Maar Eruptions 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.

Characteristics of pyroclastic deposits and evolution of Maar Bethok Lamongan Volcano Tiris Probolinggo East Java Indonesia

The Lamongan Volcano Complex has a very different character from the Bromo and Argopuro Volcano groups. Lamongan Volcano has 37 volcanic cones and 27 maars (ranu). This volcano is quarter-old and occupies part of East Java Province. Maar Bethok is composed of lava and intercalation between pyroclastic deposits from phreatomagmatic and magmatic falls that still need to be firmly consolidated. These pyroclastic deposits characterize a strombolian eruption with more lava dome or crater wall material ejected. The rocks that make up the Bethok maar include olivine-pyroxene basalt lava and pyroclastic fall deposits with fragments of olivine basalt and basalt composition. The evolution of the Bethok maar eruption started from magma with an alkaline composition and ended with a more acidic composition. The magmatism of Bethok maar has an island arc calc-alkaline affinity.

Relation between Central European Climate Change and Eifel Volcanism during the Last 130,000 Years: The ELSA-23-Tephra-Stack

The analysis of tephra layers in maar lake sediments of the Eifel shows 14 well-visible tephra during the last glacial cycle from the Holocene to the Eemian (0–130,000 yr b2k). These tephra were analyzed for their petrographic composition, which allows us to connect several tephra to eruption sites. All tephra were dated by application of the ELSA-20 chronology, developed using the late Pleistocene infilled maar lake of Auel and the Holocene lake Holzmaar (0–60,000 yr b2k). We extend the ELSA-20 chronology with this paper for the millennia of 60,000–130,000 yr b2k (ELSA-23 chronology), which is based on the infilled maar lake records from Dehner, Hoher List, and Jungferweiher. The evaluation of the tephra from the entire last glacial cycle shows that all 14 tephra were close to interstadial warming of the North Atlantic sea surface temperatures. In particular, phreatomagmatic maar eruptions were systematically associated with Heinrich events or C-events. These events represent times of warming of the Southern Hemisphere, global sea level rise, and CO2 increase, which predate the abrupt interstadial warming events of the Northern Hemisphere. This synchroneity indicates a physical relationship between endogenic and exogenic processes. Changes in the lithospheric stress field in response to changes in continental ice loads have already been suggested as a potential candidate to explain the exogenic forcing of endogenic processes. The chronology of volcanic activity in the Eifel demonstrates that intraplate mantle plumes are also affected by the exogenic forcing of endogenic processes.

Two Small Volcanoes, One Inside the Other: Geophysical and Drilling Investigation of Bažina Maar in Western Eger Rift

AbstractMaar‐diatreme volcanoes are small volcanic structures with a funnel‐shaped crater surrounded by a tephra‐ring. They are usually formed by the explosive phreatomagmatic eruptions when groundwater comes into the contact with magma. We focus on such a structure in the geodynamically active western Eger Rift (Czech Republic) and present results from multidisciplinary geophysical investigation calibrated by drilling in the newly discovered Pleistocene Bažina maar. We evaluated morphological (LiDAR‐based DEM) data and confirmed the existence of a maar‐diatreme structure by combined geophysical methods. In the map view, they revealed circular negative gravity anomaly, funnel‐shape low‐resistivity anomaly, and strong magnetic anomaly. These results allowed for the optimal location of two boreholes in the maar crater, which evinced its contact with country rocks and lithologies of the maar‐diatreme filling. The drilling revealed coherent volcanic rocks and volcaniclastic deposits, moreover, it revealed a presence of a pyroclastic cone with the olivine nephelinite feeding conduit. Further investigations disclosed maar structure and subsequent pyroclastic cone(s) with several generations of eruptions and systematic decrease of water influence on the eruption style. Different eruption styles suggest a unique evolution of two volcanoes, one inside the other. The age of the Bažina maar eruption, estimated from the reverse polarity of the detected magnetic anomaly, implies that the effusion and solidification of the lava during the eruption must be older than 0.78 Ma (Pleistocene). This points to an active volcanism in the western Eger Rift in a span of ∼0.5 Ma, where Bažina represents the oldest (maybe opening) phase.

