Maar Research Articles

Reappraising the eruptive history of the Alchichica Maar Volcano (Mexico) based on Sr–Nd isotopes: Understanding the role of the magma source region on the growth of small-volume volcanoes

Alchichica Maar is in the eastern sector of the Trans-Mexican Volcanic Belt, where volcanics range from basalts to rhyolites, with 87Sr/86Sr and 143Nd/144Nd values between 0.70293 to 0.70447 (εSr = −21.1 to −4.5) and 0.512733 to 0.512949 (εNd = +3.4 to +6.2), respectively. In this study, Nd and Sr isotopic data and new major and trace elements compositions are provided for the lava flow, the scoria cone, and the phreatomagmatic deposits composing Alchichica Maar Volcano. These results are used in combination with published data to reexamine its eruptive history, though to be formed by more than one eruptive event. The cogeneticy of the magmas that fed the eruptions of these deposits is examined to infer the role of the deep magma source region and post-magmatic alterations on the construction of the Alchichica Maar magmatic system. Alchichica samples present 87Sr/86Sr values between 0.70387 and 0.70447 (εSr = −9.0 to −0.4) and 143Nd/144Nd values from 0.512749 to 0.512773 (εNd = +2.0 to +2.8). While the 87Sr/86Sr values suggest a slight increment in radiogenic 87Sr from the pre-maar lower lava flow to the upper maar eruptions indicative of magma contamination within the upper crust and by hydrodynamic mingling during the phreatomagmatic phases, the slight variation in Nd isotopes suggest an overall homogeneous source where small batches of magmas formed episodically to feed monogenetic eruptions, which subsequently yielded the pre-maar lava flow, the pre-maar surge, the scoria cone, and the maar units. This study reinforces the idea that an already “tapped” source region of a monogenetic volcano may become fertile again, and new eruptions can occur from near or old vent sites. It also highlights the hazardous potential of small-volume volcanoes.

The volcanic architecture and tectono-magmatic framework of the Mount Grace carbonatites, southeastern Canadian Cordillera

The Mount Grace metamorphosed carbonatites (Late Devonian) outcrop as thin (0.5–4 m), laterally discontinuous, strata-bound mappable lenses within the Monashee complex of the southeastern Canadian Cordillera. The host stratigraphic sequence (Monashee cover gneiss) was metamorphosed and deformed in the Late Cretaceous to early Eocene followed immediately by exhumation of the Frenchman Cap and Thor Odin domes. We present seven stratigraphic logs for Mount Grace carbonatites including new and previously described outcroppings spanning ∼30 km. The Mount Grace carbonatite units were deposited regionally within or near the top of a shallow marine sedimentary sequence within miogeoclinal strata of the western margin of paleo-North America (Laurentia). The distribution of the Mount Grace carbonatite lithofacies and the preserved depositional structures and textures suggest that these are pyroclastic deposits resulting from phreatomagmatic eruptions. Our new data enhance the volcanological story with an eruption scenario involving phreatomagmatic reactions and deposition from pyroclastic density currents, sourced from multiple centers within a field of monogenetic maar volcanoes. The distribution of the Mount Grace carbonatites parallel to the western margin of the paleo-North American continent correlates well with regional Late Devonian alkaline magmatism associated with development of an extensional back-arc basin.

A window on the lithospheric mantle beneath the eastern Trans-Mexican volcanic belt: Insights from pargasite-bearing mantle xenocrysts from the Holocene Ocotenco maar volcano

In this short communication, we report and discuss for the first time the occurrence of pargasite-bearing pyroxenite mantle macro-xenocrysts in a basaltic-andesite juvenile material erupted from the Holocene Ocotenco maar at the Serdán-Oriental Basin Volcanic field, situated in the eastern sector of the Trans-Mexican Volcanic Belt. Through an integrated approach based on microtextural observations, mineral chemistry, geothermobarometric modelling, and comparisons with the existing literature, we determined the xenocrysts equilibrated at ca. 0.8 GPa and 1100 °C, compatible with a pargasite-bearing, upper lithospheric mantle setting. The different textures (resorbed vs. unzoned) of xenocrysts, not only provide evidence of a mantle melting event, but also enabled the discrimination between disrupted mantle fragments and peritectic solid phase generated during the pargasite breakdown. Moreover, peculiar characteristics of the xenocrysts such as the dark pargasite rims and the melt embayment in clinopyroxene, together with the lack of phenocrysts from magma fractionation, the swallow-tail texture of groundmass plagioclase, and a vesicle-rich textures, cumulatively suggest a short-term (days to months?) eruptive scenario with a fast melt extraction from the source region to a rapid ascent through the crust and a sudden explosive eruption by the intense interaction of the ascending melt with a source of water close to the surface.

