Base Surge Deposits Research Articles

First Optically Stimulated Luminescence and Radiocarbon Dating of the Late Quaternary Eruptions in the Xilinhot Volcanic Field, China

Precise dating of prehistoric volcanic eruptions is essential for reconstructing eruption sequences and assessing volcanic hazards. The timing of the onset and termination of volcanic activity in the Xilinhot volcanic field (XVF) has been a topic of debate for years. Volcanic eruptions in this area began during the Pliocene, with the K-Ar (Ar-Ar) method providing reliable ages for early formed volcanic rocks; however, this method is less effective for dating younger volcanic events that occurred since the Late Pleistocene. For younger volcanoes, sediments baked by volcanic materials, organic sediments, and silty mudstones entrapped in lava serve as excellent geological carriers for dating. In this study, suitable samples collected from the XVF were dated using 14C and optically stimulated luminescence (OSL) methods. The 14C ages obtained for the Gezishan volcano are ~6.8 cal. ka BP, while its OSL age is ~7.8 ka. The ages dated by these two methods, combined with volcano–sedimentary stratigraphic relationships and volcanic topography, confirm the Holocene eruptions of the Gezishan volcano, categorizing it as a broadly active volcano. The upper boundary age of the sandy loam layer beneath the Gezishan lava flow is ~15.5 ka, indicating that the south lava of the Gezishan effusion occurred later than the late stage of the Late Pleistocene. Additionally, the OSL ages of baked sediments at the bottom of the base surge deposits from a Maar-type volcano and aeolian sand interlayers within a Strombolian-type scoria cone in the study area are ~50 ka and ~60 ka, respectively, representing eruptions in the middle Late Pleistocene. These findings demonstrate that volcanic activity in the XVF remained vigorous during the Late Pleistocene to Holocene. This study provides significant insights for reconstructing the evolutionary history of Xilinhot volcanic activity and assessing regional volcanic hazards.

Analysis of the 2020 Taal Volcano tephra fall deposits from crowdsourced information and field data

After 43 years of dormancy, Taal Volcano violently erupted in January 2020 forming a towering eruption plume. The fall deposits covered an area of 8605 km2, which includes Metro Manila of the National Capital Region of the Philippines. The tephra fall caused damage to crops, traffic congestion, roof collapse, and changes in air quality in the affected areas. In a tropical region where heavy rains are frequent, immediate collection of data is crucial in order to preserve the tephra fall deposit record, which is readily washed away by surface water runoff and prevailing winds. Crowdsourcing, field surveys, and laboratory analysis of the tephra fall deposits were conducted to document and characterize the tephra fall deposits of the 2020 Taal Volcano eruption and their impacts. Results show that the tephra fall deposit thins downwind exponentially with a thickness half distance of about 1.40 km and 9.49 km for the proximal and distal exponential segments, respectively. The total calculated volume of erupted fallout deposit is 0.057 km3, 0.042 km3, or 0.090 km3 using the exponential, power-law, and Weibull models, respectively, and all translate to a VEI of 3. However, using a probabilistic approach (Weibull method) with 90% confidence interval, the volume estimate is as high as 0.097 km3. With the addition of the base surge deposits amounting to 0.019 km3, the volume translates to a VEI of 4, consistent with the classification for the observed height and umbrella radius of the 2020 main eruption plume. VEI 4 is also consistent with the calculated median eruption plume height of 17.8 km and sub-plinian classification based on combined analysis of isopleth and isopach data. Phreatomagmatic activity originated from a vent located in Taal Volcano’s Main Crater Lake (MCL), which contained 42 million m3 of water. This eruptive style is further supported by the characteristics of the ash grain components of the distal 12 January 2020 tephra fall deposits, consisting dominantly of andesitic vitric fragments (83–90%). Other components of the fall deposits are lithic (7–11%) and crystal (less than 6%) grains. Further textural and geochemical analysis of these tephra fall deposits contributes to better understand the volcanic processes that occurred at Taal Volcano, one of the 16 Decade Volcanoes identified by the International Association of Volcanology and Chemistry of the Earth’s Interior (IAVCEI) because of its destructive nature and proximity to densely populated areas. The crowdsourcing initiative provided a significant portion of the data used for this study while at the same time educating and empowering the community to build resilience.

