases are consistent with an overall cooling trend of the mantle, indicating the existence of longstanding heterogeneous thermal structures in the mantle.

Olivine and carbonate-rich bedrock in Gusev crater and the Nili Fossae region of Mars may be altered ignimbrite deposits

Exposures of bedrock rich in olivine and carbonate link Gusev crater and the Nili Fossae region (NFR), both of which have the highest abundance of olivine yet identified on Mars. They are recognized as possible explosive volcanic tephra deposits, but the nature of their eruption and emplacement is poorly constrained, limiting understanding of what may be a widespread volcanic process on early Mars. The compositional and morphologic similarities of these widely separated olivine and carbonate-rich rocks have not been investigated previously. We examined orbital and in situ thermal infrared spectra and find notably similar compositions between the two locations, with olivine ranging from ~Fo53 to Fo75 and carbonates composed of multiple Mg and Fe-rich phases, although with minimal magnesite in the NFR. A range of morphologic and textural features occur in the deposits of explosive volcanism on Earth that can be used to constrain the origin of those on Mars. We observed features of the olivine-rich bedrock in both locations that resemble those of welded ignimbrite deposits on Earth that formed from pyroclastic density currents in cataclysmic explosive volcanic eruptions. If correct, an ignimbrite interpretation for the olivine-rich bedrock in the NFR and Gusev crater may apply to other occurrences on Mars and indicate a style of volcanism more common in its early history.

Widespread Hydration of the Back Arc and the Link to Variable Hydration of the Incoming Plate in the Lesser Antilles From Rayleigh Wave Imaging

AbstractSubduction zone dynamics are important for a better understanding of natural hazards, plate tectonics, and the evolution of the planet. Despite this, the factors dictating the location and style of volcanism are not well‐known. Here we present Rayleigh Wave imaging of the Lesser Antilles subduction zone using the ocean bottom and land seismic data collected as a part of the VoiLA experiment. This region is an important global endmember that represents a slow (<19 mm/yr) convergence rate of old (80–120 Ma), Atlantic lithosphere formed at a slow spreading ridge. We image the fast slab, the fast‐overriding plate and the slow mantle wedge across the entire arc. We find slow velocity anomalies (∼4.1 km/s) in the mantle wedge directly beneath the arc with local minima beneath Dominica/Martinique, Montserrat and the Grenadines. We observe that slow velocities in the wedge extend 200 km into the back arc west of Martinique. The slowest mantle wedge velocity anomaly is more muted than several global wedges, likely reflecting the lower temperatures and less partial melt predicted for the Antilles. Subducted fracture zones and plate boundaries are a potential source of hydration, since they are located near the anomalies, although not directly beneath them. To match our observations, geodynamic models with a broadly hydrated mantle wedge are required, which can be achieved via deep hydration of the slab, and fluid release further into the back arc. In addition, 3‐D flow and melt migration or ponding are required to explain the shape and location of our anomalies.

Geologic and Structural Evolution of the NE Lau Basin, Tonga: Morphotectonic Analysis and Classification of Structures Using Shallow Seismicity

The transition from subduction to transform motion along horizontal terminations of trenches is associated with tearing of the subducting slab and strike-slip tectonics in the overriding plate. One prominent example is the northern Tonga subduction zone, where abundant strike-slip faulting in the NE Lau back-arc basin is associated with transform motion along the northern plate boundary and asymmetric slab rollback. Here, we address the fundamental question: how does this subduction-transform motion influence the structural and magmatic evolution of the back-arc region? To answer this, we undertake the first comprehensive study of the geology and geodynamics of this region through analyses of morphotectonics (remote-predictive geologic mapping) and fault kinematics interpreted from ship-based multibeam bathymetry and Centroid-Moment Tensor data. Our results highlight two notable features of the NE Lau Basin: 1) the occurrence of widely distributed off-axis volcanism, in contrast to typical ridge-centered back-arc volcanism, and 2) fault kinematics dominated by shallow-crustal strike slip-faulting (rather than normal faulting) extending over ∼120 km from the transform boundary. The orientations of these strike-slip faults are consistent with reactivation of earlier-formed normal faults in a sinistral megashear zone. Notably, two distinct sets of Riedel megashears are identified, indicating a recent counter-clockwise rotation of part of the stress field in the back-arc region closest to the arc. Importantly, the Riedel structures identified in this study directly control the development of complex volcanic-compositional provinces, which are characterized by variably-oriented spreading centers, off-axis volcanic ridges, extensive lava flows, and point-source rear-arc volcanoes. This study adds to our understanding of the geologic and structural evolution of modern backarc systems, including the association between subduction-transform motions and the siting and style of seafloor volcanism.