Erratum: New findings on compositionally distinct maar volcanoes: A case study from Acıgöl (Nevşehir) caldera (Central Anatolia, Turkey)

An erratum on: Uslular G, Gençalioğlu Kuşcu G, Bégué F, Ruch J, Lupi M, Higgins O and Caricchi L (2022) New findings on compositionally distinct maar volcanoes: A case study from Acıgöl (Nevşehir) caldera (Central Anatolia, Turkey). Front. Earth Sci. 10:909951. doi: 10.3389/feart.2022.909951An omission to the funding section of the original article was made in error. The following sentence has been added: "Open access funding was provided by the University of Geneva".The original version of this article has been updated.

Lateral Extent of Pyroclastic Surge Deposits at Ubehebe Crater (Death Valley, California) and Implications for Hazards in Monogenetic Volcanic Fields

AbstractHazard assessments in monogenetic volcanic fields require estimates of the runout of pyroclastic surges that result from phreatomagmatic explosive activity. Previous assessments used runout distances of 1–4 km, with large cases up to 6 km. Surge deposits at Ubehebe Crater (∼2100 y.b.p., Death Valley, California) have been traced ∼9 km from the crater center, and likely originally extended 1–3 km farther. There is no evidence that the Ubehebe Crater activity was unusually energetic; rather, its distal deposits are better preserved than those at most maar volcanoes because of its young age and the arid environment. Numerical simulations illustrate how low temperatures facilitate long runout of phreatomagmatic surges due to reduced expansion of entrained air compared to hot surges, allowing cool surges to retain higher densities than ambient air. We suggest that hazard assessments for volcanic fields with phreatomagmatic, maar‐forming eruptions should consider runout distances in the range of 10–15 km.

Pyroclastic deposits of Ubehebe Crater, Death Valley, California, USA: Ballistics, pyroclastic surges, and dry granular flows

Abstract We describe and interpret deposits associated with the final Ubehebe Crater-forming, phreatomagmatic explosive phase of the multivent, monogenetic Ubehebe volcanic center. Ubehebe volcano is located in Death Valley, California, USA. Pyroclastic deposits occur in four main facies: (1) lapilli- and blockdominated beds, (2) thinly bedded lapilli tuff, (3) laminated and cross-laminated ash, and (4) massive lapilli ash/tuff. Lapilli- and block-dominated beds are found mostly within several hundred meters of the crater and transition outward into discontinuous lenses of lapilli and blocks; they are interpreted to have been deposited by ballistic processes associated with crater-forming explosions. Thinly bedded lapilli tuff is found mainly within several hundred meters, and laminated and cross-laminated ash extends at least 9 km from the crater center. Dune forms are common within ~2 km of the crater center, while finer-grained, distal deposits tend to exhibit planar lamination. These two facies (thinly bedded lapilli tuff and laminated and cross-laminated ash) are interpreted to record multiple pyroclastic surges (dilute pyroclastic currents). Repeated couplets of coarse layers overlain by finer-grained, laminated horizons suggest that many or most of the surges were transient, likely recording individual explosions, and they traveled over complex topography in some areas. These two factors complicate the application of classical sediment-transport theory to quantify surge properties. However, dune-form data provide possible constraints on the relationships between suspended load sedimentation and bed-load transport that are consistent using two independent approaches. Massive lapilli ash/tuff beds occur in drainages below steep slopes and can extend up to ~1 km onto adjacent valley floors beneath large catchments. Although they are massive in texture, their grain-size characteristics are shared with laminated and cross-laminated ash facies, with which they are locally interbedded. These are interpreted to record concentrated granular flows sourced by remobilized pyroclastic surge deposits, either during surge transport or shortly after, while the surge deposits retained their elevated initial pore-gas pressures. Although similar surge-derived concentrated flows have been described elsewhere (e.g., Mount St. Helens, Washington, USA, and Soufriére Hills, Montserrat, West Indies), to our knowledge Ubehebe is the first case where such processes have been identified at a maar volcano. These concentrated flows followed paths that were independent of the pyroclastic surges and represent a potential hazard at similar maar volcanoes in areas with complex terrain.