Cenozoic volcanism along Dahongliutan fault in the West Kunlun Mountains, China: implication from distribution of volcanic rocks, volcanic geology, and geochemistry

Abstract In the West Kunlun Mountains, four volcanic fields (Kangxiwa, Dahongliutan, Qitaidaban and Quanshuigou) are distributed along the Dahongliutan fault, which is c. 180 km long. Based on field investigations, chronological measurements and geochemical analysis of some volcanic fields, the results of geological, geochemical and geophysical research in previous studies in the corresponding study areas are summarized. The volcanic activities in these areas were mainly effusive eruptions, explosive eruptions and phreatomagmatic eruptions. In this study, we discovered the Qitaiyanhu volcanic field for the first time and determined that the 14 C age of the lacustrine strata underlying the Qitaiyanhu lava flows is 13.110 ± 0.04 ka BP, indicating that there may still have been volcanic activities in the late Pleistocene and even the Holocene in the Dahongliutan fault area. Base surge deposits, which are the products of the interaction between magma and water, were found in the Kangxiwa volcanic field. The four shoshonitic rock fields of Kangxiwa, Dahongliutan, Qitaidaban and Quanshuigou are likely to be products of different evolution stages from the same magma source area. The magmatic origin of these volcanic fields may be related to the upwelling of the asthenosphere, triggered by the collision between the Indian and Tarim plates.

Insights Into the Eruptive Dynamics of Small Caldera-Forming Eruptions: The Case Study of the Welded Scoriae of Vulcano (Aeolian Islands, Italy)

A multi-disciplinary study, integrating volcanological field observations, petrography, whole rock geochemistry and textural and compositional analyses on plagioclase crystals has been carried out on the products of Monte Luccia, Spiaggia Lunga and Quadrara eruptions, occurred between 48 and 21 kyrs on the island of Vulcano. These products are all characterized by welded scoria blankets, and their eruptions have been generally related to the formation and/or re-activation of ring faults bordering the “Il Piano” caldera. The aim of the work is to reconstruct the pre- and syn-eruptive dynamics acting within their magma plumbing systems and the related link with the phases of caldera collapse. At the bottom of the stratigraphic sequences, the presence of base surge deposits suggests that all the eruptions started with a phreatomagmatic phase fed by a shallow reservoir. Textural and microanalytical study of plagioclase crystals of Spiaggia Lunga eruption revealed that the phreatomagmatic event activated the ascent of a volatile-rich, basaltic magma residing at 5-11 km of depth. This basaltic magma mixed with the resident shallow one, and was poured out during the course of the eruption producing a sustained lava-fountaining phase. The subsequent caldera collapse, identified by a layer of chaotic breccia interbedded in the scoriae deposit, has been linked to the partial emptying of the shallow magma reservoir. In contrast to what observed for recent eruptive events at Vulcano, the onset of the magmatic phase would be attributed to a self-activation due to volcano-tectonic events. As concerns the Mt. Luccia deposits, bordering the eastern rim of the “Il Piano” caldera, the absence of plagioclase in the mineralogical assemblage suggests the eruption of a deeper magma (>11 km b.s.l.), rapidly ascending through the re-activated ring faults of “Il Piano” caldera. At Quadrara eruption, the occurrence of a layer of white biotite-bearing latitic pumices overlying the basal phreatomagmatic deposits suggests the involvement a shallow, isolated reservoir where the increase of volatile pressure allowed the crystallization of hydrous phases; a deeper shoshonitic magma was involved later in the eruption, forming the welded scoria level at the top of the sequence.

Evolution of the submarine–subaerial edifice of Bogoslof volcano, Alaska, during its 2016–2017 eruption based on analysis of satellite imagery

The 2016–2017 eruption of Bogoslof volcano involved at least 70 detected eruptive events between mid-December 2016 and August 30, 2017. Acquisition of high-resolution satellite imagery throughout the duration of the eruptive period allowed us to document and map the various morphologic changes that occurred on the subaerial part of Bogoslof Island. The emplacement of pyroclastic-flow and surge deposits caused the island to increase in area by about 1.5 km2. The dominant volcanic landforms of the eruption were a series of tuff rings emplaced around various submarine vents. Many of the tuff rings were mantled with surface dunes and impressive amounts of ballistic ejecta, likely derived from erupting magma bodies or previously emplaced submarine lava domes. Debris-flow deposits and surface channels extending over tuff ring surfaces apparent in multiple satellite images are evidence for explosive ejection of seawater. In most cases, erupting vents were initially submarine or began at subaerial lava domes and were largely flooded by seawater suggesting that water-magma ratios were likely high. Under such conditions where water is abundant, eruptive products typically reflect a high degree of water involvement and are dominated by the formation of wet tephra jets and flows and associated deposits typically consist of fine ash and lapilli, contain accretionary lapilli and ash aggregates, and usually form tuff cones and mounds. We observed none of these features in our analysis of satellite data or during our examination of eruptive deposits on Bogoslof Island in 2018. On the contrary, the dominant landform associated with the Bogoslof eruption was tuff rings. The development of tuff rings and surface dunes are commonly associated with the formation of pyroclastic base surges that are by comparison emplaced relatively dry. Dry base surge deposits can be generated from phreatomagmatic explosions involving superheated steam. It is possible that shallow submarine, magma–wet sediment interactions were a characteristic and possibly a dominant eruptive process of the 2016–2017 Bogoslof eruption.

