Maar Crater Research Articles

Comparative optical and radiocarbon dating of laminated Holocene sediments in two maar lakes: Lake Keilambete and Lake Gnotuk, south-western Victoria, Australia

Sediment core chronologies of optical dates on single-grains/very small aliquots of sand-sized quartz are compared with Accelerator Mass Spectrometry (AMS) radiocarbon (14C) chronologies from ostracod carbonate, mixed carbonates, sedimentary organic matter and charcoal in order to establish the age of laminated Holocene sediments in maar crater lakes Keilambete and Gnotuk, Victoria, Australia. Samples for optical and AMS 14C dating were taken from the same Mackereth cores, allowing a direct comparison of the two techniques from two laminated sedimentary sequences. Additional AMS 14C samples were taken from water in Lake Keilambete and from groundwater discharging into Lake Keilambete from the crater wall, with equivalent reservoir ages of 150 ± 30 and 1940 ± 30 years respectively.AMS 14C dating of modern ostracod carbonate in Lake Keilambete demonstrates a reservoir age of 670 ± 175 years. Optical dating of ‘single-grain/very small aliquots’ of sand-sized-quartz indicate the presence of a radiocarbon reservoir in Lake Keilambete that is consistent with that measured on modern ostracods, and also demonstrate that there is no 14C reservoir in Lake Gnotuk during the Holocene. The chronology presented here supports the premise that previously published bulk conventional 14C dates from Lake Keilambete were affected by old carbon, meaning that past chronologies require revision.Limitations on the use of optical dating of single-grain/very small aliquots include the relative paucity of sand-sized quartz, which decreases the precision of the sample equivalent dose (De), and is further confounded by low environmental dose rates and resultant large uncertainties on the final age assessment. Nevertheless, evidence for partial bleaching confirms that single-grain quartz dating is the most appropriate luminescence technique, and may prove a useful alternative in situations where 14C dating is unsuitable or an alternative chronometer is required.

Origin of the supergiant Olympic Dam Cu-U-Au-Ag deposit, South Australia: Was a sedimentary basin involved?

The supergiant Olympic Dam Cu-U-Au-Ag ore deposit of South Australia occurs in a tectonic-hydrothermal breccia complex that is surrounded by Mesoproterozoic granite. The breccia is composed mainly of granite clasts and minor amounts of Mesoproterozoic volcanic clasts. Very thick (>350 m) sections of bedded sedimentary facies that occur in the breccia complex include laminated to very thin planar mudstone beds, thin to medium internally graded sandstone beds, and thick conglomerate beds. The bedded sedimentary facies extend continuously across a 1.5 km × 0.9 km area and are not limited to small separate maar craters, as previously thought. Detrital chromite and volcanic quartz in the bedded sedimentary facies cannot be matched with local sources, and imply that the provenance extended beyond the area of Olympic Dam. The lateral continuity, provenance characteristics, great thickness, below-wave-base lithofacies, and intracontinental setting suggest that the bedded sedimentary facies are remnants of a sedimentary basin that was present at Olympic Dam prior to formation of the breccia complex. We conclude that the Olympic Dam hydrothermal system operated beneath and partly within an active sedimentary basin, was not confined to maar craters, and did not vent. This new view of the setting raises the possibility that sedimentary processes were involved in ore genesis.

Origins and energetics of maar volcanoes: examples from the ultrapotassic Sabatini Volcanic District (Roman Province, Central Italy)