New findings on compositionally distinct maar volcanoes: A case study from Acıgöl (Nevşehir) caldera (Central Anatolia, Turkey)

Recent experiments have largely reshaped our knowledge of maar volcanism. A new evolutionary model promoting the role of explosion depth and vent migration during the formation of maars has provided an alternative approach to previous models. Despite a few attempts to test this model with real cases, there is still a need for field-based studies exploring the depositional characteristics of maars to better understand the factors affecting the model constraints. More investigations on less known felsic maars are required to elucidate the possible differences from their more common mafic counterparts. Here, we explore compositionally distinct monogenetic clusters within the Acıgöl caldera (NW of Central Anatolian Volcanic Province, CAVP), with four felsic maars (İnallı, Kalecitepe, Acıgöl, and Korudağ) and one mafic maar (İcik). Our field observations reveal a successive formation between rhyolitic maars and adjacent lava domes. The mugearitic İcik coalescent maar and the adjacent scoria cone are synchronously formed, which is disclosed by the intercalation of the maar and scoria cone deposits. The geochemistry of the maar juveniles suggests a parental basaltic magma source that has been possibly differentiated by varying degrees of fractional crystallization. Our findings identify the main factors in maar formation (i.e., optimum scaled depth-OSD, water-magma interaction, and basement lithology). We also determine some differences between the felsic and mafic maars in the region, such as higher juvenile content, less amount of sedimentary structure, and a clear transition from phreatomagmatic to magmatic explosions through the end of stratigraphy. All these reflect the complexity of maars, mainly formed by different depths of explosions that occurred in the shallowest few hundred meters rather than a systematic incremental decrease of the eruption locus. Further geophysical and geochronological studies will complete our proposed evolutionary model for the youngest monogenetic activity in the Acıgöl caldera that would also warrant volcanic hazard assessment due to the presence of low-velocity anomalies, shallow Curie depths, and prevalent geothermal activity.

ICDP drilling of the Eger Rift observatory: magmatic fluids driving the earthquake swarms and deep biosphere

Abstract. The new in situ geodynamic laboratory established in the framework of the ICDP Eger project aims to develop the most modern, comprehensive, multiparameter laboratory at depth for studying earthquake swarms, crustal fluid flow, mantle-derived CO2 and helium degassing, and processes of the deep biosphere. In order to reach a new level of high-frequency, near-source and multiparameter observation of earthquake swarms and related phenomena, such a laboratory comprises a set of shallow boreholes with high-frequency 3-D seismic arrays as well as modern continuous real-time fluid monitoring at depth and the study of the deep biosphere. This laboratory is located in the western part of the Eger Rift at the border of the Czech Republic and Germany (in the West Bohemia–Vogtland geodynamic region) and comprises a set of five boreholes around the seismoactive zone. To date, all monitoring boreholes have been drilled. This includes the seismic monitoring boreholes S1, S2 and S3 in the crystalline units north and east of the major Nový Kostel seismogenic zone, borehole F3 in the Hartoušov mofette field and borehole S4 in the newly discovered Bažina maar near Libá. Supplementary borehole P1 is being prepared in the Neualbenreuth maar for paleoclimate and biological research. At each of these sites, a borehole broadband seismometer will be installed, and sites S1, S2 and S3 will also host a 3-D seismic array composed of a vertical geophone chain and surface seismic array. Seismic instrumenting has been completed in the S1 borehole and is in preparation in the remaining four monitoring boreholes. The continuous fluid monitoring site of Hartoušov includes three boreholes, F1, F2 and F3, and a pilot monitoring phase is underway. The laboratory also enables one to analyze microbial activity at CO2 mofettes and maar structures in the context of changes in habitats. The drillings into the maar volcanoes contribute to a better understanding of the Quaternary paleoclimate and volcanic activity.