Sedimentary structures of the base-surge deposits in the Middle Miocene, Tango Peninsula

Sedimentary structures of the base-surge deposits in the Middle Miocene, Tango Peninsula

Subsurface structure of a maar–diatreme and associated tuff ring from a high-resolution geophysical survey, Rattlesnake Crater, Arizona

Geophysical survey techniques including gravity, magnetics, and ground penetrating radar were utilized to study the diatreme and tuff ring at Rattlesnake Crater, a maar in the San Francisco Volcanic Field of northern Arizona. Significant magnetic anomalies (+1600nT) and a positive gravity anomaly (+1.4mGal) are associated with the maar. Joint modeling of magnetic and gravity data indicate that the diatreme that underlies Rattlesnake Crater has volume of 0.8–1km3, and extends to at least 800m depth. The modeled diatreme comprises at least two zones of variable density and magnetization, including a low density, highly magnetized unit near the center of the diatreme, interpreted to be a pyroclastic unit emplaced at sufficiently high temperature and containing sufficient juvenile fraction to acquire thermal remanent magnetization. Magnetic anomalies and ground penetrating radar (GPR) imaging demonstrate that the bedded pyroclastic deposits of the tuff ring also carry high magnetization, likely produced by energetic emplacement of hot pyroclastic density currents. GPR profiles on the tuff ring reveal long (~100m) wavelength undulations in bedding planes. Elsewhere, comparable bedforms have been interpreted as base surge deposits inflated by air entrainment from eruption column collapse. Interpretation of these geophysical data suggests that Rattlesnake Crater produced highly energetic phreatomagmatic activity that gave way to less explosive activity as the eruption progressed. The positive gravity anomaly associated with the maar crater is interpreted to be caused by coherent bodies within the diatreme and possibly lava ponding on the crater floor. These dense magnetized bodies have excess mass of 2–4×1010kg, and occupy approximately 5% of the diatreme by volume. Magnetic anomalies on the crater floor are elongate NW–SE, suggesting that the eruption may have been triggered by the interaction of ascending magma with water in fractures of this orientation. GPR imaging of the tuff ring also suggests that substantial land-slip may have occurred on the western rim, perhaps causing part of the tuff ring to collapse into the crater. Strong radar reflections indicative of well-developed weathering horizons are present as well. The techniques employed at Rattlesnake Crater demonstrate the value of combining multiple geophysical techniques in areas where exposures are limited and invasive exploration is not an option.

Eruptive history of a low-frequency and low-output rate Pleistocene volcano, Ciomadul, South Harghita Mts., Romania

Based on a new set of K–Ar age data and detailed field observations, the eruptive history of the youngest volcano in the whole Carpathian-Pannonian region was reconstructed. Ciomadul volcano is a dacitic dome complex located at the southeastern end of the Călimani-Gurghiu-Harghita Neogene volcanic range in the East Carpathians. It consists of a central group of extrusive domes (the Ciomadul Mare and Haramul Mare dome clusters and the Koves Ponk dome) surrounded by a number of isolated peripheral domes, some of them strongly eroded (Balvanyos, Puturosul), and others topographically well preserved (Haramul Mic, Dealul Mare). One of the domes (Dealul Cetăţii) still preserves part of its original breccia envelope. A large number of bread-crust bombs found mostly along the southern slopes of the volcano suggest that the dome-building activity at Ciomadul was punctuated by short Vulcanian-type explosive events. Two late-stage explosive events that ended the volcanic activity of Ciomadul left behind two topographically well-preserved craters disrupting the central group of domes: the larger-diameter, shallower, and older Mohos phreatomagmatic crater and the smaller, deeper and younger Sf. Ana (sub)Plinian crater. Phreatomagmatic products of the Mohos center, including accretionary lapilli-bearing base-surge deposits and poorly sorted airfall deposits with impact sags, are known close to the eastern crater rim. A key section studied in detail south of Băile Tusnad shows the temporal succession of eruptive episodes related to the Sf. Ana (sub)Plinian event, as well as relationships with the older dome-building stages. The age of this last eruptive event is loosely constrained by radiocarbon dating of charcoal pieces and paleosoil organic matter at ca. 27–35 ka. The age of the Mohos eruption is not constrained, but we suggest that it is closely related to the Sf. Ana eruption. The whole volcanic history of Ciomadul spans over ca. 1 Myr, starting with the building up of peripheral domes and then concentrating in its central part. Ciomadul appears as a small-volume (ca. 8.74 km3) and very low-frequency and low-output rate volcano (ca. 9 km3/Myr) at the terminus of a gradually diminishing and extinguishing volcanic range. A number of geodynamically active features strongly suggest that the magma plumbing system beneath Ciomadul is not completely frozen, so future activity cannot be ruled out.

