Shallow Craters Research Articles

Drop impact on thin powder layers: pattern formation by air entrapment.

Impact of drops on thin powder layers displaces the powder particles radially outward producing shallow craters with thick rims, for example, as observed on dust layers on the floor. Here, we report that the patterns formed on thin powder layers by drop impact are not limited to such crater-like ones. Instead, depending upon the layer properties, disc or disc-plus-ring shaped patterns are formed at the impact point. We show that air entrapment and micro-bubble formation during the drop impact result in the formation of such patterns. Based on high-speed imaging, scaling analyses, and measurements with various liquids and powder layers, we propose a mechanism for the formation of such patterns. The phenomenon that we report can open further investigations on drop impact on the granular matter.

High Resolution Attenuation Images From Active Seismic Data: The Case Study of Solfatara Volcano (Southern Italy)

The quality factor Q is highly sensitive to the medium properties related to the fluid presence, like porosity, permeability and level of rock saturation. Therefore, it is very useful to image the subsoil of volcanoes in terms of anelastic images, in addition to more common elastic tomographic models. In this geological structure, indeed, fluids play a key role in controlling and governing the evolution of magmatic processes, so their accurate identification and tracking is crucial. Here, we describe a methodology that allows to obtain accurate 1-D and 3-D attenuation models from seismic active data. It has been applied to a small-scale volcanic volume of the shallow Solfatara crater, in southern Italy. Our purposes were achieved in three steps: 1/ we measured the attenuation parameter t* from active survey data, first by cross-correlating the signal with the sweep function and then by using the spectral decay method; 2/ we determined a reference 1-D attenuation model for the area through a grid model search procedure; 3/ we reconstructed the 3-D attenuation model through an iterative inversion of t* data. In order to validate the t* catalogue, we analyzed the map of t* residuals with respect to the reference 1-D model... The shallowest subsoil of Solfatara is characterized as a volume with a P-wave quality factor, QP, which increases with depth and has a high lateral anelastic heterogeneity. QP can be therefore considered as a proxy for the detection of strong variations both in fluid saturations and in the type of fluids permeating the shallowest volcanic sediments of the investigated volume.

Author Correction: Thick ice deposits in shallow simple craters on the Moon and Mercury

Author Correction: Thick ice deposits in shallow simple craters on the Moon and Mercury

Thick ice deposits in shallow simple craters on the Moon and Mercury

Permanently shadowed regions near the poles of Mercury and the Moon may cold-trap water ice for geologic time periods. In past studies, thick ice deposits have been detected on Mercury, but not on the Moon, despite their similar thermal environments. Here we report evidence for thick ice deposits inside permanently shadowed simple craters on both Mercury and the Moon. We measure the depth/diameter ratio of approximately 2,000 simple craters near the north pole of Mercury using Mercury Laser Altimeter data. We find that these craters become distinctly shallower at higher latitudes, where ice is known to have accumulated on their floors. This shallowing corresponds to a maximum infill of around 50 m, consistent with previous estimates. A parallel investigation of approximately 12,000 lunar craters using Lunar Reconnaissance Orbiter data reveals a similar morphological trend near the south pole of the Moon, which we conclude is also due to the presence of thick ice deposits. We find that previously detected surface ice deposits in the south polar region of the Moon are spatially correlated with shallow craters, indicating that the surface ice may be exhumed or linked to the subsurface via diffusion. The family of lunar craters that we identify are promising targets for future missions, and may also help resolve the apparent discrepancy between the abundance of frozen volatiles on Mercury and the Moon. Ice deposits up to around 50 m thick infill some craters near the Moon’s south pole and Mercury’s north pole, as inferred from the poleward shallowing of simple craters.