Hydro-Volcanism in the Longgang Volcanic Field, Northeast China: Insights from Topography, Stratigraphy, Granulometry and Microtexture of Xidadianzi Maar Volcano

Hydro-volcanism in the Longgang volcanic field (LVF) of Northeast China has produced a dozen maars with features of complex sequences. To better understand the formation mechanism of maar volcanos in the LVF, this study focuses on the Xidadianzi (XDDZ) maar volcano, located in the Jinchuan valley of the LVF. Based on detailed stratigraphy analysis, 14C geochronology, grain-size distribution, and scanning electron microscopy (SEM) analysis, the eruptive sequence of the XDDZ volcano, including the South Crater and the North Crater, was constructed. The whole sequence was formed after four eruptive phases, including a wet phreatomagmatic eruption, an explosive magmatic eruption, a dry and hot phreatomagmatic eruption, and a small explosive magmatic eruption. 14C geochronology indicates that the formation age of XDDZ is 15,900 ± 70 years, BP. Topographic and stratigraphic characteristics show that the landforms of two craters were damaged and buried because of the destruction of lava flows and agricultural modification. The NE- trending fissure in the hard rock area is thought to participate in the formation of the XDDZ maar volcano.

Morphological and textural analysis of basaltic pyroclasts (Atexcac maar, central Mexico): Implications for fragmentation and conduit processes

Atexcac is a maar volcano, which forms part of the monogenetic volcanic field of the Serdán-Oriental Basin, in the eastern sector of the Mexican Volcanic Belt. This volcanic landform was originated by a combination of phreatic, magmatic and phreatomagmatic explosive eruptions. During the formation of the distinct depositional facies, fluctuations in the water availability of the local aquifer, as well as in the magma flux, were probably responsible for the different mechanisms of magmatic and phreatomagmatic fragmentation and the production of the two main populations of fragments (breccia and cross-bedded deposits particles), whose morphological features, allow us to infer the dominant regime (brittle or ductile), fragmentation mechanism (magmatic or phreatomagmatic) and conditions of bubble growth dynamics and fragmentation processes during the ascent of magma through the conduit. We conclude that the breccia particles were derived from dominant magmatic explosions while the cross-bedded deposits particles are thought to be generated from explosive phreatomagmatic events. 3D imaging based on high-precision X-ray microtomography was used to determine the vesicularity of juvenile fragments of the Atexcac maar. Calculated vesicularity index, vesicle number density (VND) and vesicle morphology indicate important processes of coalescence of gas bubbles, during the ascent of the magma. VND's Atexcac clasts suggests vesicle mechanisms similar to medium-intensity eruptions, reported in the literature. The elongated and polylobular vesicle morphology of the Atexcac clasts reveals early acceleration of magma and subsequent coalescence events at most stages of gas bubble growth.

Tephrostratigraphy and morphometry of Wum Maar Volcano (Oku Volcanic Group- Cameroon Volcanic line): Implications for complex monogenetic volcanoes

Field investigations of tephra deposits and morphometric characterisation were used to constrain the eruptive history and the complexity of the Wum Maar Volcano (WMV) on the Northwestern foot of the Oku Volcanic Group (Cameroon Volcanic Line). The WMV is a compound volcanic edifice made up of an irregular horse-shoe-shaped maar-crater bordered by overlapping scoria cones and a small lava flow field. The bulk ejecta volume computed using ArcGIS.10.3 and field topographic measurements is ∼0.133 km3. The deposits were subdivided into older volcanics, a pre-maar member composed of four Strombolian scoria cones and a maar-member made up of interbedded layers of lapilli-tephra, lapilli-ash, and ash-breccia deposits. The bedding characteristics (structure, bedding dip, direction, attitude, and contact relations) coupled with the componentry analysis of maar deposits indicate phreatomagmatic-derived fallouts and pyroclastic density currents emplaced under both dry and wet conditions due to variations in magma-water ratio during the different phreatomagmatic explosions. The thick paleosol between older volcanics and the pre-maar and the maar deposits suggests that before the formation of the WMV, an eruptive activity took place and comprised an effusive outpouring of a basaltic magma batch that exploited the network of fractures within the basement rocks to reach the surface. Long after this stage, other batches of basanitic and trachybasaltic magmas richer in volatiles erupted explosively (Strombolian eruptive dynamism) around the northern and southern rims of the maar crater, leading to the emplacement of several overlapping scoria cones. The last stage was characterised by the formation of the Wum maar crater through a phreatomagmatic explosion when batches of basanitic magma came in contact with groundwater. The volcanics are sub-aphyric and host mantle xenoliths. Although evenly distributed across the different maar units, these mantle xenoliths are frequent and large enough to suggest a rapid magma ascent. The center of the maar crater compared to the location of the scoria cones suggest that the explosion locus migrated eastwards. The relative proportion of lithic granite within the pyroclastic succession increases upward (highest in MU4), and the depth of the maar varies from <10 m at the western border to about 124 m at the eastern one, suggesting a vertical and a lateral migration of the explosion locus. The intensity of the phreatomagmatic explosion reduced with time as indicated by the clast-size distribution, with a progressive reduction in water availability wherein the eruption became more internally controlled, changing in style from phreatomagmatic to Strombolian at the end. We conclude that even with very small magma volumes, monogenetic eruptions can have very complex histories, as shown by the WMV, because of the taping system, multiple vents, change in eruptive style, vertical and lateral migration of eruption foci, and multiple magma batches.