Maar volcanoes represent a common volcano type which is produced by the explosive interaction of magma with external water. Here, we provide information on a number of maars in the ultrapotassic Sabatini Volcanic District (SVD, Roman Province) as young as ∼90 ka. The SVD maars are characterised in terms of crater and ejecta ring morphologies, eruptive successions and magma compositions, in light of the local substrate settings, with the aim of assessing magma–water interaction conditions, eruption energetics and genetic mechanisms. Feeder magmas spanned the whole SVD differentiation trend from trachybasalts–shoshonites to phonolites. From the ejected lithic fragments from aquifer rocks, the range of depth of magma–water explosive interaction is estimated to have been mostly at ∼400–600 m below ground level, with a single occurrence of surficial interaction in palustrine–lacustrine environment. In particular, the interaction with external water may have triggered the explosive behaviour of poorly differentiated magmas, whereas it may have acted only as a late controlling factor of the degree of fragmentation and eruption style for the most differentiated magma batches during low-flux ascent in an incipiently fragmented state. Crater sizes, ejecta volumes and ballistic data allow a reconstruction of the energy budget of SVD maar-forming eruptions. Erupted tephra volumes from either monogenetic or polygenetic maars ranged 0.004–0.07 km3 during individual maar-forming eruptions, with corresponding total magma thermal energies of 8 × 1015–4 × 1017 J. Based on energy partitioning and volume balance of erupted magmas and lithic fractions vs. crater holes, we consider the different contributions of explosive excavation of the substrate vs. subsidence in forming the SVD maar craters. Following available models based on crater sizes, highly variable fractions (5–50%) of the magma thermal energies would have been required for crater excavation. It appears that subsidence may have played a major role in some SVD maars characterised by low lithic contents, whilst substrate excavation became increasingly significant with increasing degrees of aquifer fragmentation.

Deep explosive focal depths during maar forming magmatic-hydrothermal eruption: Baccano Crater, Central Italy

We describe the eruptive activity of the Pleistocene composite Baccano maar crater in the Sabatini Volcanic Complex (Central Italy) combining stratigraphy, grain size/componentry and rare earth element and Yttrium (REY) composition of its eruptive products with the stratigraphy and geothermal data derived from deep wells drilled on the Baccano structural high. The main lithological characteristics of the basal Baccano maar pyroclastic deposit, composed of more than 60% wt of non-thermometamorphosed lithic clasts from the sedimentary basement, show that the first eruption was magmatic-hydrothermal in nature. The lithology of the sedimentary lithic clasts indicates that the fragmentation level was at a depth of −1,000 to −1,200 m, with fragment depth verified by deep well stratigraphy. The 15% wt juvenile non-vesicular glass components suggest that magma played a minor role in powering the eruption. Assuming that the high-salinity hot hydrothermal fluids (365 −1,400 m) by the aggressive hydrothermal fluids enriched in acid components (HF, HCl, and H2SO4) may have contributed to increased CO2 partial pressure that helped to drive the hydrofracturing. This could have caused rapid vapour separation and pressure drop, allowing the almost simultaneous breaking of the aquifer cover and brecciation of the calcareous units down to −1,000 to −1,200 m depth. The relative abundance of calcareous lithics in the basal part of the first Baccano eruptive unit, representing about the upper 200 m of stratigraphy below the top of the Baccano structural high, reveals the descent of the piezometric surface during the eruption. Combining deep well information and maar product stratigraphy, using also REY data from maar pyroclastic fragments and the basement rocks we draw an interpretative model for the Baccano maar-forming eruption, concluding that a) magmatic-hydrothermal eruptions may originate deeper than previously thought, and b) hydrothermal fluids circulating in limestone aquifers may play an important role in triggering such eruptions.

Models of maar volcanoes, Lunar Crater (Nevada, USA)

Maar volcanoes are generally understood to be the result of highly energetic, explosive interaction between magma and water (groundwater or surface water). Two end-member conceptual models have been proposed to explain the dimensions (diameter, depth) of maar craters: (1) an incremental growth model, where a crater grows due to subsidence and ejection of debris over the course of many explosions, and the final size is an integrated result of multiple explosive events; (2) a model in which the dimensions of a maar crater are the result of the largest single explosion during the lifetime of the maar (major-explosion dominated model). In the latter case, the maar size can be used to estimate the energy and depth of the largest explosion, which in turn allows estimation of the magma mass involved. This paper describes Lunar Crater maar (Nevada, USA) and tests the two models as explanations for the characteristics of the volcano, in particular the major-explosion dominated model. This model implies magma mass and supply rates that are unrealistic, and the tephra at the maar do not contain key features observed in the ejecta at large single-explosion craters. The incremental growth model seems most suitable based upon geological evidence.

