Clastogenic Lava Research Articles

Geomorphological Classification of Monogenetic Volcanoes and Its Implication to Tectonic Stress Orientation in the Middle Atlas Volcanic Field (Morocco)

The Middle Atlas Volcanic Field (MAVF) covers an area of 1500 km2, with a total erupted volume of solid products (e.g., Dense Rock Equivalent or DRE) estimated to be more than 80 km3. The MAVF comprises 87 monogenetic basaltic volcanoes of Tertiary-Quaternary age as scoria cones (71%) and maars (29%). These monogenetic basaltic volcanoes have various morphologies (e.g., circular, semi-elliptic, elliptic in map views). They can be isolated or form clustered monogenetic complexes. They are largely grouped in the Middle Atlas, in an intraplate geotectonic context forming two distinct major alignments (N160–170° and N40–50°), each closely associated with regional structural elements. By the best estimates, the preserved bulk pyroclastic products do not exceed 0.7 km3, and they show large textural and componentry diversity (e.g., bedded/unbedded, coarse/fine, dense/scoriaceous fallout and pyroclastic density current deposit, etc.). Lava flows also demonstrate great variety of preserved surface textures, including pāhoehoe, ‘a’ā, and clastogenic types. Morphostructural features of lava flows linked to lava flow dynamics have also been recognized, and the presence of hornitos, columnar jointed basaltic flow units, lava tubes, tumuli, and clastogenic lava flows have been recognized and mapped. Some half-sectioned dykes expose interior parts of magmatic shallow feeding pipes. The current morphology of the volcanoes of the MAVF reflects various syn- and post-eruptive processes, including (1) erosional features due to weathering, (2) gravitational instability during and after volcanic activity, (3) vegetation impact, and (4) successive burial of lava flows. The documented volcanic features of this typical monogenetic volcanic field form the core of the region’s geoheritage elements and are considered to be unique in the new African geoheritage context. Hence, they will likely form the basis of future geotourism, geoeducation, and geoconservation ventures.

The interplay between environmental and magmatic conditions in eruption style transitions within a fissure-aligned monogenetic volcanic system of Auckland, New Zealand

Recent conceptual geological frameworks of continental monogenetic volcanism highlight that the small magma volume eruptions, resulting volcanic geology and edifice architecture in such settings are sensitive to variations in external or environmental conditions. These conditions, along with fluctuations in magma flux, can change rapidly over short time frames and cause dramatic changes in eruption style. Understanding the drivers of transitions in explosive to effusive behaviour within the short timescales of eruptions at individual volcanic centres is essential to accurately assess volcanic hazards in continental monogenetic settings. Wiri Mountain Volcanic Complex is one of the largest and most complex volcanic centres in the active Auckland Volcanic Field. Despite the significant removal of much of the original volcanic deposits, present-day exposures and historical images provide a unique opportunity to examine the growth and evolution of the volcanic complex. Wiri Mountain deposited an initial basal tuff ring (covering an area of approximately 0.67 km2) by predominantly pyroclastic density currents, followed by at least two smaller tuff rings erupted through the outer flanks of the first, in a transition from phreatomagmatic to Strombolian eruptive style. A 90 m high central scoria cone was then produced within the initial tuff ring, partially capped by lava spatter, clastogenic lava flows and lava flows that mostly covered all tuff rings, the scoria cone, and the surrounding area. A high-resolution stratigraphic study of the well-exposed tuff ring to capping magmatic succession was conducted to determine the changes in eruptive style and their driving forces. The deposit architecture of Wiri Mountain can be described using three volcanic stratigraphic units: a basal unit comprised of tuff, lapilli tuff and tuff breccia deposits, a middle unit comprised of juvenile-rich transitional tuff deposits of black scoria ash and lapilli, red scoria and spatter, and a capping unit comprised of agglutinate and lava flow successions. Most volcanic materials were either fragmented and ejected at near-optimal scaled depth, or the transition between near-optimal and shallower/deeper depths. Wiri Mountain provides a striking example of a fissure eruption likely controlled by a pre-existing tectonic fault that was fed by relatively stable melt sources over a sustained period. We infer that despite eruption within a water-saturated coastal plain, the initial phreatomagmatic phase was overridden by subsequent explosive and effusive magmatic phases through the formation of an increasingly established conduit, thus allowing sustained magma flux and melt supply. The transitions, both gradual and rapid, from an initial phreatomagmatic to subsequent magmatic explosive and effusive phases fit well to the general understanding of eruption style changes over time from other larger and more complex volcanoes of Auckland, and elsewhere worldwide. Wiri Mountain showcases the fine balance between the external and internal conditions that control eruption style variations and govern the formation of complex monogenetic volcanoes.