ВРЕМЕННЫЕ ИМПУЛЬСЫ КАЙНОЗОЙСКОГО ЭКСПЛОЗИВНО-ФРЕАТИЧЕСКОГО ВУЛКАНИЗМА В ЮГО-ЗАПАДНОМ ПРИМОРЬЕ. КОРРЕЛЯЦИЯ ИЗОТОПНЫХ И ФИТОСТРАТИГРАФИЧЕСКИХ ДАТИРОВОК

The Cenozoic tephra deposits - products of explosive phreatic eruptions of maar volcanoes in the southwest of Primorye are studied. The deposits represent rhyolitic ash and pumice pyroclastic beds with a high terrigenous component, including tephroid pseudo-conglomerates. Isotopic dating of tephra beds established two time pulses of explosive volcanism: 30–34 Mya and 23–24 Mya. The first time pulse coincided with the beginning of the formation of marginal seas and continental coal basins. It corresponds to the most productive stage of coal accumulation, the burial of wood wastes and their coalification at a faster rate, and the development of a high-temperature geothermal field and can be compared with the well-known catastrophic eruption of Mount St. Helens in the U.S.A. The second time pulse of explosive volcanism had a regional character of manifestation. It is characterized by the formation of green tuff complexes on submarine elevations of the Sea of Japan as well as along the western and eastern coasts of Japan. Synchronously with the volcanic activity started the acceleration of the sinking rate of the Sea of Japan bottom in response to the active rising of asthenospheric diapirs. The established isotopic ages do not conform to the ages determined for fossil leaves and pollen from the deposits, which may reflect the climate-forming type of such an explosive process.

Volcanic geology of the easternmost sector of the Trans-Mexican Volcanic Belt, Mexico

ABSTRACT This work presents the volcanic geology of the easternmost sector of the Trans-Mexican Volcanic Belt, including the Serdán-Oriental basin and Cofre de Perote-Citlaltépetl volcanic range, two contrasting Quaternary volcanic fields. The first comprises dominant monogenetic volcanism of bimodal composition including isolated rhyolitic domes and tuff rings, basaltic andesite maar volcanoes, cinder and lava cones, and an active caldera complex (Los Humeros). The second is dominated by large composite polygenetic volcanoes of andesitic-dacitic composition, including the shield-like compound Cofre de Perote volcano, La Gloria and Las Cumbres complexes, and the active Citlaltépetl stratovolcano. Mapping units include a pre-volcanic basement made of metamorphic (Paleozoic), and sedimentary (Jurassic-Cretaceous); intrusive (Miocene) rocks; multiple basaltic-andesitic lava flows and rhyolitic domes; volcaniclastic sequences (debris avalanches and lahars; and pyroclastic deposits (block-and-ash flows, ignimbrites, fallouts, and surges) (Miocene-Holocene). This map provides a comprehensive view of the highly diverse volcanism, which may reference for future research work.