Layering and degradation of the Rupes Tenuis unit, Mars – a structural analysis south of Chasma Boreale

Abstract The circum north-polar Rupes Tenuis unit forms the polar-proximal basal stratigraphical and morphological units that delineate the north polar cap between 180° and 300°E. In the region of the mouth of the Chasma Boreale re-entrant, the Rupes Tenuis unit is likely to extend further southwards into the northern plains. This is suggested by the occurrence of isolated remnants that have been interpreted as basaltic shield volcanoes, maar craters or mud volcanoes in the past. As key elements of this study, we assessed the quantitative characteristics of this unit using layer attitudes derived from high-resolution images and terrain-model data, and by performing cross-correlations of prominent layers whose outcrops are observed at eight cone-like remnants. The identification and unambiguous correlation of characteristic layers across the study area provided a reasonable basis for introducing at least three additional stratigraphical subunits of the Rupes Tenuis unit. Extrapolation of altitude data indicates a gentle southward dip of remnant layers, suggesting that the unit had a much larger areal extent in Martian history. The palaeo-layer contact between two subunits of the Rupes Tenuis unit correlates well with elevation values of the Hyperborea Lingula surface. Both results disagree with an interpretation of a volcanic origin for isolated mesas but underpin that they are erosional relicts of the Rupes Tenuis unit. Average erosion rates of 2.5×10 −4 ±4×10 −5 mm year −1 are relatively high when compared to Amazonian rates but are not exceptional for areas undergoing deflation. They also corroborate the idea of aeolian denudation of the Rupes Tenuis unit.

Observations on the limnology and phytoplankton community of crater Lake Kyaninga (Uganda), with special attention to its diatom flora

Background and aims - With a depth of at least 220 in, Lake Kyaninga is the deepest known maar crater lake in western Uganda. We studied its limnology and phytoplankton community to determine how the frequency and depth of water-column mixing influences nutrient cycling and seasonality in this aquatic ecosystem. Methods - Water-column temperature was measured continuously during a full annual cycle between August 2007 and August 2008. Other physical and chemical variables as well as diatom and other phytoplankton communities were investigated on three occasions, namely during the dry season in August of 2007 and 2008, and during the main wet season in April 2009. Key results and conclusions - The water column of Lake Kyaninga is permanently stratified (meromictic) below similar to 100 m depth. Above this depth, mixing frequency varies from daily (down to 8-12 m depth) over at least once per year (down to 39-47 m depth), to once in several years or decades (between 39-47 and similar to 100 m depth). Nutrient and chlorophyll concentrations as well as phytoplankton data classify the lake as low in aquatic productivity (oligotrophic). Its pelagic, open-water phytoplankton community is dominated by Cyanobacteria (blue-green algae) and Chlorophyta (green algae). Bacillariophyta (diatoms) contribute only a minor part of total phytoplankton biomass in both wet and dry seasons, and are characterized by an assemblage of small Nitzschia species. Epiphytic and epipelie diatoms are relatively few, because steep rocky crater slopes limit the littoral zone even though water-column transparency is high. The composition of recently deposited diatom assemblages preserved in offshore surface sediments gives a good, annually integrated representation of the present-day pelagic diatom community. The documented species richness of the diatom flora of Lake Kyaninga is moderate with about 150 taxa. Only similar to 17% of these are biogeographically restricted to tropical Africa; and most of these belong to the genus Nitzschia.

Growth of phreatomagmatic explosion craters: A model inferred from Suoana crater in Miyakejima Volcano, Japan