Inflated pyroclasts in proximal fallout deposits reveal abrupt transitions in eruption behaviour

During explosive eruption of low viscosity magmas, pyroclasts are cooled predominantly by forced convection. Depending on the cooling efficiency relative to other timescales, a spectrum of deposits can be formed. Deposition of hot clasts, above their glass transition temperature, can form spatter mounds, ramparts and clastogenic lava flows. Clasts may also be deposited cold, producing tephra cones and blankets. Thus, the deposit and pyroclast type can provide information about eruption dynamics and magma properties. Here we examine pyroclasts from Tseax volcano, British Columbia, Canada. These newly identified inflated pyroclasts, are fluidal in form, have undergone post-depositional expansion, and are found juxtaposed with scoria. Detailed field, chemical and textural observations, coupled with high temperature rheometry and thermal modelling, reveal that abrupt transitions in eruptive behaviour — from lava fountaining to low-energy bubble bursts — created these pyroclastic deposits. These findings should help identify transitions in eruptive behaviour at other mafic volcanoes worldwide.

Apparently ‘dry’ littoral rootless cones in Hawaiʻi formed by sustained, “confined” mixing of lava and sea water

Hydrovolcanic rootless cones form through the explosive interaction between lava and external water at locations remote from primary vents. Littoral rootless cones such as those on the coast of Hawaiʻi form at the ocean entries of basaltic lava flows and most consist dominantly of thinly bedded ash and lapilli. In contrast, the littoral rootless cones at ʻAuʻau Point and South Puʻu Kī Cone on the southwestern flank of Mauna Loa, Hawaiʻi consist of alternating layers of clastogenic lavas and thermally oxidised, tack- to moderately welded lapilli and coarse bombs with little to no ash. The pyroclastic layers have characteristics traditionally thought to be indicative of ‘dry’ explosive magmatic activity at primary vents, including the high degree of sorting in fall deposits, the paucity of ash and quench textures, extensive thermal oxidation, a high degree of welding, dominantly fluidal pyroclastic morphologies and association with clastogenic lavas. We propose that littoral rootless activity at both cones was driven by the “confined mixing” of tube-fed lava and sea water. In this configuration, the mingling of water and lava is complicated, resulting in relatively inefficient Molten Fuel Coolant Interactions (MFCIs). Cycles of more vigorous (littoral lava fountains) and less vigorous (lava bubble bursts) activity produced the alternating layers of clastogenic lava and welded bombs and lapilli. These cycles probably reflect the local depletion and replenishment of sea water injected into the lava tube. Similar apparently ‘dry’ facies found in the upper sequences of Icelandic rootless cones formed when the water content of the underlying wet sediment was depleted and MFCIs were relatively inefficient. This study suggests that both “confined mixing” of tube-fed lava and external water, and lava–water interactions with unfavourable lava–water mass ratios, such as during the late stages of formation of Icelandic rootless cones, can produce seemingly ‘dry’ deposits that nevertheless are fundamentally hydrovolcanic.

Eruptive history of La Poruña scoria cone, Central Andes, Northern Chile

New stratigraphic, lithological and petrographic analyses of La Poruna scoria cone (21° 53′ S-68° 30′ W, Central Andes, northern Chile) allow the reconstruction of the eruptive sequence of this monogenetic cone. Petrographic and lithological characteristics allow us to identify three main lithostratigraphic units at La Poruna scoria cone. The first unit consists of agglutinated lapilli, spatter beds, and clastogenic lavas that are related to the construction of the cone. The other two units are associated with a lava flow field and consist of flow of andesitic composition, which differ both in their degree of weathering and in their development of channel, surface, and internal structures (e.g., levees, ogives, and joints). With these lithostratigraphic analyses, we interpret that the construction of La Poruna occurred during four main eruption phases involving Strombolian, Hawaiian, and transitional eruptive styles. Furthermore, differences in the degree of erosion, alteration, and weathering of the lithostratigraphic units in the lava field of La Poruna suggest that this flow field was formed during two eruptive events. The excellent outcrop conditions and preservation state of the volcanic products of La Poruna allow new stratigraphic insights that advance the wider and more general understanding and the dynamics of this important type of volcanism and the potential hazards of a scoria cone eruption. The polycyclic style of the eruption needs to be included in the hazards assessment of these centers type, especially when the cone is associated with structures that can be reactivated. This process could correspond to a second phase of activity, involving ash fall, bomb, and lava emission.