Lateral migration of explosive hazards during maar eruptions constrained from crater shapes

Maar volcanoes are produced by subsurface phreatomagmatic explosions that can move vertically and laterally during an eruption. Constraining the distances that maar-forming explosions move laterally, and the number of relocations common to these eruptions, is vital for informing hazard scenarios and numerical simulations. This study uses 241 intact Quaternary maar crater shapes to establish global trends in size and spacing of explosion position relocations. Maar craters are sorted into shape classes based on the presence of uniquely identifiable combinations of overlapping circular components in their geometry. These components are used to recognize the minimum number of explosion locations responsible for observed crater shapes. Craters with unique solutions are then used to measure the size and spacing of the explosion footprints, the circular area of the largest crater produced by a single explosion of a given energy, that produce the crater shape. Thus, even in the absence of abundant observations of maar-type eruptions, the typical range, size and spacing of explosion positions are derived from maar crater shapes. This analysis indicates that most Quaternary maar eruptions involved at least three different explosion locations spanning distances of 200–600 m that did not always follow the trend of the dike feeding the eruption. Additional evaluation of larger maars, consistent with stratigraphic studies, indicates that centers of explosive activity, and thus the origin of ballistic and density current hazards, can move as many as twenty times during a maar-forming eruption. These results provide the first quantitative constraints on the scale and frequency of lateral migration in maar eruptions and these values can directly contribute to hazard models and eruption event trees in advance of future maar-type eruptions.

Features That Favor the Prediction of the Emplacement Location of Maar Volcanoes: A Case Study in the Central Andes, Northern Chile

Maar volcanoes are monogenetic landforms whose craters cut below the pre-eruptive surface and are surrounded by a tephra ring. Both the maar crater and the surrounding tephra rim deposits are typically formed due to magma–water explosive interactions. Northern Chile is located in the Central Volcanic Zone of the Andes where, in literature, 14 maars have been recognized as parasite (6) and individual (8) volcanoes. Amongst these individual maars, 3 of them, namely the Tilocálar Sur, Cerro Tujle, and Cerro Overo volcanoes, are not related to calderas and were emplaced &lt;1 Ma ago by magmatic explosive-effusive and phreatomagmatic eruptions. Based on the evolution and control of the volcanic eruptive styles of these three maars, which have been determined in previous research through fieldwork, stratigraphic, morphometric, textural (density and vesicularity), petrographic, and geochemical analyses, a set of key features that favor a prediction of the emplacement location of maar volcanoes in Central Andes, northern Chile are proposed. The set of features that permit and favor the growth mechanisms for maar formations corresponds to (i) a compressive tectonic setting (e.g., ridge structures), (ii) groundwater recharge (e.g., snowmelt and seasonal rainfall), (iii) the lithological setting (e.g., layers of low permeability), (iv) the presence of aquifer and/or endorheic basins (e.g., lakes or salars), and (v) a period of stress relaxation that permits magma ascent to the surface in volcanically active areas. Considering these characteristics, it is possible to identify places where phreatomagmatic eruption can occur. If the magma ascent flux is lower than the groundwater flux, this can lead to a phreatomagmatic eruption because of groundwater coming into contact with the magma. These eruptive features evidence internal—and external—factors that play an essential role in the transition from explosive-effusive magmatic to phreatomagmatic volcanic eruption styles during the same eruptive period that is one of the biggest challenges in volcanic hazard evaluations. Although, in this contribution, a set of features that permit and favor the growth mechanisms for a prediction of the emplacement location of maars in northern Chile is proposed, these considerations could also be applied to identify potential locations in other parts of the world where magma–water interaction eruption could occur. Therefore, this approach could be useful in the prediction of hydromagmatic volcanic eruptions and, thus, in mitigating the impact of volcanic hazard for the inhabitants of the surrounding areas.

Geochemical characterization of alchichica maar volcano, Serdán-oriental Basin, eastern Trans-Mexican Volcanic Belt: Insights on polymagmatic evolution at monogenetic volcanic clusters