Flow Dynamics of a Film Cooling Jet Issued From a Round Hole Embedded in Contoured Crater

Recent advances in gas turbine film cooling technology such as round film cooling holes embedded in craters or trenches, and shaped film cooling holes are of interest due to a marked improvement in the effectiveness of film cooling jets. Typically, shaped film cooling holes have higher manufacturing cost, while film cooling holes embedded in craters/trenches etched in thermal barrier coatings (TBC) are seen as a cost-effective alternative. In a recent numerical study Kalghatgi and Acharya (2015, “Improved Film Cooling Effectiveness With a Round Film Cooling Hole Embedded in Contoured Crater,” ASME J. Turbomach., 137(10), p. 101006) reported a novel crater shape to generate anti-counter rotating vortex pair (CRVP) beneath the film cooling jet and showed a significant improvement in film cooling performance. In the present paper, a comprehensive study of flow dynamics is presented to gain insight into the unsteady flow physics of film cooling jet issued from a round hole embedded in the contoured crater. As a baseline case, a round film cooling hole with a 35 deg inclined short delivery tube (l/D = 1.75) is used as from a previous study with freestream Reynolds number based on jet diameter set to ReD = 16,000 and density ratio of coolant to freestream fluid of ρj/ρo = 2.0. These flow conditions are used for the cases of film cooling jet embedded in contoured crater. The results are presented for two crater depths: (i) shallow crater with 0.2D depth and (ii) deep crater with 0.75D depth. First- and second-order flow statistics are presented for all the cases, including the experimental data for baseline case from the literature. Time-averaged and instantaneous flow structures are visualized to reveal the mechanisms of anti-CRVP and attenuating CRVP. The dynamics of flow structures studied using single-point spectral analysis in the shear layer and modal analysis of three-dimensional flow field shows a loss of coherency and increased time scales of shear layer structures as the crater depth is increased, primarily due to attenuating of CRVP in the downstream vicinity of the crater. The modal analysis confirmed reduced magnitude of temperature fluctuations (hot spots) on the cooling wall compared with baseline round film cooling hole. Finally, a 2–5% additional pressure loss due to the crater is reported over the existing ≈7% loss in pressure for baseline case.

Crater Density Predictions for New Horizons Flyby Target 2014 MU69

Abstract In preparation for the 2019 January 1 encounter between the New Horizons spacecraft and the Kuiper Belt object 2014 MU69, we provide estimates of the expected impact crater surface density on the Kuiper Belt object. Using the observed crater fields on Charon and Pluto down to the resolution limit of the 2015 New Horizons flyby of those bodies and estimates of the orbital distribution of the crater-forming projectiles, we calculate the number of craters per unit area formed as a function of the time a surface on 2014 MU69 has been exposed to bombardment. We find that if the shallow crater size distribution from roughly 1–15 km exhibited on Pluto and Charon is indeed due to the sizes of Kuiper Belt projectiles, 2014 MU69 should exhibit a surface that is only lightly cratered below 1 km scale, despite being bombarded for ∼4 billion years. Its surface should therefore be more clearly indicative of its accretionary environment. In addition, this object may be the first observed for which the majority of the bombardment is from exogenic projectiles moving at less than or near the speed of sound in the target materials, implying morphologies more akin to secondary craters elsewhere in the solar system. Lastly, if the shallow Kuiper Belt size distribution implied from the Pluto and Charon imaging is confirmed at 2014 MU69, then we conclude that this size distribution is a preserved relic of its state ≃4.5 Gyr ago and provides a direct constraint on the planetesimal formation process itself.

Observations of in vivo laser tissue ablation in animal models with different chromophores on the skin and modulating duration per laser exposure

Characteristics such as skin tone and pigmentation color vary among patients, but most researches on laser irradiation in laser ablation have revolved around minimizing damage to reduce pain. Chromophores are the most important factors in photon energy absorption, a key principle of laser ablation. We investigated the influences on ablation depth by different chromophores on the target and modulating duration per laser exposure using an Nd:YVO4 nanosecond 532-nm laser. We used a Fourier-domain optical coherence tomography (Fd-OCT) system combined with a 532-nm Nd:YVO4 laser to observe the ablation process. In addition, an external customized shutter and water-based color pens (red, green, blue, black) were used to determine the effects of modulating the duration per laser exposure and coloring chromophores on porcine skin and hairless mouse models. Experiments with modulating duration per laser exposure demonstrated that shorter duration generated shallower craters than longer one. Painted experimental group showed damaged region as craters in the experiments with coloring various chromophores. In this research, we investigated the effects of modulating duration per laser exposure and different chromophores on the target. Coloring chromophores with water-based dyes using pens increased tissue damage without dyeing cells or tissue.