Late-stage groundmass differentiation as a record of magma stagnation, fragmentation, and rewelding

In the 2011 eruption of Shinmoedake of the Kirishima volcano group, sub-Plinian eruptions were followed by lava extrusion with intermittent Vulcanian explosions. The interstices of microlites and relatively large nanolites (> 0.4 nm width) in the groundmass of pyroclasts (“groundmass interstices”) were studied to reveal shallow magmatic processes that lead to different eruption styles. The pumice of the sub-Plinian eruption has the least differentiated groundmass interstices composition. The groundmass interstices of the dense juvenile fragments of the Vulcanian explosions are divided into two groups. The first group is the most differentiated as expected from their lava-like texture, whereas the second group is less differentiated and covers the range of sub-Plinian pumice. The Vulcanian pumice overlaps the dense juvenile fragments despite their high vesicularity. This seemingly contradictory relationship of composition in the interstitial groundmass indicates the clastogenic lava origin of the less-differentiated group of dense juvenile fragments. In contrast, magmas stagnated in the shallow conduit maintained elevated temperature and water content, allowing groundmass interstices to differentiate by microlite growth. These magmas then erupted as pumice in the Vulcanian explosions and were effused as lava that was fragmented by subsequent explosions to be later sampled as a dense juvenile fragment. The groundmass crystallinity increased by 9.1 vol.% in 5–45 days between the sub-Plinian and Vulcanian eruptions, increasing initial melt and magma viscosity from 106.1–7.4 and 107.0–8.3 Pa s to 106.9–8.4 and 108.2–9.7 Pa s, respectively. This viscosity increase by nanolite crystallization could have facilitated the stress fragmentation of conduit magma, leading to Vulcanian explosions. Post-fragmentation expansion of the sub-Plinian pumice could have been suppressed by this viscosity increase, resulting in their low vesicularity. Late-stage groundmass differentiation can thus control shallow magmatic processes.

Late Pleistocene to present day eruptive history of the Changbaishan-Tianchi Volcano, China/DPRK: New field, geochronological and chemical constraints

Volcanic unrest during 2002–2006 renewed attention on Changbaishan-Tianchi volcano as the source of the powerful explosive eruption (VEI 6 to 7) Millennium Eruption of 946 CE (Common Era). However, progress in understanding the potential risk is complicated by discordant accounts of the most recent, explosive eruptive history. Several eruptions since the Late Pleistocene, such as the Tianwenfeng (TWF), Qixiangzhan (QXZ), Millennium (ME), Baguamiao (BGM), Liuhaojie (LHJ), 1403 CE event (1403), and even a 1903 CE event have been proposed, but the stratigraphic order, ages, distributions, and even the very existence of some of these, are still not fully ratified. Based on recent detailed stratigraphic logs of over 100 surface exposures and three deep trenches, coupled with new geochronologic and petrochemical data (including trace elements on glass and pyroxene), we can now clarify these relationships.Herein we propose that the 946 CE Millennium eruption, be formally re-named the Generalized Millennium Eruption (GME) to avoid entrenched confusion and to formally acknowledge the composite comendite-trachyte nature of the eruption. Thus three eruptions, the TWF, QXZ and the GME are confirmed. The TWF eruption is the first powerful explosive eruption at Changbaishan-Tianchi and we find evidence that this eruption may have initiated the Tianchi caldera. It produced an extensive fallout pumice deposit, that on the basis of new Optically Stimulated Luminescence (OSL) and radiocarbon (14C ages) of overlying sedimentary strata indicates an eruption age >42 ka. This new age constraint coupled with geochemical fingerprints confirms a correlation to the ~50.6 kyr B-J ash in Japan Sea. The next eruption was the predominately effusive QXZ that produced a clastogenic lava flow down the north slope of the edifice. An age of ~4–8 ka is proposed based on new radiocarbon (14C) ages of charcoal at the base and from overlying sediment. Although the age of the GME is well established as late 946 CE, there are some intriguing details in historical records of ash falls in Japan that indicate two separate events on Nov 3, 946 CE and Feb 7, 947 CE. We speculate that these correspond to the comendite and trachyte stages of the GME respectively. Subsequent to the GME, no evidence of any post-GME eruptions have been found. These findings clarify and refine the history of the explosive stage of the Changbaishan-Tianchi volcano that began ~51 kyr. This improved understanding should serve as a basis for further studies at Changbaishan-Tianchi and inform a better-constrained assessment of volcano hazard.