Monogenetic volcanic fields usually comprise short-lived isolated volcanoes. However, new vents might build over or near to the older ones, resulting in the construction of complex, compound, nested, or overlapping volcanic structures formed by the superposition of individual monogenetic edifices. In the present study, we examine the case of Alchichica Maar volcano that cut through a series of pre-volcanic materials, including sheets of lava flows, pyroclastic deposits, paleosols, and scoria cone deposits. Major and trace elements are used to characterize geochemically each of the different pre-maar and maar units that integrate the Alchichica volcano, allowing to investigate the source composition, syn-eruptive, and crustal magmatic processes and the genetic relationship between the Alchichica maar and the underlying volcanic rocks. Typical lithologies are medium-K calc-alkaline rocks, including basaltic andesites for the lava flow and the scoria cone, and basalts for the maar units and the pre-maar pyroclastic unit. They also present slight variations in La/Sm, Ba/La, Zr/Nb, Ce/Yb, Ce/Pb, and Dy/Yb ratios suggesting different petrological processes such as variable amounts of partial melting in a heterogeneous spinel mantle, fractional crystallization, and crustal contamination during magma ascent. Our results reveal a polymagmatic activity, with different magma batches fed the pre-maar and the maar eruptions. They also allowed depict and confirm a time gap between the eruptive phases of the Alchichica maar formation, which is an add on the polycyclic character of the phreatomagmatic volcanism at this maar and previously described with the volcanostratigraphy examination. In addition, we conclude that the Alchichica maar cannot be defined as a polygenetic volcano; instead, it forms part of a cluster of Late Pleistocene to Holocene overlapped monogenetic volcanoes.

Fluid inclusion systematics in porphyry copper deposits: The super-giant Grasberg deposit, Indonesia, as a case study

Fluid inclusion data provide a unique insight into the behaviour of magmatic fluids as they expand from source to surface to form porphyry copper deposits. For example, saline, liquid phase inclusions in combination with vapor phase inclusions help to define the environment of mineralization in terms of large scale, two-phase fluid flow within a magmatic vapor plume. However, the pressure at which ore formation occurs within the plume is controversial because inclusion-based estimates are commonly much higher than those based on geological estimates. Uniquely, the Grasberg supergiant porphyry copper deposit occupies the breccia-filled diatreme underlying the crater of a maar volcano and preserved a lacustrine-volcaniclastic sequence that marks the paleo-surface to within a few hundred meters. Interpreting fluid inclusion data with respect to this paleo-surface shows that porphyry mineralization extended from a paleo-depth of about 500 m to over 1500 m depth, as defined by drilling.The Grasberg deposit has three distinct populations of fluid inclusions (low salinity, hypersaline liquid and multi-solid inclusions) that are strongly clustered with respect to salinity and trapping temperature. Similar clusters of co-existing liquid- and vapor-rich inclusions are also common to porphyry copper deposits worldwide. Applying the principles of energy and mass conservation, a simple model of isenthalpic phase separation by condensation may be derived to account for these data clusters and how they are related. Vapor- and liquid-rich hypersaline inclusions may be shown quantitatively to be the consequence of expansion of a continuum low salinity, low density magmatic vapor phase to a two-phase fluid where the liquid fraction is minor (<2 vol%) and is best considered to be an aerosol or fog. The clustering of the inclusion data is the product of the different ranges of pressures that fluids encountered from the initiation of individual stockwork fractures through to their resealing.The application of mass and energy conservation principles also shows that, even though metal partitioning to the liquid phase may be favored, the proportion of total metal fractionated to the small mass of liquid is low relative to that remaining in the vapor phase. Therefore, while partitioning of metals between liquid and vapor phases may be a contributing process for metal deposition in porphyry copper deposits, other vapor-phase processes are required to efficiently deposit these major metal resources.

Stratigraphy and evolution of the Holocene Aljojuca Maar volcano (Serdán-Oriental basin, Eastern Trans-Mexican Volcanic Belt), and implications for hazard assessment