Coronene, mercury, and biomarker data support a link between extinction magnitude and volcanic intensity in the Late Devonian

The Late Devonian biodiversity crisis, one of the big five Phanerozoic diversity depletions, is composed of a series of extinction events broadly coincident with the invasion of land by plants and vertebrates. These extinctions may have been triggered by volcanism, as sedimentary mercury enrichments are associated with the two main extinctions, the Kellwasser and Hangenberg crises. However, the sources of this mercury and its relation to volcanic style are not known. Furthermore, any links between the magnitudes of volcanic emissions and biodiversity loss have not been explored. To address these issues, we develop a new suite a geochemical data from shallow water sedimentary rocks in France, Belgium, and China for three Late Devonian crises – the Kellwasser and Hangenberg events, as well as for the preceding Frasnian extinction. We report paired coronene – mercury data as a refined proxy for large igneous province (LIP) emplacement, and we use cadalene and dibenzofuran biomarkers as proxies for terrestrial ecological disturbance. Coronene is a highly condensed six-ring polycyclic aromatic hydrocarbon (PAH), which requires significantly higher energy to form as compared to smaller PAHs; coronene enrichment has been found only in LIP volcanic emission and projectile impact events. Our data show that coronene and mercury coincidentally peaked at only the environmental and biotic events, demonstrating that pulses of LIP volcanism are tightly associated with the Late Devonian crises. In addition, mercury concentration is highly correlated with cadalene and dibenzofuran abundance during the events only in nearshore settings, indicating fluxes of soil erosion. Finally, we observe a correlation between coronene index and extinction magnitude for the three events, suggesting a link between temperatures of sills and lavas during LIP emplacement, pressure of evolved volatiles, and global climatic effects.

Spatiotemporal variations in eruption style and magnitude at Yasur volcano, Vanuatu: part 2—extending Strombolian eruption classifications

In this companion study to Simons et al. (Bull Volcanol 82, 2020a), we examine Strombolian style explosive activity at Yasur (Vanuatu) over 11 weeks via seismic, thermal-infrared and visual observations. In part 1 of this study (Simons et al., Bull Volcanol 82, 2020a), we investigated the link between variations in the surface expression and style of volcanism at Yasur in relation to vent, shallow conduit and shallow magma reservoir processes. Now, in part 2, based on the database of 4998 explosions, we describe, define and classify Yasur’s Strombolian eruption styles and magnitudes, and compare and contrast to Stromboli and other similar centres. The styles of most of Yasur’s explosions fit with other known Strombolian eruptions, but they are 2–3 times more frequent and with higher average eruption heights than at Stromboli. Observed activity at Yasur also produced more ash than at Stromboli. The most powerful eruptive periods are dominated by type 2a explosions (ballistics and ash) and type 2b (ash-dominated) explosions. A new type of Strombolian style explosion was observed at Yasur and is here termed a type 3 explosion. They occur when vents are buried by a thick cap of loose, poorly sorted pyroclastic breccia. Type 3 explosions begin as a series of emergent ash-rich jets that rupture and fountain from the breccia cover, generating prolonged jetting of ash to form vigorous low plumes. The loose breccia cover apparently dissipates the violent early phase of the explosion. The greatest hazard of type comes from their occurrence with little warning from a smooth crater floor without an apparent vent and after longer than typical intervals between eruptions. The findings extend the observed range of Strombolian explosion styles and imply that surficial vent cover and burial may influence gas slug stability and contribute to variations in ash and ballistic output.