A subvertical cross section of a maar–diatreme volcano is exposed in the wall of the A.D. 2000 caldera on Miyakejima Volcano. The maar, Suoana, is one of the lateral vents of this volcano and it was inferred to be formed in the 7th century. The subvertical wall of the A.D. 2000 caldera truncated the Suoana maar crater at its center revealing the near comlete cross section of this small maar–diatreme volcano. Exposed in the cross section are a 400 m wide maar crater, an associated tuff ring with a maximum thickness of 20 m, a diatreme extending vertically to a depth of about 220 m from the floor of the maar crater, and a feeder dike connected to the base of the diatreme. The depth of the diatreme structure is about 260 m from the original ground surface. The outline of the diatreme resembles an upward-opening funnel with an almost vertical wall below 560 m asl and an upward flaring wall above 560 m asl. Coarse grained volcanic breccia fills the diatreme, the deposits of which can be divided into 6 units based on lithological and structural characteristics. The upper half of the diatreme is filled with landslide deposits, mainly derived from the surrounding crater wall. The bottom of the diatreme is occupied by massive explosion breccia. Some coherent blocks were detached from the wall of the diatreme and preserved in the diatreme fill. The Y-shaped cross-sectional geometry of the Suoana diatreme is the result of a combination of an underground subsidence in the lower part of the diatreme and the surface landslide in its upper part. The inwardly-inclined stratification of pyroclastic rock units and development of many small faults in the diatreme-filling breccia indicate successive collapse and deformation of these materials within the diatreme during the eruption. The upper part of the diatreme was formed by the landslides of the crater wall induced by the subsidence of the crater floor. Discharge of tephra from the bottom of the diatreme caused infill subsidence, which induced sliding of the inner wall of the crater. As a result, the topographic diameter of the crater became much larger than that of the diatreme itself. The tephra ring surrounding the crater consists mainly of pyroclastic fall deposits.

Emplacement dynamics of phonolite magma into maar-diatreme structures — Correlation of field, thermal modeling and AMS analogue modeling data

Emplacement mode and original shape and dimensions of a well exposed phonolite body in the České středohoří Mountains (Czech Republic) were reconstructed using combined techniques of structural analysis of magmatic fabrics and columnar jointing together with analogue and thermal mathematical modeling of cooling for different shapes of experimental bodies. Phreatomagmatic rocks in the vicinity of some phonolite stocks in the area of interest suggest that the phonolite bodies were likely emplaced into maar-diatremes. Our modeling revealed that intrusion of magma into phreatomagmatic maar-diatreme craters can result in cryptodomes, extrusive domes, lava lakes or branched intrusions. The fabric and columnar jointing pattern of the selected phonolite body reveals best fit with an asymmetric extrusive dome emplaced into the maar crater. The scaling analysis and thermal modeling also suggest that the phonolite extrusion could have formed within 6–66 days and cooled to the background temperature after 10,000 years. Combined analogue and thermal modeling also revealed that the phonolite extrusions into maar-diatreme craters are marked by upper tier (collonade) of vertical columns and lower tier of curved and outward flaring columns. Both tiers in the phonolite extrusions are divided by a subhorizontal suture.

Geophysical survey of the proposed Tsenkher impact structure, Gobi Altai, Mongolia

Abstract– We have performed forward magnetic and gravity modeling of data obtained during the 2007 expedition to the 3.7 km in diameter, circular, Tsenkher structure, Mongolia, in order to evaluate the cause of its formation. Extensive occurrences of brecciated rocks, mainly in the form of an ejecta blanket outside the elevated rim of the structure, support an explosive origin (e.g., cosmic impact, explosive volcanism). The host rocks in the area are mainly weakly magnetic, silica‐rich sandstones, and siltstones. A near absence of surface exposures of volcanic rocks makes any major volcanic structures (e.g., caldera) unlikely. Likewise, the magnetic models exclude any large, subsurface, intrusive body. This is supported by an 8 mGal gravity low over the structure indicating a subsurface low density body. Instead, the best fit is achieved for a bowl‐shaped structure with a slight central rise as expected for an impact crater of this size in mainly sedimentary target. The structure can be either root‐less (i.e., impact crater) or rooted with a narrow feeder dyke with relatively higher magnetic susceptibility and density (i.e., volcanic maar crater). The geophysical signature, the solitary appearance, the predominantly sedimentary setting, and the comparably large size of the Tsenkher structure favor the impact crater alternative. However, until mineralogical/geochemical evidence for an impact is presented, the maar alternative remains plausible although exceptional as it would make the Tsenkher structure one of the largest in the world in an unusual setting for maar craters.