Origin of the outer layer of martian low-aspect ratio layered ejecta craters

Low-aspect ratio layered ejecta (LARLE) craters are one of the most enigmatic types of martian layered ejecta craters. We propose that the extensive outer layer of these craters is produced through the same base surge mechanism as that which produced the base surge deposits generated by near-surface, buried nuclear and high-explosive detonations. However, the LARLE layers have higher aspect ratios compared with base surge deposits from explosion craters, a result of differences in thicknesses of these layers. This characteristics is probably caused by the addition of large amounts of small particles of dust and ice derived from climate-related mantles of snow, ice and dust in the areas where LARLE craters form. These deposits are likely to be quickly stabilized (order of a few days to a few years) from eolian erosion by formation of duricrust produced by diffusion of water vapor out of the deposits.

A combined field and numerical approach to understanding dilute pyroclastic density current dynamics and hazard potential: Auckland Volcanic Field, New Zealand

The most dangerous and deadly hazards associated with phreatomagmatic eruptions in the Auckland Volcanic Field (AVF; Auckland, New Zealand) are those related to volcanic base surges — dilute, ground-hugging, particle laden currents with dynamic pressures capable of severe to complete structural damage. We use the well-exposed base surge deposits of the Maungataketake tuff ring (Manukau coast, Auckland), to reconstruct flow dynamics and destructive potential of base surges produced during the eruption. The initial base surge(s) snapped trees up to 0.5m in diameter near their base as far as 0.7–0.9km from the vent. Beyond this distance the trees were encapsulated and buried by the surge in growth position. Using the tree diameter and yield strength of the wood we calculate that dynamic pressures (Pdyn) in excess of 12–35kPa are necessary to cause the observed damage. Next we develop a quantitative model for flow of and sedimentation from a radially-spreading, dilute pyroclastic density currents (PDCs) to determine the damage potential of the base surges produced during the early phases of the eruption and explore the implications of this potential on future eruptions in the region. We find that initial conditions with velocities on the order of 65ms−1, bulk density of 38kgm−3 and initial, near-vent current thicknesses of 60m reproduce the field-based Pdyn estimates and runout distances. A sensitivity analysis revealed that lower initial bulk densities result in shorter run-out distances, more rapid deceleration of the current and lower dynamic pressures. Initial velocity does not have a strong influence on run-out distance, although higher initial velocity and slope slightly decrease runout distance due to higher rates of atmospheric entrainment. Using this model we determine that for base surges with runout distances of up to 4km, complete destruction can be expected within 0.5km from the vent, moderate destruction can be expected up to 2km, but much less damage is expected up to the final runout distance of 4km. For larger eruptions (base surge runout distance 4–6km), Pdyn of >35kPa can be expected up to 2.5km from source, ensuring complete destruction within this area. Moderate damage to reinforced structures and damage to weaker structures can be expected up to 6km from source. In both cases hot ash may still cause damage due to igniting flammable materials in the distal-most regions of a base surge. This work illustrates our ability to combine field observations and numerical models to explore the depositional mechanisms, macroscale current dynamics, and potential impact of dilute PDCs. Thus, this approach may serve as a tool to understand the damage potential and extent of previous and potential future eruptions in the AVF.

No pre-eruptive uplift in the Emeishan large igneous province: New evidences from its ‘inner zone’, Dali area, Southwest China

The Permian Emeishan large igneous province (ELIP) in Southwest China has been considered a typical example of crustal domal uplift caused by mantle plume upwelling prior to the onset of volcanism. However, this model has been questioned by the discovery of hydromagmatic volcaniclastic deposits formed in a marine environment, located near the central ELIP area (the ‘inner zone’) which is inferred to be the zone of maximum uplift. The volcanology of the inner zone has thus far been poorly documented, fueling the debate about whether or not pre-eruptive uplift occurred prior to plume upwelling. Understanding the volcanology of this inner zone is therefore critical in constraining the eruption environment of the central ELIP. Our work has revealed new volcanological observations in the inner zone (Dali area), which can systematically constrain volcanism and paleoenvironment. The Basal Succession of the sequence is a thick pillow lavas pile with hyaloclastites, implying an initial deeper submarine stage of eruptions. Limestones and submarine fallout tuffs are interbedded with these pillow lavas. Above that, abundant mafic volcaniclastic products developed, which contain palagonite-rimmed lapilli-tuffs, base surge deposits and peperites, suggesting hydroclastic volcanism in a shallower submarine environment. The Upper Succession of the sequence preserves columnar-jointed lava flows and subaerial fallout tuffs, reflecting subaerial volcanism after the volcanic center emerged above the sea level. These abundant and systematic natures of this evidence suggest that the initial volcanism of the central ELIP occurred in a deep submarine environment. The submarine-to-subaerial transition is caused by progressive emplacement of voluminous magmatic products infilling the inner zone during the continuous emplacement of ELIP, rather than by crustal doming prior to the onset of volcanisms.