Hydroxyapatite mixed-electro discharge formation of bioceramic Lakargiite (CaZrO3) on Zr–Cu–Ni–Ti–Be for orthopedic application

ABSTRACTTo suit a target application, the machining mechanism of electro discharge machining (EDM) process has been hybridized to powder mixed EDM (PM-EDM) by adding various substances in the form of additives into the dielectric fluid. In this study, the application of PM-EDM to fabricate Lakargiite (CaZrO3) coating on Zr–Cu–Ni–Ti–Be substrate surface for potential orthopedic application, using hydroxyapatite (HA) powder as dielectric additive was proposed. The effect of various HA powder concentrations on deposition of CaZrO3 on Zr–Cu–Ni–Ti–Be surface was investigated. Zr–Cu–Ni–Ti–Be bulk metallic glass (BMG) has exceptional mechanical and chemical properties compared to its crystalline counterpart. It is expected that, the formation of a nanoporous bioactive CaZrO3 coating on the BMG surface will enhance its bioactivity. The FESEM investigation presents the formation of various interconnected shallow craters, nanopores, and few nanocracks and the energy dispersive X-ray spectroscopy results depict the presence of about 50% Lakargiite phase on the HA-deposited BMG surface. The X-ray powder diffraction characterization identified the phases includes ZrC, ZrO, TiC, and CaTiO3 formed on the PM-EDMed surface. The Rockwell hardness testing revealed the rise of BMG surface hardness to about 42%. The roughness of EDMed surface reduced from 6.22 nm to 2.00 nm after HA powder suspended in the dielectric fluid.

Analysis of nascent silicon phase-change gratings induced by femtosecond laser irradiation in vacuum

The formation of periodic surface structures is a general effect of femtosecond laser irradiation of solid targets showing promising interest in material science and technology. However, the experiments are typically carried out in air, a condition in which the target surface becomes densely decorated with nanoparticles that can influence the formation of the surface structures in the early stage of the irradiation process. Here we report an investigation of structures generation on a silicon surface irradiated in vacuum (10−5 mbar) with a low number of laser pulses (N ≤ 10) that exploits several microscopy techniques (optical, atomic force, electron and Raman). Our analyses allow identifying the creation of silicon phase-change gratings consisting of alternating amorphous and crystalline periodic lines, with almost no material removal, located at the periphery of a shallow ablation crater. These gratings originate from two different kinds of defects: (i) the first is characterized by a peculiar lobed shape that is produced by the first few laser pulses; (ii) the second is provided by the one-dimensional, linear singularity defined by the ablation edge of the nascent crater. Both kind of defects lead to grating structures extending outwards the amorphous central area of the crater along the direction of the laser polarization. Comparative analysis with the surface formed in air, in the same experimental conditions, evidences the important role played by nanoparticles densely decorating the target in air and the striking variation occurring in vacuum.

Diatom assemblage in the 24 cm upper sediment associated with human activities in Lake Warna Dieng Plateau Indonesia