Stratigraphy and eruption history of maars in the Clear Lake Volcanic Field, California

The Clear Lake Volcanic Field (CLVF) is the northernmost and youngest field in a chain of volcanic fields in the California Coast Range mountains. Effusive and explosive volcanic activity in the field has spanned at least 2.1 million years, with the youngest eruptions comprising a series of maar craters at the edges of, and within, Clear Lake itself. This work documents the first direct ages for many of these maar deposits and builds the stratigraphic basis for interpreting eruptive processes and dynamics of the young eruptions which produced them. Detailed stratigraphy has distinguished maar eruption products from pyroclastic deposits (monolithologic falls and flows, previously mapped together with maars as a single unit) and established a set of six eruption facies from maar deposit lithology, grain size parameters, and depositional structures. Radiocarbon dates from carbon films found on clasts at three outcrops have constrained several of these maar eruptions to ∼8,500–13,500 years BP, coinciding with eruptive periods previously estimated based on lake core tephrachronology. Part of this period also coincides with indigenous inhabitation (<12,000 years BP), which suggests that oral histories of Pomo and other local tribes may contain descriptions of volcanic phenomena experienced by local residents of the CLVF. Collaboration between volcanologists and indigenous historians may add a valuable human dimension to the youngest eruptions of the Clear Lake Volcanic Field, and help inform future volcanic hazard assessment.

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.

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.

Thermo-hydrologic processes in maar eruptions: The role of vapor transport and condensation

Thermo-hydrologic processes in maar eruptions: The role of vapor transport and condensation

Sub‐tephra archaeology and geochemistry at La Arenera, Managua, Nicaragua form a record of human activity before the Asososca Maar eruption of 2130 BP

AbstractWe recently investigated an occupational surface that lies at the base of a deep deposit of volcanic sand emplaced approximately 2130 years ago in what is now a western suburb of Managua, Nicaragua. LA4, an area of 25 × 18 meters from which overlying sand had been removed by quarrying, was cleaned so that features and artifacts could be point plotted and sediments from the pre‐eruption surface could be sampled. Portable x‐ray fluorescence analysis of surface sediments detected patterns of elemental enrichment that are interpretable as signals of pre‐eruption human activities. The geochemical data combined with feature and artifact distributions support several inferences about how people were using the space on the eve of the 2130 BP volcanic event. A small test excavation confirmed the presence of a shallow but moderately dense primary refuse deposit containing sparse but varied faunal remains, including marine species. The ubiquity of artifact scatters on other exposures of the sub‐tephra surface suggests that the site covers at least 96 hectares, and we speculate that more intact and denser deposits than LA4, possibly including structures and agricultural features, may eventually be encountered.

Linking the Ukinrek 1977 maar-eruption observations to the tephra deposits: New insights into maar depositional processes

The Ukinrek Maars erupted 30 March to 9 April 1977, forming two maars, a line of small pit craters and a tephra blanket extending to ~2 km from the vents. We combine photographic and written observations with stratigraphic analysis to reconstruct the eruption. The eruption began with very low (a few meters high) fountaining from small craters above an inferred east-west-trending dike, creating local scoria/spatter agglomerate ramparts with a sandy matrix. The eruption very quickly (in minutes to hours) centered on the West Maar. The West Maar eruption lasted 1–2 days, starting and ending with phreatomagmatic explosions with weak phreato-Strombolian activity in between. Initial explosions formed a 30-m-wide crater, enlarged by crater-wall collapse, and columns as high as 6500 m. Phreato-Strombolian activity produced ~72% of the erupted volume, including a small spatter cone and a scoria blanket around the vent. A final explosion series emplaced a lithic-rich breccia as ballistic blocks, possibly as the northern half of the final crater collapsed into the southern vent area. The East Maar formed over the last nine days of the eruption and represents ~93% of the total volume (4.6 × 106 m3) of the Ukinrek eruption. Initial explosions were probably shallower than 10–20 m but most of the eruption occurred from explosions at 50–60 m below the pre-eruptive surface, with evidence of explosions to 90 m depth only at the very end of the eruption. The East Maar eruption mostly produced columns of lapilli, ash, and steam and the deposits are mostly fallout. Winds blew fallout mostly to the north for the first 5–6 days and to the south for the last three days of the eruption. Wind-directed pyroclastic density currents collapsed from the column, producing fines-rich layers within the coarser fallout. Sporadic explosions produced weak density currents in the first few days and lithic-and juvenile-block-rich breccias in the last few days of the eruption. We interpret that collapse of the crater walls made a slurry that in part provided the water for phreatomagmatic interaction. Explosions came from depths <90 m below the pre-eruptive surface except for a few explosions at the end of the eruption, with most occurring at <70 m depth. The East Maar crater was open to 40–60 m depth throughout most of the eruption, so the explosions were rarely, if ever, deeper than 30 m below the crater floor. Thus, we infer there is no classic, well-formed diatreme structure below the maar. Collapse of the East Maar crater walls provided a supply of water-saturated sediment for much of the phreatomagmatic activity, which came from two vents that did not migrate much, if at all, during the eruption. The Ukinrek Maars deposits were nearly entirely emplaced by fallout, rather than density currents, from explosions and low columns.