Intricate episodic growth of a Hawaiian tephra deposit: case study of the 1959 Kīlauea Iki eruption

The 1959 Kīlauea Iki eruption on Hawai’i generated a succession of fountains, most reaching hundreds of meters high. The 16 episodes of fountaining persisted intermittently (with repose periods of between 7 h and 4 days) for 36 days. They produced tephra deposits that were dispersed several kilometers downwind of the vent, and a much larger volume of clastogenic lava which drained into the Kīlauea Iki crater to form a > 100 m deep lava lake. Field data from 211 tephra sample pits downwind of the vent reflect imperfectly the episodic nature of the fountaining behavior: only five composite stratigraphic subunits from the total of 16 fountaining episodes can be mapped in the field. However, isopach maps of these subunits were generated and, by the application of empirical deposit thinning relationships, volumes of each subunit were estimated. In combination with detailed observations made in 1959, our field data allow us to assign stratigraphic subunits to either single or aggregates of several fountaining episodes. The most voluminous subunits are linked with the highest fountaining, not with the longest in duration. In fact, significant downwind dispersal was possible only if the top part of the fountains reached above the crater rim, which was located about 100 m above the vent. The end of high fountaining episodes produced ash-rich partings, which are found on top of the lapilli-sized products of some episodes. However, the ash-rich intervals are strongly attenuated by wind advection and so are only present along the dispersal axes. The subunits follow slightly different dispersal axes. The maximum spread of 28 degrees between various dispersal axes is predominantly controlled by changing wind directions and spatter cone collapse into the vent, which modified its geometry. This study outlines the dependency of tephra dispersal on fountain height and emphasizes the capability of an episodic Hawaiian fountaining eruption to generate a seemingly monotonous downwind tephra deposit.

Tephra From the 3 March 2015 Sustained Column Related to Explosive Lava Fountain Activity at Volcán Villarrica (Chile)

Villarrica Volcano in the Southern Andes of Chile has sustained persistent open conduit activity, associated with a dynamic lava lake since at least 1984-85. Increased seismic activity and degassing in August 2014 was followed by increased Strombolian activity starting in February 2015 and culminated with a 1.5 km-high lava fountain on 3 March 2015. This eruption produced tephra fallout, spatter agglutination, clastogenic lavas and mixed avalanche deposits. Field work carried out in 2017 and 2018 focused upon the tephra fallout deposits from the 3 March 2015 paroxysmal eruption. Tephra fall deposits produced by lava fountain and a rapid transition to sustained eruption column at Villarrica are here described for first time. Tephra dispersion was observed mostly southeast of the crater in a narrow, elongated, nearly-elliptic area. Minimum observed tephra loading was estimated to be 80-120 g/m2 at crosswind locations, while maximum load estimation was about 11,500 g/m2, at 7-9 km distance from the vent. At 6-8 km downwind, the tephra fall deposit consisted of a single black scoria layer, fall varied from a lapilli blanket at medial distances (9-19 km) to a few individual particles at distal sites (20-43 km). All samples are vitrophyric in texture, with a mineral assemblage of plagioclase (3-30 %), clinopyroxene (2-5 %), olivine (1-4 %) and scarce orthopyroxene (<1 %) phenocrysts. We estimate that 1.4 x 109 kg, equivalent to ~2.4 x 106 m3 of basaltic andesite tephra (52.8 to 54.7 SiO2 wt. %), were erupted within a period of about 16 minutes at a mass eruption rate of ~1.5 x 106 kg/s. The sustained eruption column height was estimated at ~10.8 km, with magnitude and intensity of 2.11 and 9.13 respectively. We propose that a rejuvenated, volatile-rich magmatic intrusion is the probable triggering factor for the paroxysm. The Villarrica eruption is classified as a short-lived, large-scale lava fountain, similar to intense historical lava fountains at Mt. Etna and Kilauea. As evident by devastating 20th century Villarrica eruptions which also developed similar lava fountain behavior, this eruption style should be considered as frequent and a potential source of volcanic hazard in the future.