Geoheritage values of one of the largest maar craters in the Arabian Peninsula: the Al Wahbah Crater and other volcanoes (Harrat Kishb, Saudi Arabia)

AbstractAl Wahbah Crater is one of the largest and deepest Quaternary maar craters in the Arabian Peninsula. It is NW-SE-elongated, ∼2.3 km wide, ∼250 m deep and surrounded by an irregular near-perpendicular crater wall cut deeply into the Proterozoic diorite basement. Very few scientific studies have been conducted on this unique site, especially in respect to understanding the associated volcanic eruption processes. Al Wahbah and adjacent large explosion craters are currently a research subject in an international project, Volcanic Risk in Saudi Arabia (VORiSA). The focus of VORiSA is to characterise the volcanic hazards and eruption mechanisms of the vast volcanic fields in Western Saudi Arabia, while also defining the unique volcanic features of this region for use in future geoconservation, geoeducation and geotourism projects. Al Wahbah is inferred to be a maar crater that formed due to an explosive interaction of magma and water. The crater is surrounded by a tephra ring that consists predominantly of base surge deposits accumulated over a pre-maar scoria cone and underlying multiple lava flow units. The tephra ring acted as an obstacle against younger lava flows that were diverted along the margin of the tephra ring creating unique lava flow surface textures that recorded inflation and deflation processes along the margin of the post-maar lava flow. Al Wahbah is a unique geological feature that is not only a dramatic landform but also a site that can promote our understanding of complex phreatomagmatic monogenetic volcanism. The complex geological features perfectly preserved at Al Wahbah makes this site as an excellent geotope and a potential centre of geoeducation programs that could lead to the establishment of a geopark in the broader area at the Kishb Volcanic Field.

Amplified hazard of small-volume monogenetic eruptions due to environmental controls, Orakei Basin, Auckland Volcanic Field, New Zealand

Orakei maar and tuff ring in the Auckland Volcanic Field is an example of a basaltic volcano in which the style and impacts of the eruption of a small volume of magma were modulated by a fine balance between magma flux and groundwater availability. These conditions were optimised by the pre-85 ka eruption being hosted in a zone of fractured and variably permeable Plio-Pleistocene mudstones and sandstones. Orakei maar represents an end-member in the spectrum of short-lived basaltic volcanoes, where substrate conditions rather than the magmatic volatile content was the dominant factor controlling explosivity and eruption styles. The eruption excavated a crater ≫80 m deep that was subsequently filled by slumped crater wall material, followed by lacustrine and marine sediments. The explosion crater may have been less than 800 m in diameter, but wall collapse and wave erosion has left a 1,000-m-diameter roughly circular basin. A tuff ring around part of the maar comprises dominantly base surge deposits, along with subordinate fall units. Grain size, texture and shape characteristics indicate a strong influence of magma–water and magma–mud interactions that controlled explosivity throughout the eruption, but also an ongoing secondary role of magmatic gas-driven expansion and fragmentation. The tuff contains >70 % of material recycled from the underlying Plio-Pliestocene sediments, which is strongly predominant in the >2 ϕ fraction. The magmatic clasts are evolved alkali basalt, consistent with the eruption of a very small batch of magma. The environmental impact of this eruption was disproportionally large, when considering the low volume of magma involved (DRE < 0.003 km3). Hence, this eruption exemplifies one of the worst-case scenarios for an eruption within the densely populated Auckland City, destroying an area of ~3 km2 by crater formation and base surge impact. An equivalent scenario for the same magma conditions without groundwater interaction would yield a scoria/spatter cone with a diameter of 400–550 m, destroying less than a tenth of the area affected by the Orakei event.

Variable eruptive styles in an ancient monogenetic volcanic field: examples from the Permian Levín Volcanic Field (Krkonoše Piedmont Basin, Bohemian Massif)