Eruptive chronology of monogenetic volcanoes northwestern of Morelia – Insights into volcano-tectonic interactions in the central-eastern Michoacán-Guanajuato Volcanic Field, México

Twelve monogenetic volcanoes formed during the last 7 Ma within the Tarímbaro graben, northwest of the city of Morelia, in the central-eastern part of the Michoacán-Guanajuato volcanic field (México). These include four scoria cones (Pelón, Tetillas, Jamanal, and Parastaco), four lava domes (La Cruz, Divisadero, Estadio, and Tetillas), two small shield volcanoes (Quinceo and Tetillas) and two lava flows (Cuto and Cerritos). Based on a detailed geological map and stratigraphy aided by new 40Ar/39Ar and 14C radiometric dates, and whole-rock chemical analyses, we established the eruptive chronology of these monogenetic volcanoes. These volcanoes were built upon four early Miocene successions of regional volcanism named Cuitzeo lavas (18.7 Ma), Cuitzeo ignimbrite (17.4 Ma), Atécuaro ignimbrite (16.8 Ma), and Punhuato lavas (16.3 Ma). Late Miocene to Pliocene activity consisted of the La Cruz, Divisadero, and Estadio domes and the Cuto lava flow (6.7–3.1 Ma). Pleistocene activity included the Cuitzeo fallout pyroclastic sequence (1.48 Ma), the Quinceo small shield volcano (1.36 Ma), the Pelón scoria cone (0.84 Ma), the Tetillas small shield volcano, two lava domes and a scoria cone (0.56–0.34 Ma), the Jamanal and Parastaco scoria cones and the Cerritos lava flow (0.11 Ma). Magma volumes from these volcanoes vary from <0.1 to 4 km3. We ascribe their variety of magma volumes, morphology, volcanic styles, and spatio-temporal distribution to tectonic changes and evolution of the ENE Tarímbaro graben, whose faults belong to the Tarímbaro-Álvaro Obregón fault segment in the central part of the Morelia-Acambay Fault System. Quinceo (2 km3) and Tetillas (4 km3) small shield volcanoes are by far the most voluminous centers dominating the landscape of the city of Morelia. All rock samples have compositions varying from 49.3 to 61.6 wt % in silica with medium-K contents (0.43–1.83 wt %).

SHARAD mapping of Arsia Mons caldera

Arsia Mons – the southernmost of the Tharsis shield volcanoes – has the largest diameter single summit caldera on Mars indicating the presence of a single large magma chamber underneath. The subsidence at the summit, caused by emptying of this large chamber, followed by rift-fed volcanism on the caldera floor not observed in the other Tharsis shields point to longer duration activity and higher degree of structural evolution for Arsia Mons when compared to the other Tharsis Montes (Ascraeus Mons and Pavonis Mons). At the summit, possible ash deposits near the caldera rim, <10 km away from ~130 Myr young lava flows on the caldera floor, have been identified. The eruption history of Arsia Mons is not well-known, especially the question of whether the shield volcano underwent periods of explosive style of eruption in between prolonged effusive activity. In this study, we use orbital sounding radar data from SHAllow RADar (SHARAD) to investigate the volcanic stratigraphy within the caldera. SHARAD has been used extensively in assessing the near-subsurface geology on Mars, including volcanic provinces like Elysium and Tharsis where SHARAD has been successful in detecting buried lava flows. Using a similar approach, we use SHARAD backscatter measurements from the subsurface to map dielectric horizons. These dielectric boundaries are caused by an abrupt change in the density of the subsurface unit, thereby allowing us to delineate different lithologies. We find groups of reflectors at two locations within the caldera – west and south – with different radar backscatter characteristics. Using these distinct properties as input, we perform inverse modeling of the one-dimensional signal propagation equations to obtain the dielectric permittivity and loss tangent of the subsurface layers. We use a Markov Chain Monte Carlo approach to determine the probability distribution of these two variables that could give rise to the observed SHARAD data. The results show spatially diverse dielectric structures likely caused by spatial and temporal differences in the style of volcanism. We argue that explosive basaltic eruptions in the caldera, followed by mostly effusive post-caldera volcanism, could explain the unusual distribution of dielectric properties of the reflectors close to the caldera wall. We also examine possible causes for the uneven spatial distribution of layered radar interfaces by incorporating analysis of thermal inertia observations. This provides further insight into preferential detection of subsurface reflectors by SHARAD in volcanic units.