Proximal lava drainage controls on basaltic fissure eruption dynamics

Hawaiian basaltic eruptions commonly initiate as a fissure, producing fountains, spattering, and clastogenic lava flows. Most fissures rapidly localize to form a small number of eruptive vents, the location of which may influence the subsequent distribution of lava flows and associated hazards. We present results from a detailed field investigation of the proximal deposits of episode 1 of the 1969 fissure eruption of Mauna Ulu, Kīlauea, Hawai‘i. Exceptional preservation of the deposits allows us to reconstruct vent-proximal lava drainage patterns and to assess the role that drainage played in constraining vent localization. Through detailed field mapping, including measurements of the height and internal depth of lava tree moulds, we reconstruct high-resolution topographic maps of the pre-eruption ground surface, the lava high-stand surface and the post-eruption ground surface. We calculate the difference in elevation between pairs of maps to estimate the lava inundation depth and lava drainage depth over the field area and along different segments of fissure. Aerial photographs collected during episode 1 of the eruption allow us to locate those parts of the fissure that are no longer exposed at the surface. By comparing with the inundation and drainage maps, we find that fissure segments that were inundated with lava to greater depths (typically 1–6 m) during the eruption later became foci of lava drainage back into the fissure (internal drain-back). We infer that, in these areas, lava ponding over the fissure suppressed discharge of magma, thereby favouring drain-back and stagnation. By contrast, segments with relatively shallow inundation (typically less than ~ 1 m), such as where the fissure intersects pre-eruptive topographic highs, or where flow away from the vent (outflow) was efficient, are often associated with sub-circular vent geometries in the post-eruption ground surface. We infer that these parts of the fissure became localization points for ongoing magma ascent and discharge. We conclude that lava inundation and drainage processes in basaltic fissure eruptions can play an important role in controlling their localization and longevity.

New insights on lithofacies architecture, sedimentological characteristics and volcanological evolution of pre-caldera (> 22 ka), multi-phase, scoria- and spatter-cones at Somma-Vesuvius

Pre-caldera (>22ka) lateral activity at Somma-Vesuvius is related to scoria- and spatter-cone forming events of monogenetic or polygenetic nature. A new stratigraphic, sedimentological, textural and lithofacies investigation was performed on five parasitic cones (Pollena cones, Traianello cone, S. Maria a Castello cone and the recently found Terzigno cone) occurring below the Pomici di Base (22ka) Plinian products emplaced during the first caldera collapse at Somma-Vesuvius. A new Ar/Ar age of 23.6±0.3ka obtained for the Traianello cone as well as the absence of a paleosol or reworked material between the S. Maria a Castello cone and the Pomici di Base deposits suggest that such cone-forming eruptions occurred near the upper limit of the pre-caldera period (22–39ky). The stratigraphy of three of these eccentric cones (Pollena cones and Traianello cone) exhibits erosion surfaces, exotic tephras, volcaniclastic layers, paleosols, unconformity and paraconformity between superimposed eruptive units revealing their multi-phase, polygenetic evolution related to activation of separate vents and periods of quiescence. Such eccentric cones have been described as composed of scoria deposits and pure effusive lavas by previous authors. Lavas are here re-interpreted as welded horizons (lava-like) composed of coalesced spatter fragments whose pyroclastic nature is locally revealed by relicts of original fragments and remnants of clast outlines. These welded horizons show, locally, rheomorphic structures allowing to define them as emplaced as clastogenic lava flows. The lava-like facies is transitional, upward and downward, to less welded facies composed of agglutinated to unwelded spatter horizons in which clasts outlines are increasingly discernible. Such textural characteristics and facies variation are consistent with a continuous fall deposition of Hawaiian fire-fountains episodes alternated with Strombolian phases emplacing loose scoria deposits. High enrichment factor values, measured in the scoria deposits, imply the ejection of large proportion of ash even during Strombolian events.

Syn-eruptive breakdown of pyrrhotite: a record of magma fragmentation, air entrainment, and oxidation

Air entrainment in fragmented magmas controls the dynamics of volcanic eruptions. Pyroclast oxidation kinetics may be applied to quantify the degree of magma–air interaction. Pyrrhotite (Po) in volcanic rocks is often oxidized to form magnetite (Mt) and hematite (Hm), and its reaction mechanisms are well constrained. To test utilizing Po oxidation as a marker for magma–air interactions, we compared the occurrence of Po oxidation products from three different eruption styles during the Sakurajima 1914–1915 eruption. Pumices from the Plinian eruption include columnar-type Fe oxides (Mt with subordinate width of Hm) often accompanied by relict Po. This columnar type is also found in clastogenic lava, where it is almost completely oxidized to Hm. The effusive lava contains framboidal aggregates of subhedral to anhedral Mt crystals without Hm. The formation mechanisms of columnar and framboidal Fe oxides were estimated. The columnar type Fe oxides were formed syn-eruptively through gaseous reactions, as opposed to the melt in a magma chamber, as demonstrated by the Ti-free nature of the columnar Mt and its synchronous oxidation to Hm. By contrast, the framboidal type was formed in a melt with decreasing fS2. The calculation of Hm growth in a conductively cooling pumice clast constrains the surface temperature of pumice in the eruption column. The paragenesis and oxidation degree of Po and Fe oxides are consistent with the eruption processes in terms of magma fragmentation, air entrainment, and welding, and can, therefore, be a responsive marker for the magma–air interaction.