Cladoceran-inferred environmental change during the LGM to Holocene transition from Onepoto maar paleolake, Auckland, New Zealand

Subfossil cladocerans have rarely been used for paleoenvironmental reconstruction from New Zealand lake sediments, and here we detail the first examination of the response of cladocerans to past environments from an Auckland maar paleolake. Cladoceran remains were examined in the upper 230 cm of the lacustrine sediments spanning c. 9–31 cal. ka BP from Onepoto maar during which time the lake underwent significant changes. Lake level was relatively high during the Last Glacial Coldest Period (c. 28–18 cal. ka BP), with limited forest in the catchment, shoreline vegetation and low water temperature indicated by the presence of a planktonic taxon, Bosmina meridionalis, and a cold tolerant chydorid cladoceran Alona sp. (affinis type). However, the climate during the last glacial termination after c. 17.9 cal. ka BP changed abruptly, perhaps becoming extremely dry and windy, resulting in increased production of cladoceran resting eggs. Between c. 17.6 and 14.1 cal. ka BP, a gradually ameliorating climate with sustained windiness and dryness might have been unfavourable for hatching of chydorid cladoceran eggs and their recolonisation. After c. 14.1 cal. ka BP, continued climatic amelioration was accompanied by reduced lake levels inferred from high cladoceran littoral: planktonic ratios as well as increased cladoceran diversity and abundance of less cold tolerant chydorid taxa: Alona guttata, Alona sp. (intermedia type) and Alonella excisa. At c. 9 cal. ka BP, the maar crater rim was breached by the marine waters commensurate with postglacial sea-level rise, resulting in degraded water quality and production of a large number of cladoceran ephippia in the sediments.

Holocene climate change and human impact, central Mexico: a record based on maar lake pollen and sediment chemistry

This paper presents multiproxy (pollen, magnetic susceptibility, sediment chemistry) records from two maar crater lakes in the Valle de Santiago of Guanajuato, Mexico: Hoya San Nicolás and Hoya Rincon de Parangueo. Sediment cores from the two sites have basal dates of ca 11,600 and 9600 cal yr BP, respectively. The San Nicolás results show that the lake desiccated several times during the Holocene, and this resulted in the selective destruction of the less resistant pollen types. A comparative analysis of data from both sites indicates that during the earliest Holocene (ca 11,600–9000 cal yr BP) there were short term oscillations of wetter and drier climate; during the early Holocene (ca 9000–5700 cal yr BP) it was on average wetter; during the middle Holocene (ca 5700–3800 cal yr BP) drier; and during the middle to late Holocene (ca 3800–2200 cal yr BP) wetter. Increasing human disturbance during the late Holocene of environment obscures the climatic record. Agricultural activities in the area began as early as ca 5700 cal yr BP and intensified around 2400 cal yr BP.

The sedimentology and dynamics of crater-affiliated wind streaks in western Arabia Terra, Mars and Patagonia, Argentina

Wind streaks comprise recent aeolian deposits that have been extensively documented on Venus, Earth and Mars. Martian wind streaks are among the most abundant surface features on the planet and commonly extend from the downwind margins of impact craters. Previous studies of wind streaks emerging from crater interior deposits suggested that the mode of emplacement was primarily related to the deposition of silt-sized particles as these settled from plumes. We have performed geologic investigations of two wind streaks clusters; one situated in western Arabia Terra, a region in the northern hemisphere of Mars, and another in an analogous terrestrial site located in southern Patagonia, Argentina, where occurrences of wind streaks emanate from playas within maar craters. In both these regions we have identified bedforms in sedimentary deposits on crater floors, along wind-facing interior crater margins, and along wind streaks. These observations indicate that these deposits contain sand-sized particles and that sediment migration has occurred via saltation from crater interior deposits to wind streaks. In Arabia Terra and in Patagonia wind streaks initiate from crater floors that contain lithic and evaporitic sedimentary deposits, suggesting that the composition of wind streak source materials has played an important role in development. Spatial and topographic analyses suggest that regional clustering of wind streaks in the studied regions directly correlates to the areal density of craters with interior deposits, the degree of proximity of these deposits, and the craters' rim-to-floor depths. In addition, some (but not all) wind streaks within the studied clusters have propagated at comparable yearly (Earth years) rates. Extensive saltation is inferred to have been involved in its propagation based on the studied terrestrial wind streak that shows ripples and dunes on its surface and the Martian counterpart changes orientation toward the downslope direction where it extends into an impact crater.