New thermoluminescence age estimates for the Nyos maar eruption (Cameroon Volcanic Line).

Nyos maar is located in the Cameroon Volcanic Line and generates a multitude of primary and secondary hazards to the local population. For risk assessment and hazard mitigation, the age of the Nyos maar eruption provides some vital information. Since previous dating efforts using a range of techniques resulted in vastly varying eruption ages, we applied thermoluminescence (TL) methods to obtain independent and direct chronological constraints for the time of maar formation. Target minerals were granitic quartz clasts contained in pyroclastic surge deposits. Thermoluminescence plateau results prove that heat and/or pressure during the phreatomagmatic eruption was sufficient to reset the inherited luminescence signal of granitic bedrock quartz. Parallel application of three TL measurement protocols to one of the two samples gave consistent equivalent doses for the quartz ultra-violet emission. Despite the robustness of our dose estimates, the assessment of the dose rate was accompanied by methodological challenges, such as estimation of the original size distribution of quartz grains in the pyroclastic deposits. Considering results from additional laboratory analyses to constrain these uncertainties, we calculate an average maximum TL age of 12.3 ± 1.5 ka for the Nyos maar eruption. Based on these new data, a more solid risk assessment can be envisaged.

Mid‐Wisconsin to Holocene Permafrost and Landscape Dynamics based on a Drained Lake Basin Core from the Northern Seward Peninsula, Northwest Alaska

AbstractPermafrost‐related processes drive regional landscape dynamics in the Arctic terrestrial system. A better understanding of past periods indicative of permafrost degradation and aggradation is important for predicting the future response of Arctic landscapes to climate change. Here, we used a multi‐proxy approach to analyse a ~ 4 m long sediment core from a drained thermokarst lake basin on the northern Seward Peninsula in western Arctic Alaska (USA). Sedimentological, biogeochemical, geochronological, micropalaeontological (ostracoda, testate amoebae) and tephra analyses were used to determine the long‐term environmental Early‐Wisconsin to Holocene history preserved in our core for central Beringia. Yedoma accumulation dominated throughout the Early to Late‐Wisconsin but was interrupted by wetland formation from 44.5 to 41.5 ka BP. The latter was terminated by the deposition of 1 m of volcanic tephra, most likely originating from the South Killeak Maar eruption at about 42 ka BP. Yedoma deposition continued until 22.5 ka BP and was followed by a depositional hiatus in the sediment core between 22.5 and 0.23 ka BP. We interpret this hiatus as due to intense thermokarst activity in the areas surrounding the site, which served as a sediment source during the Late‐Wisconsin to Holocene climate transition. The lake forming the modern basin on the upland initiated around 0.23 ka BP and drained catastrophically in spring 2005. The present study emphasises that Arctic lake systems and periglacial landscapes are highly dynamic and that permafrost formation as well as degradation in central Beringia was controlled by regional to global climate patterns as well as by local disturbances. Copyright © 2015 John Wiley &amp; Sons, Ltd.