Disruption of tephra fall deposits caused by lava flows during basaltic eruptions

Observations in the USA, Iceland and Tenerife, Canary Islands reveal how processes occurring during basaltic eruptions can result in complex physical and stratigraphic relationships between lava and proximal tephra fall deposits around vents. Observations illustrate how basaltic lavas can disrupt, dissect (spatially and temporally) and alter sheet-form fall deposits. Complexity arises through synchronous and alternating effusive and explosive activity that results in intercalated lavas and tephra deposits. Tephra deposits can become disrupted into mounds and ridges by lateral and vertical displacement caused by movement (including inflation) of underlying pāhoehoe lavas and clastogenic lavas. Mounds of tephra can be rafted away over distances of 100 s to 1,000 s m from proximal pyroclastic constructs on top of lava flows. Draping of irregular topography by fall deposits and subsequent partial burial of topographic depressions by later lavas can result in apparent complexity of tephra layers. These processes, deduced from field relationships, have resulted in considerable stratigraphic complexity in the studied proximal regions where fallout was synchronous or alternated with inflation of subjacent lava sheets. These mechanisms may lead to diachronous contact relationships between fall deposits and lava flows. Such complexities may remain cryptic due to textural and geochemical quasi-homogeneity within sequences of interbedded basaltic fall deposits and lavas. The net effect of these processes may be to reduce the usefulness of data collected from proximal fall deposits for reconstructing basaltic eruption dynamics.

Eruption conditions of spatter deposits

Spatter is an eruptive product that forms within a narrow range of thermal conditions: it must be hot enough to deform and agglutinate, but not so hot that clasts completely re-fuse and remobilize as clastogenic lava. This narrow thermal window of spatter-forming conditions allows for quantitative prediction of cooling rates and accumulation rates. Cooling and accumulation rates then provide information that enables estimates of eruption parameters for inaccessible and prehistoric deposits. High-temperature experiments conducted on basaltic scoria from Devil’s Garden, Oregon have revealed the eruption temperature was ~1130°C. The strength welds formed between experimental clasts is shown to depend on cooling rate. Natural samples are compared to the experimental samples by measuring tensile strength and welded area between clasts. The weld strength in natural deposits yields estimates of cooling rates that range between 2.5°C and 48°C/min, with the majority of the samples grouping between 7°C and 14°C/min. Thermal models based on these cooling rates yield spatter accumulation rates of 0.5–1.8m/h in the Devil’s Garden spatter deposits. We provide a general model for cooling and accumulation rates for spatter cones, ramparts, and hornitos, which allow estimation of the factors that control basaltic eruptive products.

The 2013 eruption of Pavlof Volcano, Alaska: a spatter eruption at an ice- and snow-clad volcano

The 2013 eruption of Pavlof Volcano, Alaska began on 13 May and ended 49 days later on 1 July. The eruption was characterized by persistent lava fountaining from a vent just north of the summit, intermittent strombolian explosions, and ash, gas, and aerosol plumes that reached as high as 8 km above sea level and on several occasions extended as much as 500 km downwind of the volcano. During the first several days of the eruption, accumulations of spatter near the vent periodically collapsed to form small pyroclastic avalanches that eroded and melted snow and ice to form lahars on the lower north flank of the volcano. Continued lava fountaining led to the production of clastogenic lava flows that extended to the base of the volcano, about 3–4 km beyond the vent. The generation of fountain-fed lava flows was a dominant process during the 2013 eruption; however, episodic collapse of spatter accumulations and formation of hot spatter-rich granular avalanches was a more efficient process for melting snow and ice and initiating lahars. The lahars and ash plumes generated during the eruption did not pose any serious hazards for the area. However, numerous local airline flights were cancelled or rerouted, and trace amounts of ash fall occurred at all of the local communities surrounding the volcano, including Cold Bay, Nelson Lagoon, Sand Point, and King Cove.

Historia eruptiva de los volcanes monogenéticos de El Toro (23º05’S-66º42’W), Puna norte, Argentina