Lake Warna is a small shallow crater lake on the Dieng Plateau, Central Java, the second highest plateau in the world after Nepal. A 24 cm sediment core was extracted from Lake Warna tore construct environmental changes in the Lake and its catchment from preserved diatom assemblages. Diatoms are microalgae in the Bacillariophyte that have silicious cell walls that can be preserved in sediments. As diatom species are sensitive to water quality changes in the assemblages upcore reflect changes in lake condition. Sediment cores were collected from two sites,sliced at 1 cm intervals for diatom analysis and bulked across 3 cm for 210 Pb radiometric dating. Examination of diatoms in a 24 cm sediment core from Lake Warna reveals clear correlation with human activities in the catchment area over the past 124 years. The record is divided into 2 zones based on sustained changes in the diatom assemblages. The lowest zone Zone I (21–15 cm, estimated 1935–1954) was dominated by Frustulia crassinervia (Brebisson ex W. Smith) Ross, Gomph onema parvulum (Kutzing) Kutzing, Pinnularia valdetolerans Mayama & H. Kobayasi, P. viridis (Nitzsch) Ehrenberg, and Aulacoseira distans (Ehrenberg) Simonson. Zone II (15–0 cm, estimated 1980–2013), the uppermost zone was dominated by P. viridiformis Krammer, P. latevittata Cleve, E. monodonvar.tropica(Hustedt)Hustedt,S.seminulumGrunow,P acrosphaeriaW.Smith. E. zygodon Ehrenberg, P. gibba Ehrenberg, and P. viridiformis Krammer. The 15 cm core from TW3 commenced sediment accumulation 114 years ago and is divided into 2 zones as well. Zone I (15–7 cm, estimated 1901–1981) was dominated by Brachysira brebissonii R. Ross, G. parvulum (Kutzing) Kutzing, E. monodon var. tropica (Hustedt) Hustedt, E. zygodon Ehrenberg and S. seminulum Grunow. Zone II (7–0 cm, estimated 1981–2006) wasdominated by P.viridi for mis Krammer, P.gibba Ehrenberg, Pinnularia viridis(Nitzsch) Ehrenberg, Sellaphora bacillum (Ehrenberg) Mann, S.seminulum Grunow,and Stauroneissp. These assemblages reflect on going acid,clear water conditions for the time represented by thecores. The recent rise in S.seminulum Grunow reflects recent lake eutrophication likely owing to the accelerated agricultural and urban development in the lake’s catchment in recentdecades.

Competence and lithostratigraphy of host rocks govern kimberlite pipe morphology

We use analogue experimentation to test the hypothesis that host rock competence primarily determines the morphology of kimberlite pipes. Natural occurrences of kimberlite pipes are subdivided into three classes: class 1 pipes are steep-sided diatremes emplaced into crystalline rock; class 2 pipes have a wide, shallow crater emplaced into sedimentary rock overlain by unconsolidated sediments; class 3 pipes comprise a steep-sided diatreme with a shallow-angled crater emplaced into competent crystalline rock overlain by unconsolidated sediments. We use different configurations of three analogue materials with varying cohesions to model the contrasting geological settings observed in nature. Pulses of compressed air, representing the energy of the gas-rich head of a kimberlitic magma, are used to disrupt the experimental substrate. In our experiments, the competence and configuration of the analogue materials control the excavation processes as well as the final shape of the analogue pipes: eruption through competent analogue strata results in steep-sided analogue pipes; eruption through weak analogue strata results in wide, shallow analogue pipes; eruption through intermediate strength analogue strata results in analogue pipes with a shallow crater and a steep-sided diatreme. These experimental results correspond with the shapes of natural kimberlite pipes, and demonstrate that variations in the lithology of the host rock are sufficient to generate classic kimberlite pipe shapes. These findings are consistent with models that ascribe the pipe morphologies of natural kimberlites to the competence of the host rocks in which they are emplaced.

Study of Machining of Inconel 825 Super Alloy Using Powder Mixed EDM Process

Powder mixed electrical Discharge Machining (PMEDM) plays vital role in machining process of some very difficult to machine materials. Inconel 825 are highly demanding materials for today’s industries due to their unique properties. Properties like high creep resistance under high stress condition, high resistant to corrosion, good machinability, ductility, weldibility, low value of thermal expansion makes it durable for high speed aeroplanes and supersonic jets. In present work Inconel 825 (nickel alloy) which is widely used in aerospace industries, power plants, nuclear power plants, petro chemical, chemical rockets has been used as work piece and machined with using powder mixed electrical discharge machining process (PMEDM). Peak current (IP), gap voltage (GV), powder concentration (PC) and pulse on time (TON) are opted as input experimental parameters. Whereas, surface roughness (SR) and material removal rate (MRR) are considered as output response parameters. Tool used during experiment is tungsten carbide. Tungsten carbide is hard materials in which wear is minimum. It could be concluded from the experiment that there is significant improvement in MRR as well as SR. Tool wear is negligible. Further, surface machined by PMEDM is compared with conventional EDM and analysed using field emission electron microscopy (FESEM) and RSM (response surface methodology). Images obtained by FESEM also confirm improvement in surface integrity. Moreover, less micro-crack, micro-holes and shallower craters were observed on the surface generated by PMEDM as compare to conventional EDM