The Fossil-Lagerstätte Enspel — a crater lake in a volcanic-influenced terrestrial environment of the Westerwald Basin (late Oligocene, Western Germany)

The Westerwald region belongs to the Central European Volcanic Province (CEVP). As seen here, theWesterwald Basin is an aggregation of small basins (the smallest ones cover an area as little as 10,000 m), mainly situated within the Variscan Mosel, Lahn and Dill Synclines. Sedimentation started in the middle Eocene (Hottenrott 2002). A pre-existing landscape was filled first by coarse sediments, followed by clays and brown coal (Schafer et al. 2011). Tectonic activity seems to have increased during the middle late Oligocene, resulting in an increasing relief, displayed by sandy sediments (Butz and Vortisch 1987). Two watersheds separate the Westerwald Basin from the Lower Rhine Embayment (Schindler and Wuttke 2013) (Fig. 1). In the late Oligocene, volcanic activity commenced with trachytic intrusions, phreatomagmatic maar eruptions (Enspel) and pyroclastic eruptions. They were followed by basic shallow intrusions, lava flows, and continued maar formation (Elbtal, ?Elz) and the deposition of thick pyroclastics (Schreiber 1996; Haase et al. 2004). Locally, the eruptions and minor intercalated sediments covered the landscape up to 50 m high (Schafer et al. 2011). Reischmann (2011) relates the Oligocene volcanism to the Alpine orogeny and North Atlantic rifting; the main reason is extension and thinning of the lithosphere, resulting in a passive uplift of the asthenosphere; partial melting was caused by pressure release, which resulted in the eruptions in the CEVP. The volcanic activity led to changes in the local drainage systems, e.g. dammed valleys with subsequent lake formation. Caused by contemporaneous temporarily high Chattian sea level, palustrine facies became widespread, so that thick lignite seams were formed along with laminated lake sediments (e.g. Lake Norken, Uhl et al. 2011; fossil site Breitscheid, Schindler and Kolb 2010). Hygrophilous plant communities covered the lowlands. The macroflora of this riparian vegetation is dominated by leaves of Acer cf. tricuspidatum BRONN, twigs of Glyptostrobus europaeus (BRONGNIART) UNGER and charcoalified wood of Cupressaceae (Taxodiaceae s.l.), as seen in the lacustrine to swampy sediments of Lake Norken (Uhl et al. 2011). Upland or dryland floras and faunas were also influenced by volcanic activity. Forests died under heavy ash load, and their remnants were washed into the lowlands and partly preserved as silicified trunks. Isolated reptile and mammal bones and teeth (e.g. Teike and Tobien 1950) can be found within coarse volcanic ash (personal field observation, and unprepared material of the collections of the Museums of Breitscheid, Hachenburg, Wiesbaden and the Hessisches Landesamt fur Umwelt und Geologie, Wiesbaden, all Germany), demonstrating the influence of pyroclastic and/or lahar events on the biota. Floral recovery starts with pioneering plants like Cyperaceae, Poaceae and Betulaceae. This pioneering flora was succeeded by zonal assemblages of a mesophytic forest with strong East Asian influence (Herrmann et al. 2003, 2010; Kohler and Uhl 2014). This article is a contribution to the special issue “The Fossil-Lagerstatte Enspel reconstructing the palaeoenvironment with new data on fossils and geology” M. Wuttke (*) : T. Schindler Direktion Landesarchaologie, Referat Erdgeschichte, Generaldirektion Kulturelles Erbe Rheinland-Pfalz, Grose Langgasse 29, 55116 Mainz, Germany e-mail: michael.wuttke@gdke.rlp.de

Origin of temporal - compositional variations during the eruption of Lake Purrumbete Maar, Newer Volcanics Province, southeastern Australia

The Pleistocene Lake Purrumbete maar eruption sequence of the Newer Volcanics Province of southeastern Australia records stratigraphic, temporal and geochemical variations that cannot be explained by crustal processes, such as fractional crystallisation and crustal contamination. Samples from the Lake Purrumbete maar have distinct trace element trends that correlate with stratigraphic height, with decreasing incompatible element concentrations from the bottom of the sequence to the top, indicating that the most enriched melts were ejected during the earliest eruption stages, with the least enriched melts erupted in the final stages. Variations in rare earth element ratios indicate that these melts were formed by dynamic melting of garnet lherzolite mantle material, consistent with other volcanic centres within the basaltic cones subprovince of the Newer Volcanics Province. The geochemistry of the erupted units at Lake Purrumbete also record minor differentiation of the melts by olivine fractionation, although these processes alone cannot explain the observed geochemical variations in the eruption sequence. Further differentiation of the melts by different processes such as deep clinopyroxene fractionation or assimilation of metasomatised lithosphere may be responsible for the observed geochemical trends at Lake Purrumbete. This shows the complexity of the plumbing system of monogenetic volcanoes, but also shows the opportunities that these volcanoes provide to study these processes in detail, as the geochemical variations that are preserved in this type of volcanism by fast magma ascent through the crust, as evidenced by the presence of fresh mantle xenoliths, predominantly reflect processes that occurred in the mantle rather than in the crust.

