Mono Craters Research Articles

Return of the pika: American pikas re-occupy long-extirpated, warm locations.

American pikas (Ochotona princeps), small mammals related to rabbits, occur in mountainous regions of western North America, where they live in shattered‐rock habitats (talus). Aspects of their physiology and life history create situations that appear to put pikas at risk from warming climates. Some low‐elevation, warm sites that historically harbored pikas have become extirpated, and the assumption is that these will not be re‐colonized under current climate trends. Unexpectedly, in 2021, we found that pikas had re‐colonized two very warm, low‐elevation, dry sites in eastern California, USA, in the Bodie Mountains and Mono Craters. Resident pikas appear to have been absent at both sites for ≥10 years. These findings suggest that pikas, which are normally diurnally active, are able to overcome thermal dispersal barriers and re‐colonize long‐extirpated sites, perhaps by moving during cool nights. Our data also highlight the often unrecognized suitability of pika habitat in warm regions where the interiors of taluses can remain stably cool even when external air temperatures are hot.

Emergence of wet conditions in the Mono Basin of the Western USA coincident with inception of the Last Glaciation

Abstract At present, the Basin and Range of the western USA is arid, but geologic studies show evidence of past wetness. The timing of these wetter conditions reveals a close association with glacial conditions. This association has led to the hypothesis of a causal link between glacial climate and regional wetness, but poor age control on the onset of regional wetness thwarts a test of this hypothesis. Here we determine the start of the most recent interval of persistent wetness in the Mono Basin, which is a hydrologically closed depression that sits at the west-central edge of the Basin and Range. The most recent emergence of persistent wetness in the Mono Basin is stratigraphically correlated with the depositional age of Ash 19—a rhyolitic ash bed that represents the oldest tephra of the Wilson Creek Formation and one of the earliest-known products of explosive volcanic activity from the Mono Craters. We constrain the depositional age of Ash 19 by using the U/Th disequilibrium dating method to date carbonates that are younger and older than Ash 19. Our U/Th dating results show that Ash 19 was deposited before the formation of a cross-cutting carbonate bed dated to 69.2 ± 0.3 ka but after an underlying carbonate tufa dated to 67.4 ± 3.5 ka, which suggests that the start of wetness in the Mono Basin was contemporary with the inception of the Last Glaciation—the beginning of Marine Isotope Stage 4—at ca. 70 ka. This finding corroborates the hypothesis of a link between glacial climate and regional wetness.

No ring fracture in Mono Basin, California

AbstractIn Mono Basin, California, USA, a near-circular ring fracture 12 km in diameter was proposed by R.W. Kistler in 1966 to have originated as the protoclastic margin of the Cretaceous Aeolian Buttes pluton, to have been reactivated in the middle Pleistocene, and to have influenced the arcuate trend of the chain of 30 young (62–0.7 ka) rhyolite domes called the Mono Craters. In view of the frequency and recency of explosive eruptions along the Mono chain, and because many geophysicists accepted the ring fracture model, we assembled evidence to test its plausibility. The shear zone interpreted as the margin of the Aeolian Buttes pluton by Kistler is 50–400 m wide but is exposed only along a 7-km-long set of four southwesterly outcrops that subtend only a 70° sector of the proposed ring. The southeast end of the exposed shear zone is largely within the older June Lake pluton, and at its northwest end, the contact of the Aeolian Buttes pluton with a much older one crosses the shear zone obliquely. Conflicting attitudes of shear structures are hard to reconcile with intrusive protoclasis. Also inconsistent with the margin of the ovoid intrusion proposed by Kistler, unsheared salients of the pluton extend ∼1 km north of its postulated circular outline at Williams Butte, where there is no fault or other structure to define the northern half of the hypothetical ring. The shear zone may represent regional Cretaceous transpression rather than the margin of a single intrusion. There is no evidence for the Aeolian Buttes pluton along the aqueduct tunnel beneath the Mono chain, nor is there evidence for a fault that could have influenced its vent pattern. The apparently arcuate chain actually consists of three linear segments that reflect Quaternary tectonic influence and not Cretaceous inheritance. A rhyolitic magma reservoir under the central segment of the Mono chain has erupted many times in the late Holocene and as recently as 700 years ago. The ring fracture idea, however, prompted several geophysical investigations that sought a much broader magma body, but none identified a low-density or low-velocity anomaly beneath the purported 12-km-wide ring, which we conclude does not exist.