*Corresponding author The Permian pyroclastic deposits of the Levin Volcanic Field within the Krkonose Piedmont Basin were studied in terms of volcanology. Pyroclastic rocks are exposed in two quarries and the study was supported with a 30 m deep borehole K1 penetrating these rocks. The pyroclastic rocks are altered but preserved textures enable reconstruction of eruptive styles. The volcanic sequence exposed in the abandoned Hvězda quarry starts with a phreato-Strombolian pyroclastic rocks rich in basaltic scoria, cuspate glass shards, armoured- and accretionary lapilli overlain with mafic lava. Subsequent activity was phreatomagmatic in style and produced fall-out of accretionary lapilli and accumulation of base surge deposits. Overlying subhorizontally bedded matrix-supported pyroclastic deposits are rich in scoria and contain spindle-shaped bombs. These rocks are interpreted as mafic pyroclastic flow deposits related to Strombolian eruptions. A similar succession capped by scoriaceous fall-out deposit was documented in the K1 borehole. A coherent mafic volcanic rock (lava or sill) terminates the succession exposed in the Hvězda quarry. Agglutinates at the base of the Studenec quarry were produced during Hawaiian eruptions building up a spatter cone. The cone most probably dammed a stream and created an ephemeral lake. Increasing influence of water on eruptive styles is documented in overlying pyroclastic deposits of phreatomagmatic eruptions. Subsequent lava flowed into the lake. Quenching of the lava resulted in formation of pillows enclosed in hyaloclastite breccia. Further up in the exposure, transition of pillow to massive lava has been recognized. Overlaying pyroclastic deposits are matrix-supported and rich in scoria and spindle-shaped bombs. Similarly to the Hvězda quarry, these are interpreted as mafic pyro clastic flow deposits. Volcanic activity in the Levin Volcanic Field was characterized by Hawaiian, Strombolian, phreato-Strombolian and phreatomagmatic eruptions accompanying lava effusions and possibly sills emplacement. The lavas were emitted in both subaerial and subaquatic conditions. The character and distribution of volcaniclastic facies suggests existence of several monogenetic volcanoes.

Block‐and‐ash flow deposit of the Narcondam Volcano: Product of dacite–andesite dome collapse in the Burma–Java subduction complex

AbstractNarcondam Island in the Andaman Sea represents a dacite–andesite dome volcano in the volcanic chain of the Burma–Java subduction complex. The pyroclasts of andesitic composition are restricted to the periphery of the dome predominantly in the form of block‐and‐ash deposits and minor base surge deposits. Besides pyroclastic deposits, andesitic lava occurs dominantly at the basal part of the dome whereas dacitic lava occupies the central part of the dome. The pyroclasts are represented by non‐vesiculated to poorly vesiculated blocks of andesite, lapilli, and ash. The hot debris derived from dome collapse was deposited initially as massive to reversely‐graded beds with the grain support at the lower part and matrix support at the upper part. This sequence is overlain by repetitive beds of lapilli breccia to tuff breccia. These deposits are recognized as a basal avalanche rather than lahar deposit. This basal avalanche was punctuated by an ash‐cloud surge deposit representing a sequence of thinly bedded units of normal graded unit to parallel laminated beds.

Syn- and post-eruptive volcanic processes in the Yubileinaya kimberlite pipe, Yakutia, Russia, and implications for the emplacement of South African-style kimberlite pipes

The Yubileinaya kimberlite pipe, with a surface area of 59 ha, is one of the largest pipes in the Yakutian kimberlite province. The Devonian pipe was emplaced under structural control into Lower Paleozoic karstic limestone. The pipe complex consists of several smaller precursor pipes which are cut by the large, round Main pipe. While the precursor pipes show many features typical for root zones, Main pipe is younger, cuts into the precursor pipes and exposes well-bedded volcaniclastic sediments. The maximum estimated erosion since emplacement is 250 m. Open pit mapping of a 180 m thick kimberlite sequence documents the waning phases of the volcanic activity in the kimberlite pipe and the onset of its crater infill by resedimentation. Three volcanic lithofacies types can be differentiated. The deepest and oldest facies type is a massive volcaniclastic rock (“AKB”) only accessible in drill core. It is equivalent to Tuffisitic Kimberlite in South African pipes and thought to be related to the main volcanic phase which was characterized by violent explosions. The overlying lithofacies type comprises primary and resedimented volcaniclastic sediments as well as rock avalanche deposits sourced from the exposed maar crater collar. It represents the onset of sedimentation onto the crater floor during the waning phase of volcanic eruptions, where primary pyroclastic deposition was contemporaneous with resedimentation from the tephra wall and the widening maar crater. Ongoing volcanic activity is also testified by the presence of a vertical feeder conduit marking the area of the last volcanic eruption clouds piercing through the diatreme. This feeder conduit is overlain by the third and youngest lithofacies type which consists mainly of resedimented volcaniclastic material and lake beds. During the sedimentation of this facies, primary volcanic activity was only minor and finally absent and resedimentation processes dominated the crater infill. The Yubileinaya pipe complex exposes root zones, contact breccias as well as diatreme and crater infill sediments. It has all features typical of large South African-style pipes and much can be learned from Yubileinaya about the emplacement sequence and behaviour of these pipes. Emplacement of the pipe occurred over an extended time span with intermittent phases of volcanic quiescence and consolidation. The AKB reveals little direct evidence of what sort of emplacement process was dominant during the main period of volcanic activity. There is neither textural evidence that violent degassing of a juvenile gas phase has caused pipe excavation, nor that external water was present during the main phase of volcanic eruptions. However, there is clear evidence in rock textures that meteoric surface water was present during crater infill. Base surge deposits forming part of the bedded crater infill sequence indicate that water was present in the eruption clouds and, hence, the root zone of the pipe. There is no reason to assume that groundwater did not also have access to the ascending magma during the main phase of volcanic activity that excavated the pipe and formed the AKB.