Volcaniclastic stratigraphy of the Tiscapa maar crater walls (Managua, Nicaragua): implications for volcanic and seismic hazards and Holocene climate changes

The Tiscapa maar in the center of Managua city formed by a phreatomagmatic eruption 60 ka old basaltic–andesitic formation F1 comprises mafic ignimbrites and phreatomagmatic tephras derived from the Las Sierras volcanic complex south of Managua. Formation F2 contains the ~60 ka basaltic–andesitic Fontana tephra erupted from the Las Nubes Caldera of the Las Sierras complex 15 km to the S, the 25 ka Upper Apoyo tephra from the Apoyo Caldera 35 km to the SE, and the Lower (~17 ka) and Upper (12.4 ka) Apoyeque tephras from the Chiltepe volcanic complex 15 km to the NW. These tephras are separated by weathering horizons and paleosols indicating dry climatic conditions. Fluvial deposits of a SSW-NNE running paleo-river system build formation F3. The fluvial sediments contain, from bottom to top, scoriae from the ~6 ka basaltic San Antonio tephra, pumice lapilli from the Apoyo and Apoyeque tephras and the 6.1 ka Xiloa tephra, and scoriae derived from the Fontana tephra. The fluvial sediment succession thus reflects progressively deeper carving erosion in the southern highlands (where a large-amplitude regional erosional unconformity exists at the appropriate stratigraphic level) that began after ~6 ka. This suggests that the mid-Holocene tropical high-precipitation climatic phase affected western Nicaragua about a thousand years later than other circum-Caribbean regions. The end of the wet climate phase ~3 ka ago is recorded by a deep weathering zone and paleosol atop formation F3 prior to the Tiscapa eruption. Formation F4 is the Tiscapa tuffring composed of pyroclastic surge and fallout deposits that cover a minimum area of 1.2 km2. The 4 × 109 kg of erupted basaltic magma is compositionally and genetically related to the low-Ti basalts of the N–S striking Nejapa-Miraflores volcanic–tectonic alignment 5 km to the West of Tiscapa. Ascent and eruption mode of the Tiscapa magma were controlled by the Tiscapa fault that has a very active seismic history as it achieved 12 m displacement in about 3000 years. Managua city is thus exposed to continued seismic and volcanic risks.

Morphotectonic setting of maar lakes in the Campo de Calatrava Volcanic Field (Central Spain, SW Europe)

In the Campo de Calatrava Volcanic Field (CCVF, Central Spain), the eruption of Pliocene–Pleistocene maar craters into two clearly distinct types of pre-volcanic rocks allows the observation and comparison of hard-substrate and soft-substrate maar lakes. Hard-substrate maars formed when phreatomagmatic processes affected the jointed, Paleozoic igneous and metamorphic rocks (hard substrate), giving rise to funnel-like maar lake basins. Soft-substrate maars resulted from phreatomagmatic volcanic processes affecting poorly-consolidated Pliocene sediments, forming bowl-like maar lake basins. Pre-volcanic bedrock determined the post-eruptive lacustrine architecture in the craters and favored a higher preservation of hard-substrate maars in comparison to soft-substrate maars. This is because the hard-substrate maars, surrounded by a deep stable crater wall, are more capable of collecting sediments in their basins. These sediments could be preserved for longer than similar deposits in broad, shallow maars with a soft substrate. Ancient soft-substrate maars do not usually preserve their original morphology well and can be identified only by their lacustrine deposits. Carbonate lacustrine/palustrine deposits surrounding a bowl-like depression are the remnants of this second type of maar lake, and allow reconstruction of the original morphology of ancient soft-substrate maar craters. Geophysical (electrical tomography ground surveys) and geomorphologic–geologic mapping techniques were combined with fieldwork and facies analysis in order to locate and accurately characterize the Pliocene–Pleistocene soft-substrate maar volcanic structures of the CCVF.