Conservation status of American pikas (Ochotona princeps)

AbstractThe American pika (Ochotona princeps) is commonly perceived as a species that is at high risk of extinction due to climate change. The purpose of this review is two-fold: to evaluate the claim that climate change is threatening pikas with extinction, and to summarize the conservation status of the American pika. Most American pikas inhabit major cordilleras, such as the Rocky Mountain, Sierra Nevada, and Cascade ranges. Occupancy of potential pika habitat in these ranges is uniformly high and no discernible climate signal has been found that discriminates between the many occupied and relatively few unoccupied sites that have been recently surveyed. Pikas therefore are thriving across most of their range. The story differs in more marginal parts of the species range, primarily across the Great Basin, where a higher percentage of available habitat is unoccupied. A comprehensive review of Great Basin pikas revealed that occupied sites, sites of recent extirpation, and old sites, were regularly found within the same geographic and climatic space as extant sites, and suggested that pikas in the Great Basin tolerated a broader set of habitat and climatic conditions than previously understood. Studies of a small subset of extirpated sites in the Great Basin and in California found that climate variables (most notably measures of hot temperature) were associated more often with extirpated sites than occupied sites. Importantly, upward contraction of the lower elevation boundary also was found at some sites. However, models that incorporated variables other than climate (such as availability of upslope talus habitat) often were better predictors of site persistence. Many extirpations occurred on small habitat patches, which were subject to stochastic extinction, as informed by a long-term pika metapopulation study in Bodie, California. In addition, several sites may have been compromised by cattle grazing or other anthropogenic factors. In contrast, several low, hot sites (Bodie, Mono Craters, Craters of the Moon National Monument and Preserve, Lava Beds National Monument, Columbia River Gorge) retain active pika populations, demonstrating the adaptive capacity and resilience of pikas in response to adverse environmental conditions. Pikas cope with warm temperatures by retreating into cool interstices of their talus habitat and augment their restricted daytime foraging with nocturnal activity. Pikas exhibit significant flexibility in their foraging tactics and are highly selective in their choice of available vegetation. The trait that places pikas at greatest risk from climate change is their poor dispersal capability. Dispersal is more restricted in hotter environments, and isolated low-elevation sites that become extirpated are unlikely to be recolonized in a warming climate. The narrative that American pikas are going extinct appears to be an overreach. Pikas are doing well across most of their range, but there are limited, low-elevation losses that are likely to be permanent in what is currently marginal pika habitat. The resilience of pikas in the face of climate change, and their ability or inability to persist in marginal, hot environments, will continue to contribute to our understanding of the impact of climate change on individual species.

Constraining the Early Eruptive History of the Mono Craters Rhyolites, California, Based on 238U‐230Th Isochron Dating of Their Explosive and Effusive Products

AbstractThe Mono Craters are an overlapping chain of at least 28 domes and coulees located south of Mono Lake, east central California, and represent the most recent eruptions of high‐silica rhyolite magma in the Mono Lake‐Long Valley volcanic region. Regionally widespread tephra fall deposits from the Mono Craters serve as important chronostratigraphic markers for correlations of late Quaternary terrestrial archives in the western United States. A well‐resolved eruption chronology for the Mono Craters is thus not only critical for volcanic hazard considerations but also for paleoclimatic and paleomagnetic studies. Here we constrain the timing of early Mono Craters volcanism using ion microprobe 238U‐230Th dating of unpolished crystal faces of autocrystic zircon and allanite microphenocrysts, which samples the final stage of crystallization prior to eruption. The stratigraphically oldest domes (biotite‐bearing rhyolites) yield 238U‐230Th isochron dates between ca. 42 and ca. 19 ka and are unambiguously linked to dated tephras in the Wilson Creek formation of ancestral Mono Lake via coupled 238U‐230Th geochronology and titanomagnetite geochemistry. The second oldest set of domes (orthopyroxene‐bearing rhyolites) have overlapping 238U‐230Th isochron dates that are within uncertainty of published K/Ar and 40Ar/39Ar dates for the fayalite‐bearing rhyolite domes, suggesting a period of intense effusive activity for the Mono Craters near the Pleistocene‐Holocene boundary. Our new and previously published 238U‐230Th dates for tephras in the Wilson Creek formation provide robust geochronologic constraints for the controversial geomagnetic excursion recorded in the Wilson Creek formation.