Review of post-collisional volcanism in the Central Anatolian Volcanic Province (Turkey), with special reference to the Tepekoy Volcanic Complex

Neogene-Quaternary post-collisional volcanism in Central Anatolian Volcanic Province (CAVP) is mainly characterized by calc-alkaline andesites-dacites, with subordinate tholeiitic-transitional-mildly alkaline basaltic volcanism of the monogenetic cones. Tepekoy Volcanic Complex (TVC) in Nigde area consists of base surge deposits, and medium to high-K andesitic-dacitic lava flows and basaltic andesitic flows associated with monogenetic cones. Tepekoy lava flows petrographically exhibit disequilibrium textures indicative of magma mixing/mingling and a geochemisty characterized by high LILE and low HFSE abundances, negative Nb–Ta, Ba, P and Ti anomalies in mantle-normalized patterns. In this respect, they are similar to the other calc-alkaline volcanics of the CAVP. However, TVC lava flows have higher and variable Ba/Ta, Ba/Nb, Nb/Zr, Ba/TiO2 ratios, indicating a heterogeneous, variably fluid-rich source. All the geochemical features of the TVC are comparable to orogenic andesites elsewhere and point to a sub-continental lithospheric mantle source enriched in incompatible elements due to previous subduction processes. Basaltic monogenetic volcanoes of CAVP display similar patterns, and HFS anomalies on mantle-normalized diagrams, and have incompatible element ratios intermediate between orogenic andesites and within-plate basalts (e.g. OIB). Accordingly, the calc-alkaline and transitional-mildly alkaline basaltic magmas may have a common source region. Variable degrees of partial melting of a heterogeneous source, enriched in incompatible elements due to previous subduction processes followed by fractionation, crustal contamination, and magma mixing in shallow magma chambers produced the calc-alkaline volcanism in the CAVP. Magma generation in the TVC, and CAVP in general is via decompression melting facilitated by a transtensional tectonic regime. Acceleration of the extensional regime, and transcurrent fault systems extending deep into the lithosphere favoured asthenospheric upwelling at the base of the lithosphere, and as a consequence, an increase in temperature. This created fluid-present melting of a fluid-enriched upper lithospheric mantle or lower crustal source, but also mixing with asthenosphere-derived melts. These magmas with hybrid source characteristics produced the tholeiitic-transitional-mildly alkaline basalts depending on the residence times within the crust. Hybrid magmas transported to the surface rapidly, favored by extensional post-collision regime, and produced mildly alkaline monogenetic volcanoes. Hybrid magmas interacted with the calc-alkaline magma chambers during the ascent to the surface suffered slight fractionation and crustal contamination due to relatively longer residence time compared to rapidly rising magmas. In this way they produced the mildly alkaline, transitional, and tholeiitic basaltic magmas. This model can explain the coexistence of a complete spectrum of q-normative, ol-hy-normative, and ne-normative monogenetic basalts with both subduction and within-plate signatures in the CAVP.

The architecture, eruptive history, and evolution of the Table Rock Complex, Oregon: From a Surtseyan to an energetic maar eruption

The Table Rock Complex (TRC; Pliocene–Pleistocene), first documented and described by Heiken [Heiken, G.H., 1971. Tuff rings; examples from the Fort Rock-Christmas Lake valley basin, south-central Oregon. J. Geophy. Res. 76, 5615-5626.], is a large and well-exposed mafic phreatomagmatic complex in the Fort Rock–Christmas Lake Valley Basin, south-central Oregon. It spans an area of approximately 40 km 2, and consists of a large tuff cone in the south (TRC1), and a large tuff ring in the northeast (TRC2). At least seven additional, smaller explosion craters were formed along the flanks of the complex in the time between the two main eruptions. The first period of activity, TRC1, initiated with a Surtseyan-style eruption through a 60–70 m deep lake. The TRC1 deposits are dominated by multiple, 1-2 m thick, fining upward sequences of massive to diffusely-stratified lapilli tuff with intermittent zones of reverse grading, followed by a finely-laminated cap of fine-grained sediment. The massive deposits are interpreted as the result of eruption-fed, subaqueous turbidity current deposits; whereas, the finely laminated cap likely resulted from fallout of suspended fine-grained material through a water column. Other common features are erosive channel scour-and-fill deposits, massive tuff breccias, and abundant soft sediment deformation due to rapid sediment loading. Subaerial TRC1 deposits are exposed only proximal to the edifice, and consist of cross-stratified base-surge deposits. The eruption built a large tuff cone above the lake surface ending with an effusive stage, which produced a lava lake in the crater (365 m above the lake floor). A significant repose period occurred between the TRC1 and TRC2 eruptions, evidenced by up to 50 cm of diatomitic lake sediments at the contact between the two tuff sequences. The TRC2 eruption was the last and most energetic in the complex. General edifice morphology and a high percentage of accidental material suggest eruption through saturated TRC1 deposits and/or playa lake sediments. TRC2 deposits are dominated by three-dimensional dune features with wavelengths 200–500 m perpendicular to the flow, and 20–200 m parallel to the direction of flow depending on distance from source. Large U-shaped channels (10–32 m deep), run-up features over obstacles tens of meters high, and a large (13 m) chute-and-pool feature are also identified. The TRC2 deposits are interpreted as the products of multiple, erosive, highly-inflated pyroclastic surges resulting from collapse of an unusually high eruption column relative to previously documented mafic phreatomagmatic eruptions.