ANTIMONY FIXATION IN SOLID PHASES AT THE HYDROTHERMAL FIELD OF KOLUMBO SUBMARINE ARC-VOLCANO (SANTORINI): DEPOSITION MODEL AND ENVIRONMENTAL IMPLICATIONS

Antimony, an emergent global contaminant, that is hydrothermally discharged along with other epithermal metals(-loids) (Au, As, Hg, Ag, Tl, Ag) onto Kolumbo volcano’s shallow (<500 m water depth) crater seabed, is fixed either in pyrite, orpiment-like As-sulfides, and ferrihydrite-like Fe-oxy(hydro)oxides, or forms independent Pb(Zn)- Sb sulfosalts, of layered Sb-rich (up to 2.2 wt%) chimneys. High concentrations of Sb (≤ 27.2 wt%) are found in early colloform chemically-zoned hydrothermal pyrite, and later orpiment (As2S3)-type As sulfide phases(≤16.09 wt %), along individual micronscale growth zones. Antimony in pyrite may occur in the relatively more toxic trivalent (or lower valence) (Sb3+) rather than pentavalent (Sb5+) forms. Lead (Pb) always occurs with Sb in growth zones where the abundances of Sb and Pb vary inversely with Fe and S, suggesting that Sb and Pb occur either as homogeneously distributed sulfosalt nanoparticles of Sb and Pb and/or lattice bound trace elements. These findings indicate the solid phases that fix Sb on the seafloor are crucial for high- grade concentration during shallow-water hydrothermal polymetallic mineralization, and reducing the high hydrothermal flux of this notorious environmental toxin to seawater, near the fishing area of Santorini that is also one of the most popular tourist places in the world.

The role of pre-impact topography in impact melt emplacement on terrestrial planets

On terrestrial planets, flow-like deposits of impact melt are commonly found exterior to fresh impact craters. Often, these deposits are asymmetric about the crater rim, and the direction of flow may relate to either the pre-impact topography or the impact azimuth. In this work, we seek to determine the primary mechanism responsible for impact melt emplacement on two very different terrestrial worlds: the Moon and Venus. Newly derived stereo topography data allows us to investigate the role of pre-impact topography in melt emplacement on Venus for the first time. We determine that pre-impact topography plays an important role in melt emplacement around complex craters on the Moon but not on Venus. This difference may relate to the differences in gravity (and hence, crater depth and melt volume) on the two worlds. Melt trapped in the relatively deep lunar craters may require additional momentum to be pushed over the crater rim, and therefore emerges preferentially from the region of rim crest low. This added momentum might come from uplift during the crater modification stage. For the shallower craters on Venus, the downrange momentum imparted in oblique impacts may be sufficient to push the melt up and over the crater rim, explaining the correlation between melt direction and impact azimuth. Understanding the emplacement of impact melt on terrestrial planets of different sizes and geologic histories provides added insight into the impact cratering process.

3D ultra-high resolution seismic imaging of shallow Solfatara crater in Campi Flegrei (Italy): New insights on deep hydrothermal fluid circulation processes

Seismic tomography can be used to image the spatial variation of rock properties within complex geological media such as volcanoes. Solfatara is a volcano located within the Campi Flegrei, a still active caldera, so it is of major importance to characterize its level of activity and potential danger. In this light, a 3D tomographic high-resolution P-wave velocity image of the shallow central part of Solfatara crater is obtained using first arrival times and a multiscale approach. The retrieved images, integrated with the resistivity section and temperature and the CO2 flux measurements, define the following characteristics: 1. A depth-dependent P-wave velocity layer down to 14 m, with Vp < 700 m/s typical of poorly-consolidated tephra and affected by CO2 degassing; 2. An intermediate layer, deepening towards the mineralized liquid-saturated area (Fangaia), interpreted as permeable deposits saturated with condensed water; 3. A deep, confined high velocity anomaly associated with a CO2 reservoir. These features are expression of an area located between the Fangaia, water saturated and replenished from deep aquifers, and the main fumaroles, superficial relief of the deep rising CO2 flux. Therefore, the changes in the outgassing rate greatly affect the shallow hydrothermal system, which can be used as a “mirror” of fluid migration processes occurring at depth.