Stratigraphic and sedimentologic framework for tephras in the Wilson Creek Formation, Mono Basin, California, USA

Numerous tephra layers occur within the late Pleistocene Wilson Creek Formation, where they are interbedded with lacustrine deposits of Lake Russell, the ancestor of present-day Mono Lake. Most of the tephra layers are rhyolitic in composition, and were produced from the Mono Craters. We present detailed stratigraphy and sedimentology of the tephra layers, sampled at twelve outcrops near the shoreline of Mono Lake and the Mono Craters, and implement grain size, componentry, and surface morphology analysis to characterize their physical properties. Sub-unit correlation is proposed for certain tephra units. Noticeable features of the tephras, such as the occurrence of low-density rounded or highly vesicular pumice within certain sub-units, are highlighted. The abundant obsidian, lithics, and ostracods within many sub-units suggest that the associated eruption pulses involved water-magma interaction.Eruptions from the Mono Craters during the late Pleistocene were consistent neither in frequency nor volume. The Mono Craters were most active during the eruption of tephras in Sequences C (∼42.5–39.4 ka) and A (∼14.1–12.9 ka), and Pleistocene volcanic activity reached its peak during the eruption of tephra layer C11. Detailed interpretation of tephra layer B7 and tephras in Sequence A is given, which includes the number of eruption pulses, vent location, dispersal direction, occurrence of pyroclastic flow and surge, and other features and processes. Tephras in Sequence B (∼26.2–23.1 ka) may have had their vents located in the southern half of the Mono Craters, or are smaller in volume (except for B7), compared to the other tephras in the Wilson Creek Formation. Vents for tephra layers A4, A3, and A1 are located near the northern end of the Mono Craters. The stratigraphy of tephra layers B7 and A1 suggests an unstable depositional environment, which can be used to help constrain the water-level history of Lake Russell during the late Pleistocene.

Formation of obsidian pyroclasts by sintering of ash particles in the volcanic conduit

The ranges in intensity and style of volcanic eruptions, from highly explosive Plinian eruptions to quiescent lava extrusions, depend on the style and efficiency of gas loss from ascending magma. Obsidian pyroclasts – small, glassy pieces of quenched magma found in some volcanic tephra beds – may preserve valuable information about magma degassing in their vesicle textures and volatile contents. Accurate interpretation of their textures and volatiles, however, requires understanding the mechanism of formation of the pyroclasts. Obsidian pyroclasts from the ca. 1325–1350 C.E. North Mono eruption of Mono Craters (CA, USA) were analyzed and found to have H2O and CO2 contents indicating that they were formed at pressures in the approximate range of 3–40 MPa. Many also contain domains with differing vesicle textures, separated by boundaries containing xenocrystic material, indicating that they are composed of smaller fragments that have sutured together. More than half of the pyroclasts analyzed contained small (∼10 μm), highly distorted vesicles, with multi-cuspate morphology, interpreted as the remnants of interstitial gas trapped amongst sintered fragments of melt/glass. Rounded vesicles are also common and are interpreted to result from surface tension-driven relaxation of the distorted vesicles. Calculated timescales of sintering and relaxation are consistent with timescales for pyroclast formation indicated by H2O re-equilibration within the heterogeneous pyroclasts. This sintering model for the origin of obsidian pyroclasts is further supported by the observation that spherical vesicles are found mainly in H2O-rich pyroclasts, and distorted vesicles mainly in H2O-poor pyroclasts. We conclude that obsidian pyroclasts generated during the North Mono eruption were formed by cycles of fragmentation, sintering/suturing, and relaxation, over a very wide range of depths within the conduit; we find no evidence to support pumice (foam) collapse as the formation mechanism. Similar textures, and the occurrence of xenolithic material, in obsidian pyroclasts in other eruptions suggest that sintering may be generally responsible for the origin of obsidian pyroclasts. Our conceptual model indicates that volatile contents in obsidian pyroclasts reflect both degassing of bubbly magma and the composition of gas trapped between sintering particles.