Diatreme Breccias at the Kelian Gold Mine, Kalimantan, Indonesia: Precursors to Epithermal Gold Mineralization

Early Miocene volcanism associated with a maar-diatreme breccia complex preceded main-stage epithermal gold mineralization at the Kelian gold mine, Kalimantan, Indonesia. Prior to brecciation, andesite intrusions (19.7 ± 0.06 Ma) were emplaced into a package of felsic volcaniclastic rocks and overlying carbonaceous sand-stones and mudstones, and a weakly mineralized geothermal system was established. Intrusion of quartz-phyric, (19.8 ± 0.1 Ma) and quartz-feldspar-phyric rhyolite (19.5 ± 0.1 Ma) into the active geothermal system triggered widespread fragmentation and formation of the maar-diatreme complex. Subsurface phreatomagmatic, and phreatic explosions disrupted the preexisting hydrothermal system, producing three composite diatreme breccia, bodies (the Tepu, Runcing, and Burung Breccias). The diatremes consist of polymict breccias and sandstones that contain abundant carbonaceous matrix. A distinctive facies association comprising coherent rhyohte, jigsaw-fit rhyolite breccia, and matrix-rich breccias that contain wispy to blocky juvenile rhyolite clasts define the root zones of the diatremes. The surficial products of maar-diatreme volcanic activity at Kelian are preserved as large blocks of well-stratified breccias. They contain accretionary lapilli and were deposited by a combination of wet, pyroclastic basesurge, fallout, and cosurge fallout processes. Evidence for syneruptive resedimentation of the pyroclastic: deposits is preserved in poorly stratified breccia beds. Megablocks of phreatomagmatic base-surge deposits were dropped down several hundred meters from the maar environment into the underlying diatremes. Volcanism in the Kelian maar-diatreme complex was dominated by a combination of phreatomagmatic and phreatic processes, with subordinate hydraulic, tectonic, and dry magmatic fragmentation. The carbonaceous -matrix-rich diatreme breccias acted as aquicludes during subsequent hydrothermal activity, focusing fluid flow into the wall rocks adjacent to the diatremes, where epithermal gold mineralization and hydrothermal brecciation occurred. © 2008 Society of Economic Geologists, Inc.

Cretaceous basaltic phreatomagmatic volcanism in West Texas: Maar complex at Peña Mountain, Big Bend National Park

A structurally complex succession of basaltic pyroclastic deposits produced from overlapping phreatomagmatic volcanoes occurs within Upper Cretaceous floodplain deposits in the Aguja Formation in Big Bend National Park, West Texas. Together with similar basaltic deposits recently documented elsewhere in the Aguja Formation, these rocks provide evidence for an episode of phreatomagmatic volcanism that predates onset of arc magmatism in the region in the Paleogene. At Peña Mountain, the pyroclastic deposits are ≥ 70 m thick and consist dominantly of tabular beds of lapillistone and lapilli tuff containing angular to fluidal pyroclasts of altered sideromelane intermixed with abundant accidental terrigenous detritus derived from underlying Aguja sediments. Tephra characteristics indicate derivation from phreatomagmatic explosions involving fine-scale interaction between magma and sediment in the shallow subsurface. Deposition occurred by pyroclastic fall and base-surge processes in near-vent settings; most base-surge deposits lack tractional sedimentary structures and are inferred to have formed by suspension sedimentation from rapidly decelerating surges. Complexly deformed pyroclastic strata beneath a distinct truncation surface within the succession record construction and collapse of an initial volcano, followed by a shift in the location of the conduit and excavation of another maar crater into Aguja strata nearby. Preserved portions of the margin of this second crater are defined by a zone of intense soft-sediment disruption of pyroclastic and nonvolcanic strata. U–Pb isotopic analyses of zircon grains from three basaltic bombs in the succession reveal the presence of abundant xenocrysts, in some cases with ages > 1.0 Ga. The youngest concordant analyses for all three samples yield a weighted mean age of 76.9 ± 1.2 Ma, consistent with the presence of Late Campanian vertebrate fossils in the upper Aguja Formation. We infer that the volcanism is related to the intraplate Balcones igneous province, which was emplaced in the same time frame in a marine setting farther east.