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.

The ∼ 1245 yr BP Asososca maar eruption: The youngest event along the Nejapa–Miraflores volcanic fault, Western Managua, Nicaragua

The Asososca Tephra comprises the asymmetrically distributed, well-stratified phreatomagmatic products of the 1245 + 125/− 120 yr BP Asososca maar eruption West of Managua, Nicaragua. It is the youngest eruptive unit related to the monogenetic volcanism concentrated along the N–S trending Nejapa–Miraflores active fault and intersecting E–W to NE–SW and NW–SE minor faults. The close relationship between monogenetic volcanism and the structural-local tectonic framework is evidenced by the lineament domains and vent alignment patterns identified by remotely sensed images and DEMs of the research area. The Asososca maar lake bathymetry suggests that the crater was formed by coalescing vents migrating during the eruptive event. The Asososca Tephra dominantly consists of accidental lithics disrupted from the underlying stratigraphic units observed inside and around the maar crater. Dry base-surge bedsets are dominant throughout the eruptive unit, showing facies variation from proximal, cross-stratified beds to mid-distal plane-parallel and wavy-parallel beds. Grain-size analyses indicate that base surges transported dominantly − 1, 0, and 1ϕ sized particles at the bedload in form of traction carpets, while fine ash material in continuous suspension was minimal. Juvenile fragments were only identified in the finer fractions, varying from vesiculated scoriae to dense sideromelane fragments. SEM analyses suggest that the Asososca eruption resulted from a highly efficient fuel–coolant interaction between a tholeiitic basaltic melt and an aquifer hosted in a shallow level (between 50 and 200 m) of olivine-bearing scoria ash and lapilli beds. The explosion triggered the country-rock comminution and the production of moss-like, fused-shaped, and blocky ash-shards with stepped surfaces, quenching cracks, pitting, alteration skins, and adhered particles, all indicative of phreatomagmatic fragmentation. The very recent age of the Asososca maar eruption confirms that the densely inhabited Managua area is volcanically active and that hazard studies should focus on the potential for phreatomagmatic eruptions.

Stable carbon and oxygen isotopes in sub-fossil Sphagnum: Assessment of their applicability for palaeoclimatology

To investigate the potential of stable isotopes of Sphagnum peat deposits for palaeoclimate research and to inform sampling strategies, we present results from a study of selected Sphagnum plant constituents. We report a combined stable carbon and oxygen isotope record of cellulose separately extracted from Sphagnum branches and stem sections manually sampled from a ∼ 4000 year old peat, deposited in a small dry maar crater located in the Westeifel Volcanic Field, Germany. We have determined the species composition of each individual sample of Sphagnum branches to address the sensitivity of the stable isotope records to potential changes in Sphagnum assemblages. The youngest approximately 3000 years old peat section consists of scarcely decomposed Sphagnum plant material. From the bog's surface to a depth of ∼ 60 cm the predominant species is Sphagnum magellanicum. Between ∼ 60 and ∼ 550 cm the peat predominantly consists of Sphagnum capillifolium var. rubellum. At greater depths the decomposition status increases, species identification is, however, solely achievable if the record under examination consists of moderately decomposed peat. The stable carbon and oxygen isotope values of cellulose from Sphagnum stem sections are significantly lighter than those of the branches. Both isotopic offsets between the different plant compounds exhibit a strong degree of correlation, are statistically highly significant and observable down-core. The stable carbon isotope offset averages to 1.5‰, however, presumably decreases with increasing age of the plant material. In contrast, the averaged oxygen isotope offset of 0.9‰ is consistent in time. Our results imply that if no differentiation into Sphagnum branches and stem sections prior to stable isotope analyses is possible, erroneous interpretations of the isotope records are likely, since down-core changes in the ratio of branches to stem sections in the peat profile are most likely. This also implies that the removal of all non- Sphagnum plants or plant fragments is insufficient to retrieve stable isotope signals from peat deposits exclusively reflecting palaeo-environmental conditions. Even isotopic records from bulk Sphagnum cellulose comprise of two different signals: firstly, an environmental signal based on the plant response to external controls. This signal is, however, masked by a second plant physiological signal originating from the isotopic offset between branches and stem sections.