Nonequilibrium degassing, regassing, and vapor fluxing in magmatic feeder systems

Magma degassing models typically invoke volatile depletion of a single parental melt, with permeable loss of exsolved gas having served for many years as the paradigm for the transition from volatile-rich, explosive eruptions to volatile-depleted lava flows. These degassing models are guided by measurements of H2O, CO2, and hydrogen isotope variations retained in melt that quenched to glass, but the existing models are not uniquely constrained by the data. There also remains uncertainty surrounding the origin and significance of volcanic glass fragments. We show that individual obsidian pyroclasts from Mono Craters, California (USA), are heterogeneous in dissolved H2O and CO2, suggesting that clasts are assembled from juvenile melt and rewelded ash during magma ascent. This is in contrast to the conventional view that clasts are chemically homogeneous and sample the chilled, glassy margins of conduit walls. The new measurements of dissolved H2O and CO2 help reconcile existing open-system degassing models used to explain elevated CO2/H2O ratios, provide time scales based on diffusion modeling for pyroclast formation, and show that magma does not necessarily lose volatiles monotonically during ascent-driven decompression.

Behavioral Ecology of American Pikas (Ochotona princeps) at Mono Craters, California: Living on the Edge

The behavioral ecology of the American pika (Ochotona princeps) was investigated at a relatively hot south-facing, low-elevation site in the Mono Craters, California, a habitat quite different from the upper montane regions more typically inhabited by this species and where most prior investigations have been conducted. Mono Craters pikas exhibited a behavioral profile that contrasted significantly with that of pikas found in upper montane regions. Mono Craters pikas were less surface active than pikas in studies at high-elevation sites, although their rate of short-call vocalizations was similar. Mono Craters pikas did not exhibit typical foraging behavior: they fed and collected hay at significantly reduced rates, and did not construct large central-place hay piles. Social behaviors (conspecific aggression, social tolerance, avoidance) were infrequent compared with data from prior studies in upper montane environments. The Mono Craters site appears to be one of the warmest localities in which pikas have been observed. Recorded talus surface temperatures consistently exceeded 30 °C, and temperatures >40 °C were commonly recorded. In contrast, temperatures measured in the matrix of the talus were consistently cooler, and the apparent insulating effect of talus, as measured by the difference between surface and matrix temperatures, was typically most pronounced on the hottest days. Although pika activity was most frequent in early morning, late afternoon, and at night, pikas were also active during the hottest part of the day, presumably because of their ability to behaviorally thermoregulate by retreating into the cooler talus matrix. Data on populations of pikas which inhabit marginal sites can help us understand how pikas and other montane animals might respond in a world of climate change so that we may more effectively plan for their conservation.

Satellite-Based Thermophysical Analysis of Volcaniclastic Deposits: A Terrestrial Analog for Mantled Lava Flows on Mars

Orbital thermal infrared (TIR) remote sensing is an important tool for characterizing geologic surfaces on Earth and Mars. However, deposition of material from volcanic or eolian activity results in bedrock surfaces becoming significantly mantled over time, hindering the accuracy of TIR compositional analysis. Moreover, interplay between particle size, albedo, composition and surface roughness add complexity to these interpretations. Apparent Thermal Inertia (ATI) is the measure of the resistance to temperature change and has been used to determine parameters such as grain/block size, density/mantling, and the presence of subsurface soil moisture/ice. Our objective is to document the quantitative relationship between ATI derived from orbital visible/near infrared (VNIR) and thermal infrared (TIR) data and tephra fall mantling of the Mono Craters and Domes (MCD) in California, which were chosen as an analog for partially mantled flows observed at Arsia Mons volcano on Mars. The ATI data were created from two images collected ~12 h apart by the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) instrument. The results were validated with a quantitative framework developed using fieldwork that was conducted at 13 pre-chosen sites. These sites ranged in grain size from ash-sized to meter-scale blocks and were all rhyolitic in composition. Block size and mantling were directly correlated with ATI. Areas with ATI under 2.3 × 10−2 were well-mantled with average grain size below 4 cm; whereas values greater than 3.0 × 10−2 corresponded to mantle-free surfaces. Correlation was less accurate where checkerboard-style mixing between mantled and non-mantled surfaces occurred below the pixel scale as well as in locations where strong shadowing occurred. However, the results validate that the approach is viable for a large majority of mantled surfaces on Earth and Mars. This is relevant for determining the volcanic history of Mars, for example. Accurate identification of non-mantled lava surfaces within an apparently well-mantled flow field on either planet provides locations to extract important mineralogical constraints on the individual flows using TIR data.