Using microlites to gain insights into ascent conditions of differing styles of volcanism at Soufrière Hills Volcano

Microlite textural characteristics are analyzed in the products of Vulcanian explosions, ash venting occurring synchronously with lava effusion (syn-extrusive ash venting), a precursory explosion, and lava dome effusion associated with Phases 3, and 5 of the current Soufrière Hills Volcano (SHV) eruption. 2D microlite population statistics and crystal size distributions are used to infer decompression pathways for each style of volcanism. In addition, magma ascent rates for each eruptive style were calculated using microlite number density (Toramaru et al. 2008).Strong differences between pyroclastic and effusive microlites are observed in 2D population statistics and crystal size distributions, indicating higher amounts of smaller microlites (<0.0054 mm) in the lava dome compared with pyroclastic products. Crystal size distributions show that the lava dome and Vulcanian explosion samples have similar amounts of large microlites (>0.0084 to 0.012 mm). However, syn-extrusive ash venting samples have comparatively greater amounts of large microlites. The similarity between lava dome and Vulcanian explosions at larger microlite sizes suggests comparable crystallization conditions in the deeper conduit system (>2 km depth). The contrast in eruption style despite similarity at depth suggests a shallow control to eruption processes at SHV. The greater amounts of large microlites in syn-extrusive ash venting compared to the lava dome indicates two distinct decompression pathways within a single conduit, inferred to result from differences in ascent rate. Sluggish conduit margin ascent leading to lower decompression rates is inferred to produce the increased degree of crystal growth observed in syn-extrusive ash venting compared with the lava dome.Calculated magma ascent rates for Vulcanian explosions and syn-extrusive ash venting have similar ranges, while lava effusion is up to a factor of 10 faster. Differences in magma ascent rate are inferred to result from variations in the depth at which ascent rate is calculated for each eruptive style. Ascent rate is calculated at microlite nucleation depth using the method of Toramaru et al. (2008). However, the average microlite nucleation depth will vary because of differences in microlite nucleation rate during ascent. More small microlites in the lava dome suggest greater nucleation rates in the shallow conduit, recording an on average shallower magma ascent rate (<1 km depth). Rapid decompression during pyroclastic eruptions restricts shallow crystallization, suggesting the microlite population is on average recording the deeper conduit system (>2 km depth).

Rhyolite Domes and Subsequent Offlap of Pliocene Carbonates on Volcanic Islets at San Basilio (Baja California Sur, Mexico)