Viscous relaxation of Ganymede’s impact craters: Constraints on heat flux

Measurement of crater depths in Ganymede’s dark terrain have revealed substantial numbers of unusually shallow craters indicative of viscous relaxation [see companion paper: Singer, K.N., Schenk, P. M., Bland, M.T., McKinnon, W.B., (2017). Relaxed impact craters on Ganymede: Regional variations and high heat flow. Icarus, submitted]. These viscously relaxed craters provide insight into the thermal history of the dark terrain: the rate of relaxation depends on the size of the crater and the thermal structure of the lithosphere. Here we use finite element simulations of crater relaxation to constrain the heat flux within the dark terrain when relaxation occurred. We show that the degree of viscous relaxation observed cannot be achieved through radiogenic heating alone, even if all of the relaxed craters are ancient and experienced the high radiogenic fluxes present early in the satellite’s history. For craters with diameter ≥ 10 km, heat fluxes of 40–50 mW m−2 can reproduce the observed crater depths, but only if the fluxes are sustained for ∼1 Gyr. These craters can also be explained by shorter-lived “heat pulses” with magnitudes of ∼100 mW m−2 and timescales of 10–100 Myr. At small crater diameters (4 km) the observed shallow depths are difficult to achieve even when heat fluxes as high as 150 mW m−2 are sustained for 1 Gyr. The extreme thermal conditions required to viscously relax small craters may indicate that mechanisms other than viscous relaxation, such as topographic degradation, are also in play at small crater diameters. The timing of the relaxation event(s) is poorly constrained due to the sparsity of adequate topographic information, though it likely occurred in Ganymede’s middle history (neither recently, nor shortly after satellite formation). The consistency between the timing and magnitude of the heat fluxes derived here and those inferred from other tectonic features suggests that a single event caused both Ganymede’s tectonic deformation and its crater relaxation. Future observations should permit more robust determination of the relative timing of the heating event that caused viscous relaxation.

Relative depths of simple craters and the nature of the lunar regolith

We assessed the morphologies of more than 930 simple impact craters (diameters 40m–10km) on the Moon using digital terrain models (DTMs) of a variety of terrains in order to characterize the variability of fresh crater morphology as a function of crater diameter. From Lunar Reconnaissance Orbiter Camera (LROC) Narrow Angle Camera (NAC) DTMs, we determined depth-to-diameter (d/D) ratios for an extremely fresh set of these craters with diameters less than 400m and found that their d/D ratios range from 0.11 to 0.17. Using both NAC and Kaguya Terrain Camera DTMs, we also determined the d/D ratios for the set of fresh simple craters larger than 400m in diameter. The d/D ratios of these larger craters are typically near 0.21, as expected of gravity-dominated crater excavation. Fresh craters less than ∼400m in diameter, on the other hand, exhibit significantly lower d/D ratios. Various possible factors affect the morphologies and relative depths (d/D ratios) of small strength-dominated craters, including impactor and target properties (e.g., effective strength, strength contrasts, porosity, pre-existing weaknesses), impact angle and velocity, and degradation state. While impact conditions resulting from secondary impacts can also affect crater morphologies, we found that d/D ratio alone was not a unique discriminator of small secondary craters. To investigate the relative influences of degradation and target properties on the d/D ratios of small strength-dominated craters, we examined a subset of fresh craters located on the geologically young rim deposits of Tycho crater. These craters are deeper and steeper than other craters of similar diameter and degradation state, consistent with their relative freshness and formation in the relatively coherent, melt-rich deposits in this region. The d/D ratios of globally distributed small craters of similar degradation state and size range, on the other hand, are relatively shallow with lower average wall slopes, consistent with crater excavation in a weak or poorly cohesive layer. The widespread predominance of these small, shallow craters is consistent with the pervasive, poorly cohesive upper regolith.