Imaging the magmatic system of Mono Basin, California, with magnetotellurics in three dimensions

AbstractA three‐dimensional (3‐D) electrical resistivity model of Mono Basin in eastern California, unveils a complex subsurface filled with zones of partial melt, fluid‐filled fracture networks, cold plutons, and regional faults. In 2013, 62 broadband magnetotelluric stations were collected in an array around southeastern Mono Basin from which a 3‐D electrical resistivity model was created with a resolvable depth of 35 km. Multiple robust electrical resistivity features were found that correlate with existing geophysical observations. The most robust features are two 300 ± 50 km3 near‐vertical conductive bodies (3–10 Ω m) that underlie the southeast and northeastern margin of Mono Craters below 10 km depth. These features are interpreted as magmatic crystal‐melt mush zones of 15 ± 5% interstitial melt surrounded by hydrothermal fluids and are likely sources for Holocene eruptions. Two conductive east dipping structures appear to connect each magma source region to the surface. A conductive arc‐like structure (< 0.9 Ω m) links the northernmost mush column at 10 km depth to just below vents near Panum Crater, where the high conductivity suggests the presence of hydrothermal fluids. The connection from the southernmost mush column at 10 km depth to below South Coulée is less obvious with higher resistivity (200 Ω m) suggestive of a cooled connection. A third, less constrained conductive feature (4–10 Ω m) 15 km deep, extending to 35 km is located west of Mono Craters near the eastern front of the Sierra Nevada escarpment and is coincident with a zone of sporadic, long‐period earthquakes that are characteristic of a fluid‐filled (magmatic or metamorphic) fracture network. A resistive feature (103–105 Ω m) located under Aeolian Buttes contains a deep root down to 25 km. The eastern edge of this resistor appears to structurally control the arcuate shape of Mono Craters. These observations have been combined to form a new conceptual model of the magmatic system beneath Mono Craters to a depth of 30 km.

Ascent and emplacement dynamics of obsidian lavas inferred from microlite textures

To assess the eruption and emplacement of volumetrically diverse rhyolite lavas, we measured microlite number densities and orientations from samples collected from nine lavas in Yellowstone Caldera and two from Mono Craters, USA. Microlite populations are composed of Fe-Ti oxides ± alkali feldspar ± clinopyroxene. Number densities range from 108.11 ± 0.03 to 109.45 ± 0.15 cm−3 and do not correlate with distance from the vent across individual flows and are remarkably similar between large- and small-volume lavas. Together, those observations suggest that number densities are unmodified during emplacement and that ascent rates in the conduit are similar between small domes and large lava flows. Microtextures produced by continuous decompression experiments best replicate natural textures at decompression rates of 1–2 MPa hr−1. Acicular microlites have a preferred orientation in all natural samples. Because the standard deviation of microlite orientation does not become better aligned with distance travelled, we conclude that microlites exit the conduit aligned and that strain during subaerial flow was insufficient to further align microlites. The orientations of microlite trend and plunge in near-vent samples indicate that pure shear was the dominant style of deformation in the conduit. We speculate that collapsing permeable foam(s) provides a mechanism to concurrently allow microlite formation and alignment in response to the combination of degassing and flattening by pure shear.