San Basilio basin in Baja California Sur (Mexico) exhibits distinct styles of volcanism that interrupted phases of normal sedimentation correlated with the Zanclean Stage (Lower Pliocene). Sea cliffs around a 4-km2 bay opening onto the Gulf of California are dominated by rhyolite, mudstone, sandstone, and limestone. Volcanism associated with re-sedimented hyaloclastite is regionally uncommon and the goal was to investigate interactions between volcanic events and intervals of stability represented by fossil-rich strata. Methods of study involved a combination of microfossil and macrofossil analyses. Relating the basin’s faults to Pliocene development in the greater Gulf of California was a secondary goal. Microfossils Bolivina bicostata and B. interjuncta recovered from mudstone indicate an initial water column of 150 m. An abrupt hydromagmatic explosion ruptured the mudstone cover, followed by banded rhyolite flows inter-bedded with sandstone. Outlying limestone beds with the index fossil Clypeaster bowersi are separated from rhyolite by conglomerate eroded under intertidal conditions. A renewed phase of activity saw eruption of smaller volcanoes in the basin center semi-contemporaneous with pecten limestone deposited on unstable slopes. Normal faults conform to a pattern of extensional rifting in the proto-gulf, followed by cross-cutting faults indicating the onset of transtensional tectonics beginning about 3.5 Ma.

Comparison of lake and land tephra records from the 2015 eruption of Calbuco volcano, Chile

Tephra layers in lake sediment cores are regularly used for tephrostratigraphy as isochronous features for dating and recording eruption frequencies. However, their value for determining volcanic eruption size and style may be complicated by processes occurring in the lake that modify the thickness and grain size distributions of the deposit. To assess the reliability of data from lake cores, we compare tephra deposited on land during the 2015 eruption of Calbuco volcano in Chile to records in sediment cores from three lakes of different sizes that are known to have received primary fall deposits. In general, the thickness and granulometry of the deposit in lake cores and nearby terrestrial sections are very similar. As anticipated, however, cores sampled close to (here, within 300 m of) fluvial inflows were affected by sediment deposition from the lake’s catchment; they differed from primary deposits not only in their greater thickness and organic content but also in poor sorting and lack of grading. Cores 850 m away from the inlet were not affected. We consider our results in the context of the particle settling regime as well as each lake’s location, bathymetry and catchment area. We find that the particle settling regime is important in more distal settings where the ash particles are small and particle settling occurs in density plumes rather than as individual particles. We conclude that lake cores can be useful for physical volcanology providing consideration is given to eruption parameters such as particle size and mass flux, as well as lake features such as bathymetry and catchment area.

Volcanic Geosites and Their Geoheritage Values Preserved in Monogenetic Neogene Volcanic Field, Bahariya Depression, Western Desert, Egypt: Implication for Climatic Change-Controlling Volcanic Eruption

Bahariya monogenetic volcanic field is characterized by important geomorphological features (geomorphosites), namely, sub-circular maar-tuff ring, scoria cones, and domal-shaped tumuli. These geomorphosites constitute an asset for geoeducation, geotourism and miscellaneous social activities. They offer important knowledge into the paleoenvironmental and climatic factors that affected the style of volcanism at the occasion, and eventually shaped the diverse landforms found in the volcanic field. Bahariya Oasis is exclusive for its excellent locations where many volcanic heritages of high value give evidence of phreatomagmatic and effusive-controlled phases which formed volcanic landscapes under humid to dry climate. The geoheritage and archeological sites of early settlements are abundant in the Bahariya Oasis, accentuating the scientific magnitude of this region. There have been seven geosites recognized such as (1) the scoria cone, (2) the lava flows and their surface morphological features, (3) the pseudopillow fractures, (4) columnar joints, (5) peperites, (6) tumuli, and (7) rootless cones. These geosites coupled with other unique sites define the Oasis as global geopark. The latter will consider as an excellent logistical network to endorse volcanic geosciences and raise the economic growth in this part of Bahariya Oasis. The diverse geological characteristics at the Bahariya make this area a high volcanic geodiversity that can be used for geoeducational programs and geotourism. Excursions and research programs carried out by universities will contribute to enhanced geoconservation for local sustainable development. Currently, in the Bahariya region, tourism is not well developed, but it is recommended that, roads be improved to give better accessibility to the geomorphosites, and interpretative panels, informative brochures, multi-media presentations, seminars and workshops, scientific lectures, and postcards be produced to inform tourists about the geology of the region.