Eifel maars: Quantitative shape characterization of juvenile ash particles (Eifel Volcanic Field, Germany)

The Eifel region in western central Germany is the type locality for maar volcanism, which is classically interpreted to be the result of explosive eruptions due to shallow interaction between magma and external water (i.e. phreatomagmatic eruptions). Sedimentary structures, deposit features and particle morphology found in many maar deposits of the West Eifel Volcanic Field (WEVF), in contrast to deposits in the East Eifel Volcanic Field (EEVF), lack the diagnostic criteria of typical phreatomagmatic deposits.The aim of this study was to determine quantitatively the shape of WEVF and EEVF maar ash particles in order to infer the governing eruption style in Eifel maar volcanoes. The quantitative shape characterization was done by analyzing fractal dimensions of particle contours (125–250μm sieve fraction) obtained from Scanning electron microscopy (SEM) and SEM micro-computed tomography (SEM micro-CT) images.The fractal analysis (dilation method) and the fractal spectrum technique confirmed that the WEVF and EEVF maar particles have contrasting multifractal shapes. Whereas the low small-scale dimensions of EEVF particles (Eppelsberg Green Unit) coincide with previously published values for phreatomagmatic particles, the WEVF particles (Meerfelder Maar, Pulvermaar and Ulmener Maar) have larger values indicating more complex small-scale features, which are characteristic for magmatic particles. These quantitative results are strengthening the qualitative microscopic observations, that the studied WEVF maar eruptions are rather dominated by magmatic processes. The different eruption styles in the two volcanic fields can be explained by the different geological and hydrological settings found in both regions and the different chemical compositions of the magmas.

High level triggers for explosive mafic volcanism: Albano Maar, Italy

Colli Albani is a quiescent caldera complex located within the Roman Magmatic Province (RMP), Italy. The recent Via dei Laghi phreatomagmatic eruptions led to the formation of nested maars. Albano Maar is the largest and has erupted seven times between ca 69–33ka. The highly explosive nature of the Albano Maar eruptions is at odds with the predominant relatively mafic (SiO2=48–52wt.%) foiditic (K2O=9wt.%) composition of the magma. The deposits have been previously interpreted as phreatomagmatic, however they contain large amounts (up to 30%vol) of deep seated xenoliths, skarns and all pre-volcanic subsurface units. All of the xenoliths have been excavated from depths of up to 6km, rather than being limited to the depth at which magma and water interaction is likely to have occurred, suggesting an alternative trigger for eruption. High precision geochemical glass and mineral data of fresh juvenile (magmatic) clasts from the small volume explosive deposits indicate that the magmas have evolved along one of two evolutionary paths towards foidite or phonolite. The foiditic melts record ca. 50% mixing between the most primitive magma and Ca-rich melt, late stage prior to eruption. A major result of our study is finding that the generation of Ca-rich melts via assimilation of limestone, may provide storage for significant amounts of CO2 that can be released during a mixing event with silicate magma. Differences in melt evolution are inferred as having been controlled by variations in storage conditions: residence time and magma volume.

Experimental birth of a maar–diatreme volcano

Maar–diatreme eruptions are hazardous to people and infrastructure, and are also linked to the formation of the kimberlitic variety of diatremes, which is important economically. Processes occurring in the subsurface diatreme and their relation to surface eruptions are not yet well understood. We conducted field-scale experiments using analog materials to shed more light on these processes, especially the formation of the proto-diatreme during the first explosions of a maar eruption. Specifically, a series of buried explosions in a prepared, layered substrate (pad) produced craters, extra-crater deposits and sub-crater deposits analogous to volcanic maar craters, tephra rings and incipient diatremes. Post-explosion substrate excavation revealed that single large explosions produce sub-crater deposits extending nearly to the crater-rim crest. The same energy divided into three blasts, either co-located or at different depths with the same epicenter, produced narrower and sometimes deeper sub-crater deposits even though the final sizes of the craters were similar to that produced by the single large blast. The sub-crater deposits have an upper zone with domains from different substrate depths, and an underlying zone distinguished primarily by being more loosely packed than the original substrate. Videos show surface motion extending beyond the post-shot crater rim, and largely vertical ejection and fallback of material into the footprint of these deposits, especially for the explosions that occurred below optimal depth of burial. We infer that much of the loosely packed material was disassembled, vertically transported to different heights during the explosions, then fell back without significant relative lateral movement of grains. However, subvertical fallback did produce apparent cross-cutting structures in shallow sub-crater deposits. One explosion ejected material from the deepest substrate horizon, but it was redeposited only within the crater and is unrepresented in the ejecta rim. Implications of the experiments for maar–diatreme volcanoes, including some kimberlite pipes, are as follows: (1) vertical focusing of deep explosions in the diatreme explains the deficit of deep wallrock lithics observed at maar volcanoes; (2) direct vertical fallback is possibly an important process forming diatreme deposits, especially during the earliest stages; and (3) even in our limited simulation the number and scaled depth of explosions clearly affect proto-diatreme size and structure.