A simple model for the viscosity of rhyolites as a function of temperature, pressure and water content

In order to better constrain the viscosity (η) of high-silica rhyolite at low to moderate water contents (X), which represent water saturation at near-surface pressure–temperature (P–T) conditions, we made 211 viscosity measurements on Mono Craters rhyolites containing between 0.01 and 1.2wt.% H2O, at temperatures between 796 and 1774K using parallel plate and concentric cylinder methods at atmospheric pressure. We then developed and calibrated a new empirical model for the rhyolitic melt viscosity, where non-linear variations due to temperature (T), and water content (X) are nested within linear and exponential dependencies of logη on pressure (P). The model was fitted to a total of 691 data points including published data on rhyolites, granites and haplogranites. The significance of model parameters was evaluated at the 95% confidence level. The model is simple enough for use in numerical models of conduit or lava flow dynamics:logη=-4.40+11609-1248ln(w+0.17)T-(140.1-62.3ln(w+0.17))-P0.00082+0.000051w-0.95Twhere η is viscosity in Pas, w is water content in wt.%, P is pressure in MPa and T is temperature in K. The root mean square error (RMSE) between the model and the 691 data points used in calibration is 0.43log units, and analysis of the residuals shows that the model fits all modeled regions of P–T–X (H2O) space to a similar degree of quality. In both regards, the new model outperforms previous models for rhyolite viscosity. Multi-level modeling enabled us to show that higher temperatures and higher water contents both independently favor a more negative pressure-dependence of viscosity. The model suggests that the effect of pressure on viscosity undergoes a transition from a positive to a negative effect as temperatures rise above ∼1175K for anhydrous rhyolites, and above ∼865K for melts containing 5wt.% H2O. We validated the model by examination of the few published viscosity data where P is varied but T and X(H2O) remain approximately constant. Experimental constraints have led to spurious correlations between P, T, X(H2O) and η in viscosity datasets, so that models may struggle to correctly resolve the individual effects of P, T and X(H2O), and especially their cross-correlations.

Edaphic Specialization in the Cryptic Species Mentzelia monoensis (Loasaceae)

Mentzelia monoensis J. M. Brokaw & L. Hufford is a cryptic hexaploid species known only from Mono County, California. Previous studies have suggested a recent allopolyploid origin of M. monoensis but have not investigated the causes of its narrow distribution. Here we report the discovery of a unique haplotype from the chloroplast intergenic spacer ndhF-rpl32 that is sufficient to distinguish M. monoensis from all other species in Mentzelia sect. Trachyphytum using preserved specimens from any developmental stage. Based on soils collected with verified voucher specimens, we show that the edaphic niche of M. monoensis is significantly different from those of all other species in Mentzelia sect. Trachyphytum. A principal components analysis suggests the edaphic niche of M. monoensis is also one of the most extreme in Mentzelia sect. Trachyphytum. Most populations of M. monoensis were collected in soils derived from silica-rich rhyolite tephra produced by the Mono Craters volcanic chain. These coarse textured soils are lower in cation exchange capacity and plant available nutrients than those of any other species in Trachyphytum. Our findings suggest that edaphic specialization may have played a significant role in the establishment and geographic distribution of M. monoensis and other species in the Mono Basin.

Magmatic gas emissions at Holocene volcanic features near Mono Lake, California, and their relation to regional magmatism

Silicic lavas have erupted repeatedly in the Mono Basin over the past few thousand years, forming the massive domes and coulees of the Mono Craters chain and the smaller island vents in Mono Lake. We report here on the first systematic study of magmatic CO2 emissions from these features, conducted during 2007–2010. Most notably, a known locus of weak steam venting on the summit of North Coulee is actually enclosed in a large area (~0.25km2) of diffuse gas discharge that emits 10–14t/d of CO2, mostly at ambient temperature. Subsurface gases sampled here are heavily air-contaminated, but after standard corrections are applied, show average δ13C-CO2 of −4.72‰, 3He/4He of 5.89RA, and CO2/3He of 0.77×1010, very similar to the values in fumarolic gas from Mammoth Mountain and the Long Valley Caldera immediately to the south of the basin. If these values also characterize the magmatic gas source at Mono Lake, where CO2 is captured by the alkaline lake water, a magmatic CO2 upflow beneath the lake of ~4t/d can be inferred. Groundwater discharge from the Mono Craters area transports ~13t/d of 14C-dead CO2 as free gas and dissolved carbonate species, and adding in this component brings the estimated total magmatic CO2 output to 29t/d for the two silicic systems in the Mono Basin. If these emissions reflect intrusion and degassing of underlying basalt with 0.5wt.% CO2, a modest intrusion rate of 0.00075km3/yr is indicated. Much higher intrusion rates are required to account for CO2 emissions from Mammoth Mountain and the West Moat of the Long Valley Caldera.