The Global Distribution of Active Ionian Volcanoes and Implications for Tidal Heating Models

Abstract Tidal heating is the major source of heat in the outer solar system. Because of its strong tidal interaction with Jupiter and the other Galilean satellites, Io is incredibly volcanically active. We use the directly measured volcanic activity level of Io’s volcanoes as a proxy for surface heat flow and compare it to tidal heating model predictions. Volcanic activity is a better proxy for heat flow than simply the locations of volcanic constructs. We determine the volcanic activity level using three data sets: the Galileo Photopolarimeter Radiometer (PPR), Galileo Near-Infrared Mapping Spectrometer (NIMS), and New Horizons LEISA. We also present a systematic reanalysis of the Galileo NIMS observations to determine the 3.5 μm brightness of 51 active volcanoes. We find that potential differences in volcanic style between high and low latitudes make high-latitude observations unreliable for distinguishing between tidal heating models. Observations of Io’s polar areas, such as those by Juno, are necessary to unambiguously understand Io’s heat flow. However, all three of the data sets examined show a relative dearth of volcanic brightness near 180 W (anti-Jovian point) and the equator, and the only data set with good observations of the sub-Jovian point (LEISA) also shows a lack of volcanic brightness in that region. These observations are more consistent with the mantle-heating model than the asthenospheric-heating model. Furthermore, all three of the data sets are consistent with fourfold symmetry in longitude and peak heat flow at mid-latitudes, which best matches with the combined heating case of Tackley et al.

Does large igneous province volcanism always perturb the mercury cycle? Comparing the records of Oceanic Anoxic Event 2 and the end-Cretaceous to other Mesozoic events

Mercury (Hg) is increasingly being used as a sedimentary tracer of Large Igneous Province (LIP) volcanism, and supports hypotheses of a coincidence between the formation of several LIPs and episodes of mass extinction and major environmental perturbation. However, numerous important questions remain to be answered before Hg can be claimed as an unequivocal fingerprint of LIP volcanism, as well as an understanding of why some sedimentary records document clear Hg enrichment signals whilst others do not. Of particular importance is evaluating the impact of different volcanic styles on the global mercury cycle, as well as the role played by depositional processes in recording global Hg-cycle perturbations. Here, new mercury records of Cretaceous Oceanic Anoxic Event 2 (OAE 2: ∼94 Ma) and the latest Cretaceous (∼67–66.0 Ma) are presented. OAE 2 is associated with the emplacement of multiple, predominantly submarine, LIPs; the latest Cretaceous with subaerial volcanism of the Deccan Traps. Both of these connections are strongly supported by previously published trends towards unradiogenic osmium- (Os) isotope values in globally distributed sedimentary records. Hg data from both events show considerable variation between different locations, attributed to the effectiveness of different sediment types in registering the Hg signal, with lithologically homogeneous records documenting more clear Hg enrichments than sections with major changes in lithology such as limestones to claystones or organic-rich shales. Crucially, there is no geographically consistent signal of sedimentary Hg enrichment in stratigraphic records of either OAE 2 or the latest Cretaceous that matches Os-isotope evidence for LIP emplacement, indicating that volcanism did not cause a global Hg perturbation throughout the entire eruptive history of the LIPs formed at those times. It is suggested that the discrepancy between Os-isotope and Hg trends in records of OAE 2 is caused by the limited dispersal range of Hg emitted from submarine volcanoes compared to the global-scale distribution of Os. A similar lack of correlation between these two proxies in uppermost Cretaceous strata indicates that, although subaerial volcanism can perturb the global Hg cycle, not all subaerial eruptions will do so. These results highlight the variable impact of different volcanogenic processes on the efficiency of Hg dispersal across the globe. Factors that could influence the impact of LIP eruptions on the global mercury cycle include submarine versus subaerial volcanism, volcanic intensity or explosivity, and the potential contribution of thermogenic mercury from reactions between ascending magma and surrounding organic-rich sediments.