Experimental craters formed by single and multiple buried explosions and implications for volcanic craters with emphasis on maars

Craters at many volcanoes, including most maars, are formed by multiple subsurface explosions. Experiments compared the crater formed by a single large, buried explosion, with craters formed by multiple explosions with the same cumulative energy. Explosive charges were detonated in pads composed of layered aggregates, in three configurations: (1) a single large charge buried near its optimal crater excavation depth; (2) three charges, each with 1/3 the energy of the first one, buried at approximately the same depth with respect to the original pad surface; (3) the same three charges buried successively deeper. Final crater size in the multiple explosion cases is not a good indicator of the energy of individual explosions. However, crater morphology, and ejecta volume and distribution can be good indicators of explosion energy and depth. These results directly impact the estimate of the energy released by past maar eruptions and future hazard assessments.

Direct dating of Quaternary phreatic maar eruptions by luminescence methods

The suitability of quartz optically stimulated luminescence (OSL) and feldspar infrared stimulated luminescence (IRSL) for the direct dating of phreatic eruptions was tested on examples from the Eifel Volcanic Field, Germany. The mean IRSL age of 11.6 ± 0.5 ka for Ulmener Maar Tephra is in excellent agreement with the independent age control (11 ± 0.1 ka), but the mean OSL age of 14.2 ± 0.6 ka overestimates the known age by 3 ka. For Meerfelder Maar Tephra, consistent IRSL (mean 74.9 ± 5.0 ka) and OSL ages (74.9 ± 5.5 ka) have been observed. The consistent results from Meerfelder Maar imply that the overestimation observed for Ulmener Maar quartz OSL might not be relative to the eruption age, but rather represents a small absolute offset. In samples taken from deposits of the eruption of Laacher See Volcano, no resetting of the OSL signal and highly scattered IRSL estimates were found. This implies that phreatomagmatic eruptions are less well suited for this dating approach compared to pure phreatic maar eruptions, where the effect of high-pressure shock waves probably dominates the process of resetting the luminescence signal.

Depósitos de metales preciosos de gran tonelaje en la región del Caribe

Shallow epithermal stockworks hosted by maar eruption craters contain over ninety percent of the bulk minable gold identified to date in the Circum-Caribbean region. Pueblo Viejo (Dominican Republic) and Santa Rosa (Panama) contains combined gold reserves of over 600 tonnes. Bulk mineable gold deposits are also being discovered in and around veins that were formely mined underground. Bellavista, El Recio and San Martin (Costa Rica) contain combined announced gold reserves of over 50 tonnes.Two hundred ninety one representative samples collected at 8 bulk mineable gold deposits were analyzed for Au, Ag, As, Hg and Sb and include veins, vein stockworks, hydrothermal eruption breccias, sinter, variably altered host, unaltered host, nearby intrusive rocks, and basement. Analyses completed to date show that Circum-Caribbean epithermal ores are enriched in arsenic, mercury and antimony as well as gold and silver.Exploration efforts in the region naturally focus on districts with past prodution. However, an order of magnitude more bulk mineable gold has been discovered in maar-hosted shallow epithermal stockworks outside of the old bonanza vein camps. Explorations guides include basal island arc volcanic sequences, hydrothermal and hydrovolcanic (maar) eruption breccias, thinly-bedded carbonaceous maar sediments, and hot spring-related silicification and argillic alteration