Dome collapse mechanisms and block-and-ash flow emplacement dynamics inferred from deposit and impact mark analysis, Mono Craters, CA

Characteristics of the Panum block-and-ash flow (BAF) deposit, Mono Craters, CA, were analyzed to determine the mechanisms of collapse of the parent dome and dynamics of emplacement of the BAF. Granulometry, componentry, and obsidian water content data were used to define distinct facies of the Panum BAF deposit. These suggest a sequential, three-stage collapse model for the ancestral dome of the Panum vent, with destabilization first of its cold, brittle outer margins and then of its hot, ductile interior to an estimated depth of ~260m below the free-air surface.Impact marks on clast faces that resulted from clast-to-clast interactions recorded details of the impacting mechanism. We analyzed ~15,000 marks and report for the first time their size, shape, and sorting to infer the flow regime of the BAF during emplacement, to a distance of ~2.6km from the vent. Clast-to-clast interactions occurred over a broad spectrum from purely collisional (normal to clast faces) to purely frictional (clasts shearing in a unidirectional flow), and recorded a transition to a more frictional flow regime in distal reaches as the sliding head of the BAF decelerated and halted. Finally, we briefly differentiate a similar, but previously undescribed older deposit that predates the 1325–1350A.D. North Mono eruptive episode.

Deposits of the most recent eruption in the Southern Mono Craters, California: Description, interpretation and implications for regional marker tephras

The penultimate eruption in the Mono Craters, Mono County, CA, USA, occurred in the southern section of the volcanic chain, and is herein named the South Mono eruption. The South Mono eruption occurred in 594–648calA.D., and its products consist of widespread Plinian and phreatomagmatic fall, surge and pyroclastic flow deposits. The explosive deposits can be broken into Basal, Orange–Brown (surge dominated) and Upper subunits. The eruptive phase represented by the Upper beds was the most intense and voluminous, dispersing tephra over a wide region of eastern CA and western NV. South Coulee was the only effusive product of the eruption, and comprises the vast majority of the c. 0.4cukm dense-rock equivalent (DRE) volume. The tephra overlies the deposits of Wilson Butte to the south, and is correlated herein with Wood's Tephra 2, and Walker Lake and Turupah Flats regional marker tephra layers. Other dates for these regional tephras may be the result of dating ash redeposited in debris flow events following fire.

Stable chlorine isotope behavior during volcanic degassing of H2O and CO2 at Mono Craters, CA