The Middle-Pleistocene (~300 ka) Rodderberg maar-scoria cone volcanic complex (Bonn, Germany): eruptive history, geochemistry, and thermoluminescence dating

The Rodderberg volcanic complex (RVC) is located within the city limits of Bonn (Germany) approximately 20 km to the north of the Quaternary East Eifel Volcanic Field (EEVF). It is the product of intense phreatomagmatic volcanism forming a 90 m deep maar crater and strombolian eruptions. Deposit features indicate that the location of the vent(s) shifted from N to S during the strombolian phase. The erupted leucite-nephelinite magma (on the order of ca. 1 × 10−2 km3) was largely homogenous with minor, stratigraphically controlled, variation in olivine and clinopyroxene microphenocryst content. Stratigraphic evidence and thermoluminescence dating indicate that the RVC erupted during the glacial MIS 8 at around 300 ka. During this time, the EEVF experienced a transitional stage between two major phases of volcanic activity involving a change in magma sources. This is consistent with the RVC geochemical data which show affinities to both the older EEVF leucite-nephelinite association (430–380 ka) and the younger basanite association (<215 ka). In the Eifel, magma ascent through the upper crust is apparently linked to tectonic fractures. It may be speculated that a tectonically controlled diking event channeled magma to the north of the main EEVF and that the RVC represents an exceptional surficial expression of a significantly larger subsurface intrusion. This scenario would be consistent with recent observations of diking-related volcanism in the East African Rift zone and previously inferred models for magma ascent in similar intraplate volcanic fields.

Reconstructing paleoenvironment, eruption mechanism and paleomorphology of the Pliocene Pula maar, (Hungary)

Pula maar is a partially eroded Pliocene maar-diatreme volcano, part of the Mio-Pliocene Bakony-Balaton Highland Volcanic Field. The surficial remnant of the maar-diatreme volcano consists of (1) a distinct depression with a thick post-eruptive lacustrine alginite sediment infill interbedded with coarse-grained volcaniclastic sediments, (2) a narrow marginal zone inside the depression consisting of primary pyroclastic rock units that are interpreted to be partly collapsed and subsided blocks of entire sections from the tephra ring formerly surrounding the maar crater depression, and (3) coarse-grained volcaniclastic debris-flow deposits closely associated with the collapsed primary pyroclastic rock units in the marginal zone. The presence of coherent lava rocks below the crater-fill units, their distribution pattern and their association with scoriaceous beds indicate that, after the maar-diatreme-forming phreatomagmatic explosive activity, small (100 m-scale) scoria and/or spatter cones erupted in the maar crater. These cones are the likely source of the lava flows that partially filled the maar crater basin. The widespread dm-to-m thick basaltic sand and/or silt units at the base of the post-eruptive crater-filling sedimentary succession are interpreted to be reworked volcaniclastic material from the intra-maar scoria/spatter cones as well as from the tephra ring. Based on comparative analyses of 53 core descriptions, this study reveals that the original maar crater basin was larger than previously suggested. The deep level of the maar crater is reconstructed to be a northeast–southwest elongated depression, currently forming a c. 50-m-deep basin. Geomorphological considerations suggest that most of the phreatomagmatic pyroclastic rocks are composed of base surge and tephra fall deposits around the deep maar depression. These allochthonous rock units form a 50–400 m wide zone of proximal tuff-ring sequences. The formation of this zone is inferred to be a result of a combination of syn-eruptive subsidence due to mass deficit in the rigid Triassic dolomite basement caused by the phreatomagmatic explosions as well as post-eruptive subsidence of the crater- and diatreme-filling successions due to diagenetic compaction. The facies in the centre of the maar lake is a soft laminated “alginite” (mainly Botryococcus colonies, diatom frustles, calcium carbonate crystals, clay minerals). In the section exposed in the Pula open cast mine, a single turbiditic layer is present. This layer originated in a landslide, which possibly could have been caused by either syn-eruptive earthquake and/or a sudden post-eruptive subsidence event of the diatreme fill.