The monogenetic volcanism from the El Toro region (23&ordm;05&rsquo;S-66&ordm;42&rsquo;W) in NW Argentina comprises a group of low-volume (&lt;5 km2) mafic volcanic rocks erupted during the Late Miocene-Pliocene in the northern Puna (Andean Central Volcanic Zone). The activity of at least four eruptive centers resulted in the discharge of several lava flows from a cluster of cinder cones, whose architecture coincides with the typical facies model defined for edifices constructed during Strombolian style eruptions. In some cases, volcanoes recorded brief phases of transitional Hawaiian activity that resulted in small and localized fire-fountaining (occasionally with oblique development), responsible for the generation of agglutinated deposits as spatter and clastogenic lava. These highly welded products, still hot and ductile, may have acted as zones of weakness from which the edifices collapsed gravitationally during lava effusions (rafting process). The latter suggests that effusive and pyroclastic eruptions sometimes may have been coeval. Volcanic rocks are geochemically classified as arc-like, high-K calc-alkaline basalts, basaltic andesites and andesites. Mobile/inmobile trace element ratios (e.g., Ba/Nb) suggest that the El Toro rocks have arc-like signatures, transitional between frontal arc andesites (Le&oacute;n Muerto) and mafic to intermediate Andean rocks with intraplate affinities (Las Burras). The latter confirms a near-arc backarc position for the western northern Puna during the Upper Miocene to Pliocene. Although geochemically indistinguishable, rocks from El Toro volcanoes have significantly different petrographic characteristics (phenocryst assemblage, % of crystals). This suggests that some volcanic rocks from El Toro represent the eruption of very similar magma batches that show strong differences in temperature and degree of crystallization, i.e., batches evolving at different crustal depths and/or with different magma chamber residence times. On the other hand, magmas of different chemical compositions were erupted almost simultaneously from the same vents. This intriguing compositional pattern demonstrates that short-lived monogenetic volcanoes may have a complex polymagmatic character.

From feeder dykes to scoria cones: the tectonically controlled plumbing system of the Rauðhólar volcanic chain, Northern Volcanic Zone, Iceland

The Rauðholar volcanic chain, located in the Northern Volcanic Zone of Iceland, has been variably eroded such that, in the northern part, the original scoria cones are preserved, while the central and southern parts expose their shallow feeders. The chain thus offers insight into the inner workings of the near-surface feeder system of scoria cones. The volcanic chain was mapped in 3D using GPS. The en echelon-arranged volcanic chain can be divided into three parts: The southernmost part contains only plugs and necks with a thin pyroclastic cover as well as multi-tiered lava flows. The central part combines partially eroded scoria cones, (feeder) dyke intersections, and welded scoria interbedded within rootless and clastogenic lava flows; the welded scoria is composed of different kinds of lithics and bombs. The northern part preserves almost intact, overlapping scoria cones with voluminous lapilli-sized scoriaceous deposits. The overall dyke trend is orthogonal but shows radial patterns in individual cone complexes. Feeder dykes observed to depths of about 200 m below the volcanic chain are up to 8 m thick and flare in to conduits in the uppermost 20–50 m. The exposed shallow plumbing system shows that magma pathways through the volcanic edifice are very complex with incremental, repeated intrusions. We interpret the arcuate shape to be the result of a local change in the orientation of the stress field because the Rauðholar volcanic chain is located within a major relay structure between volcanoes on the eastern Fremrinamur rift arm and a rift extension with grabens on the western periphery.

The Intra-Continental Al Madinah Volcanic Field, Western Saudi Arabia: A Proposal to Establish Harrat Al Madinah as the First Volcanic Geopark in the Kingdom of Saudi Arabia

The United Nations Educational, Scientific and Cultural Organization promotes conservation of geological and geomorphological heritage through the promotion and protection of sites of importance and the development of educational programs under the umbrella of geoparks. Here, we identify significant volcanic features that could be organised and promoted as the first geopark in the Kingdom of Saudi Arabia. The Al Madinah Volcanic Field, or Harrat Al Madinah, has numerous volcanic geosites relevant to the understanding of the evolution of intraplate volcanic fields dominated by Hawaiian and Strombolian style eruptions, and includes the location of the last historically erupted volcanoes in the Arabian Peninsula. The unique volcanic features of the proposed Harrat Al Madinah Volcanic Geopark (HAMVG) are organised within three precincts, each with specific volcanic phenomena. The first and most accessible precinct, defined as the “Historic Eruption Precinct—1256 AD and 641 AD Historic Eruption Sites”, contains two major geotops with numerous individual geosites representing the youngest volcanoes of the Arabian Peninsula, including the lava spatter and scoria cones of the 1256 AD eruption just 10 km SE of Al Madinah city. The second precinct, “Lava Lakes, Lava Fountains and Volcano Spreading Precinct—The Mosawdah Volcano”, provides an in-depth view of an eruption that produced low lava fountains, clastogenic lava flows, agglutinated lava spatter cones and extensive lava flows from a central, lava lake-occupied crater. In the third precinct, referred to as “Silicic Lava Domes and Explosion Craters Precinct”, the results of explosive eruptions are visible, with deep craters and ash blankets around the vents. Here, there is also evidence for how trachytic lava can protrude from single and multiple vents to build lava domes. This precinct also offers the most dramatic landscape and adventure volcano tourism opportunity in this arid environment. This three-level hierarchy of the proposed HAMVG fits well with the gradual educational program proposed here to demonstrate the recent and potential future volcanism of the region, from the most common but less destructive to the less common but more hazardous eruptions. The proposed HAMVG will promote the protection of this globally unique, young volcanic landscape and offer geoeducational opportunities to the general public and to the scientific community. The Harrat Al Madinah is also located in a culturally significant place near to Al Madinah city, which is one of the holiest places to Muslims. The proposed geopark is easily accessable through highways (and by train in the near future) and it could provide a significant economic benefit to Al Madinah city.