Aljojuca maar is located in the eastern Trans-Mexican Volcanic Belt, within the Serdán-Oriental Basin, which is characterized by Quaternary bimodal monogenetic volcanism. Aljojuca maar has an irregular shape, with an eastern embayment structure that forms an E-W alignment with at least three older scoria cones to the east. The crater walls expose a sequence of pre-maar volcaniclastic deposits, with a lava flow linked to the older scoria cones and a paleosol at its top; the latter indicates that a significant hiatus occurred between the growth of the scoria cones and the eruption of the Aljojuca maar, whose deposits overlie the soil. Strong interactions of the ascending magma with groundwater led to the emplacement of a ~30-m-thick maar sequence. Our detailed stratigraphic study shows that the eruption can be grouped into two main stages, comprising eight stratigraphic units deposited during five eruptive phases. The first stage reflects the development of the main crater (a maar sensu stricto) and is recorded by six stratigraphic units (from A to F). The eruption began with a strombolian pulse, followed by phreatomagmatic activity (unit A), forming dilute pyroclastic density currents. In the second phase, phreatomagmatic explosions prevailed with different degrees of fragmentation, from juvenile and lithic-rich lapilli fallout (unit B), to less diluted pyroclastic density currents (unit C), and a spatially restricted strombolian pulse. The third phase includes a main-vent enlargement event represented by a chaotic and heterolithic breccia (unit D). In the fourth phase, dilute pyroclastic density current deposits (unit E) dominate, with minor chaotic breccias (unit F). The second stage (fifth phase) includes the occurrence of a possibly fissural E-W structure, which formed a scallop in the eastern flank of the maar (eastern embayment structure), breaking the previously formed main crater's eastern flank and producing a 4-m-chaotic breccia (unit G) followed by the occurrence of a dilute pyroclastic density current (unit H), whose energy enabled the current to climb to the closest scoria cone. The migration of the explosive loci toward the eastern flank of the maar reflects a tectonic control by the dominant regional east-trending structural pattern. Several radiocarbon ages support that the eruption of Aljojuca took place between 2.7 ka (age of charcoal fragments under the ejecta ring) and ~2.2 ka (age of the oldest lake sediments in the maar crater). This has important implications for the hazard assessment of volcanism in this area, which had not been considered previously as a potentially active system.

Late Pleistocene to Holocene activity of Alchichica maar volcano, eastern Trans-Mexican Volcanic Belt

Detailed volcano-stratigraphy and 40Ar/39Ar dating are used for the first time to reconstruct the eruptive evolution of the Alchichica maar and to contribute to the understanding of maar volcanism within the eastern Trans-Mexican Volcanic Belt (TMVB). The maar excavated in an area where previous eruptions included Hawaiian, Strombolian, and phreatomagmatic types. Alchichica is characterized by an irregular shape tephra ring with a bulk volume of ∼0.088 km3 based on high-resolution digital terrain model calculations. The maar deposits comprises two stratigraphic units (lower and upper) with contrasting lithofacies characteristics suggesting different depositional conditions and variations in eruption behavior. These units correspond to: (1) vent-clearing stage with explosions producing a succession of massive tuff-breccia intercalated with massive lapilli-tuffs; (2) pulsating phreatomagmatic eruptions producing a series of accretionary lapilli-rich tuffs that alternate with dominantly well-stratified tuffs and lapilli-tuffs deposited from pyroclastic density currents. The geochronological data provide first constraints about the young age (Late Pleistocene-Holocene) of the Alchichica maar. The presence of the maar and some of the other pre-maar eruptions suggest long-lasting dispersed volcanism in a well-defined, but small area, where rejuvenated monogenetic vents cut into earlier ones, having significant implications for hazard assessments in the region.

Maar Stories: How Oral Traditions Aid Understanding of Maar Volcanism and Associated Phenomena during Preliterate Times

Ancient stories recalling catastrophic events were developed, sometimes encoded in myth, and passed down across several millennia in largely oral contexts. Volcanism is well suited to such stories and there are examples of extant stories recalling eruptions that occurred several millennia ago. This study focuses on a subset of these stories—those that recall the formation and subsequent (hazard-related) manifestations of maar volcanoes. Because these form as a result of the mixing of magma and groundwater, which produces explosive phreatomagmatic eruptions, they are among the most memorable catastrophic volcanic phenomena. Ancient stories recalling maar formation are known from Australia where cultural isolation for most of the past 65,000 years explains the extraordinary longevity and replication fidelity of such stories. Stories referring to the postformation developments of maars from Lake Albano in Italy are also described, together with less readily interpreted stories from elsewhere. Motif analysis suggests that preliterate peoples incorporated their observations of maar formation into stories as the shrieks of birds (escaping gas) and the approach of demons (eruptions), as well as narrative details such as the sky turning red and the ground surface twisting and cracking. Motifs referring to posteruption activity at maars include those that recall craters filling with water and ones that recall associated breaches of crater rims, lahars, and flooding downslope. The existence of maar stories of the kinds described and their demonstrable potential for adding detail and explanation to particular events several millennia ago should encourage geographers to treat such information sources with more respect than has been customary. Key Words: lahar, local knowledge, maar, maar lake, oral traditions, volcanism.