Trends in CO2 and H2O concentrations and δD values of obsidian clasts from Mono Craters volcanic field, California demonstrate clear chemical and isotopic evidence for eruptive degassing. However, neither Cl concentrations nor stable isotopes (35Cl and 37Cl) track the degassing process, which is likely because of disequilibrium due to slow diffusion of Cl in the cooling melt. Obsidian pyroclasts (n = 29) were collected from tuff layers representing a single eruptive sequence that occurred circa 1340 A.D., as well as, rhyolitic obsidian samples (n = 12) were collected from three high-silica (>74 % SiO2) flows forming the domes and coulees in the region. The Cl, H2O, and CO2 concentrations recorded by the eruptive pyroclastic obsidians track the chemical evolution of the magmatic system during eruption, whereas the concentrations of the dome samples represent the final degassed product. The H2O and CO2 concentrations of the pyroclastic samples range from 0.49 to 2.13 wt% and 2 to 35 ppm, respectively; whereas concentrations in the dome glasses range from 0.17 to 0.33 wt% and 1 to 3 ppm, respectively. H2O and CO2 concentrations in the pyroclastic fall and dome samples are strongly correlated and reflect the degassing trend of the eruptive sequence. Chlorine concentrations of the pyroclastic fall samples and the domes range from 609 to 833 ppm and 681 to 872 ppm, respectively. Cl concentrations do not display a strong correlation with either H2O or CO2 concentrations. δD values of the pyroclastic fall obsidians vary between −84 ‰ and −55 ‰, whereas the δD values of the dome obsidians vary between −117 ‰ and −91 ‰. D/H ratios decrease with total water content following a distillation trend controlled by both closed and open system degassing. δ37Cl values of pyroclastic fall obsidians (−1.9 ‰ to −0.1 ‰) overlap with those of dome samples (−1.2 ‰ to 0.0 ‰). The similar Cl concentrations between the pyroclastic fall and dome obsidians argue for lack of Cl degassing, despite H2O and CO2 loss. These observations can be explained by disequilibrium effects due to the slow diffusion rate of Cl compared to H2O and CO2 in silicate melt, buffering by a separate brine phase, or by fluxing of Cl from a deeper magma source, with the slow diffusion rate of Cl being the preferred explanation. The wide range in δ37Cl values may be indicative of isotopic compositional heterogeneities in the magma source due to assimilation of sedimentary material or fluxing of mantle-derived Cl to a crustal melt.

Geochemical fingerprinting of Wilson Creek formation tephra layers (Mono Basin, California) using titanomagnetite compositions

Nineteen tephra layers within the Wilson Creek formation near Mono Lake provide a record of late Pleistocene to early Holocene volcanic activity from the nearby Mono Craters and are important chronostratigraphic markers for paleomagnetic, paleoclimatic, and paleoecologic studies. These stratigraphically important tephra deposits can be geochemically identified using compositions of their titanomagnetite phenocrysts. Titanomagnetite compositions display a broad range (XUsp 0.26–0.39), which allow the tephra layers to be distinguished despite the indistinguishable major-element glass compositions (76–77wt% SiO2) of their hosts. The concentrations of Ti and Fe in titanomagnetite display geochemical and stratigraphic groupings that allow clear discrimination between older (>57ka) and younger (<41ka) tephras. Some individual tephra layers can be uniquely identified on the basis of titanomagnetite MgO, MnO, and Al2O3 contents. In addition, a few tephra layers can be correlated to their source vents by their titanomagnetite compositions. The unique geochemical fingerprint of the Mono Craters-sourced titanomagnetites also allows the discrimination of two tephra layers apparently sourced from nearby Mammoth Mountain volcano in Long Valley.

New Records of Marginal Locations for American Pika (Ochotona princeps) in the Western Great Basin

We describe 46 new site records documenting occupancy by American pika (lOchotona princeps) at 21 locations from 8 mountain regions in the western Great Basin, California, and Nevada. These locations comprise a sub- set of sites selected from regional surveys to represent marginal, isolated, or otherwise atypical pika locations, and to provide information for assessing environmental tolerance limits. Several locations are known from historic observations (Madeline Plain, Bodie Mtns., Wassuk Mtns., Mono Craters) and are included here to update current status. Site eleva- tions range from 1848 m to 3392 m; relative to the broad range of pika sites in the region, the new locations have cli- mates that are 2—4 °C warmer and receive approximately half the annual precipitation. Sites are located in lava flows and domes, inselbergs (isolated, rocky exposures on a small hill), eroding bedrock, rock-glacier till, talus slopes, and anthro- pogenic roadbed armaments and mining ore dumps. Several sites are situated in uncommon vegetation contexts, for example, montane desert scrub communities or locations where vegetation adjacent to taluses is sparse or lacking. Prox- imity to surrounding pika habitats (as a measure of marginality) was evaluated based on relative talus distribution pat- terns for 0.5-km, 2.5-km, and 5.0-km circular areas nested around each site. Seven idealized, schematic spatial patterns were used to assess potential connectivity among sites, ranging from “island” (no other talus within the respective areas) to “even” (many talus patches regularly distributed). Applying this approach to the 21 sites demonstrated a simple method for qualitatively assessing pika habitat relative to dispersal potential and metapopulation viability and also revealed complexities of biogeographic patterns related to marginality.