The Cerro Morado Andesites: Volcanic history and eruptive styles of a mafic volcanic field from northern Puna, Argentina

The Upper Miocene Cerro Morado Andesites constitutes a mafic volcanic field (∼100 km 2) composed of andesite to basaltic andesite rocks that crop out ∼75 km to the east from the current arc, in the northern Puna of Argentina. The volcanic field comprises lavas and scoria cones resulting from three different eruptive phases developed without long interruptions between each other. Lavas and pyroclastic rocks are thought to be sourced from the same vents, located where orogen-parallel north-south faults crosscut transverse structures. The first eruptive phase involved the effusion of extensive andesitic flows, and minor Hawaiian-style fountaining which formed subordinate clastogenic lavas. The second phase represents the eruption of slightly less evolved andesite lavas and pyroclastic deposits, only distributed to the north and central sectors of the volcanic field. The third phase represents the discharge of basaltic andesite magmas which occurred as both pyroclastic eruptions and lava effusion from scattered vents distributed throughout the entire volcanic field. The interpreted facies model for scoria cones fits well with products of typical Strombolian-type activity, with minor fountaining episodes to the final stages of eruptions. Petrographic and chemical features suggest that the andesitic units (SiO 2 > 57%) evolved by crystal fractionation. In contrast, characteristics of basaltic andesite rocks are inconsistent with residence in upper-crustal chambers, suggesting that batches of magmas with different origins or evolutive histories arrived at the surface and erupted coevally. Based on the eruptive styles and lack of volcanic quiescence gaps between eruptions, the Cerro Morado Andesites can be classified as a mafic volcanic field constructed from the concurrent activity of several small, probably short-lived, monogenetic centers.

Record of complex scoria cone eruptive activity at Red Mountain, Arizona, USA, and implications for monogenetic mafic volcanoes

Scoria cone eruptions are generally modeled as a simple succession from explosive eruption to form the cone to passive effusion of lava, generally from the base of the cone. Sector collapse of scoria cones, wherein parts of the cone are rafted on a lava flow, is increasingly recognized as common, but the reasons that a cone may not be rebuilt are poorly understood. Red Mountain volcano is a Pleistocene scoria cone in the San Francisco Volcanic Field of northern Arizona, USA. The cone lies along the trace of a major steeply dipping normal fault that originated during Proterozoic tectonism and was reactivated in Tertiary time. The earliest phase of eruption at Red Mountain was typical “Strombolian”, forming a cone that was followed by or possibly synchronous with lava effusion, toward the west from the base of the cone. Rafting then ensued as the west side of the cone collapsed; approximately 15% of the cone is preserved in mounds as much as 30 m high. Rafting was extensive enough to remove most of the cone over the vent area, which effectively reduced the pressure cap on the magma conduit. Resultant low fountaining fed clastogenic lava flows and minor scoria fallback. Clastogenic flows traveled as far as 4 km and now form a cliff 30–40 m high at the edge of the lava platform. Although several possibilities explain the change in vent dynamics and eruptive style, we favor the interpretation that an increase in magma-rise rate caused collapse of the cone. The abrupt removal of 300 m of material over the vent removed a conduit “cork” and low fountaining began. Magma that had erupted effusively suddenly became explosive. This aspect of scoria cone rafting at Red Mountain is broadly similar to sector collapse followed by explosive eruption in larger systems. A steep-walled, 150-m-high amphitheatre on the northeast side of Red Mountain exposes weakly to strongly altered scoria cemented by calcite, iron, and zeolites. We suggest that vapor-phase alteration was responsible for sealing fine-grained ash beds in the cone, and a pressurized system developed. Residual heat from a dike that was emplaced as part of the magmatic activity provided heat that drove groundwater along the regional fault up into the cone. Eventually the overpressurized system exploded in a phreatic eruption that created the amphitheatre, which has subsequently been enlarged by water and wind erosion. The combined sequence of events at Red Mountain illustrates some of the complexities in monogenetic scoria cone eruptions that have received little attention to date.