Experimental investigations on electrical discharge diamond peripheral surface grinding (EDDPSG) of hybrid metal matrix composite

In this paper, authors have endeavored to develop an experimental setup of Electrical Discharge Diamond Peripheral Surface Grinding which is a configuration of Electrical Discharge Diamond Grinding process. The machining performance characteristics were experimentally studied using the developed experimental setup during machining of a challenging material Al/SiCp/B4Cp hybrid metal matrix composite. Gap current, on-time, off-time, abrasive grit number, speed of wheel, and table speed were input process parameters considered. The influence of input process parameters were experimentally investigated on material removal rate and surface roughness using one parameter at a time approach. Scanning Electron Microscopy was used to study the surface texture and recast layers on machined surface. The surface finish is better achieved at a critical pulse off-time with appropriate declogging of diamond wheel. A specific abrasive grit size is suitable for better removal of thermal infested surface deteriorations with minimal marks of grinding traces. The presence of shallow craters and negligible thickness of recast layers on machined surface substantiate the potential capability and applicability of developed EDDPSG experimental system.

Eruptive history of the Ubehebe Crater cluster, Death Valley, California

A sequence of late Holocene eruptions from the Ubehebe Crater cluster in Death Valley was short-lived, emplacing several phreatomagmatic and magmatic deposits. Seven craters form the main group, which erupted along a north-south alignment 1.5km long. At least five more make a 500-m east-west alignment west of the main crater group. One more is an isolated shallow crater ~400m south of that alignment. All erupted through Miocene fanglomerate and sandstone, which are now distributed as comminuted matrix and lithic clasts in all Ubehebe deposits. Stratigraphic evidence showing that all Ubehebe strata were emplaced within a short time interval includes: (1) deposits from the many Ubehebe vents make a multi-package sequence that conformably drapes paleo-basement topography with no erosive gullying between emplacement units; (2) several crater rims that formed early in the eruptive sequence are draped smoothly by subsequent deposits; and (3) tack-welded to agglutinated spatter and bombs that erupted at various times through the sequence remained hot enough to oxidize the overlying youngest emplacement package. In addition, all deposits sufficiently consolidated to be drilled yield reliable paleomagnetic directions, with site mean directions showing no evidence of geomagnetic secular variation. Chemical analyses of juvenile components representing every eruptive package yield a narrow range in major elements [SiO2 (48.65–50.11); MgO (4.98–6.23); K2O (2.24–2.39)] and trace elements [Rb (28–33); Sr (1513–1588); Zr (373–404)]. Despite lithologic similarities, individual fall units can be traced outward from vent by recording layer thicknesses, maximum scoria and lithic sizes, and juvenile clast textural variations. This permits reconstruction of the eruptive sequence, which produced a variety of eruptive styles. The largest and northernmost of the craters, Ubehebe Crater, is the youngest of the group. Its largely phreatomagmatic deposits drape all of the others, thicken in paleogullies and thin over several newly created crater rims. Evidence in-hand virtually requires that the Ubehebe cluster of craters erupted over a brief time interval, not protracted over centuries.

A COMPARATIVE MORPHOLOGICAL STUDY OF THE KOS-NISYROS-TILOS VOLCANOSEDIMENTARY BASINS

A swath bathymetric map of Kos-Nisyros-Tilos Volcanic field was created with 50m grid interval, with 10 m isobaths at a scale 1:100.000 using SEABEAM 1180 (180 kHz) multibeam system for depths &lt;500m and SEABEAM 2120 (20 kHz) multibeam system for depths &gt;500m. Five basins have been distinguished in the circum-volcanic area of Nisyros: 1)Eastern Kos basin, the larger and deeper one, with an average sea-bottom depth of 630m. Submarine canyons within the basin occur along the southern coastline of Kos cutting the isobaths from 150 up to 400m depth. A shallow crater with relative topography of ±70m has been discovered at the bottom of the basin (600-670m) 2)Western Kos basin with average depth of 520m. The basin is separated from the Eastern Kos Basin by a rise between Yali and Kos at 400m depth. This basin is separated from the Western Nisyros basin by the Kondeliousa rise. 3)The Western Nisyros basin is located between Kondeliousa rise and western Kos platform with depths of 550m. 4)The Southern Nisyros basin constitutes the northern end of the large Karpathos basin which reaches more than 2000m depth towards the south. 5)The Tilos basin with depths of 600m occurs southeast of Nisyros Island, separated from South Nisyros Basin through a rise of less than 400m depth. The Pachia-Pergoussa and Yali-Nisyros basins are shallow structures within the intra-volcanic relief of Nisyros and surrounding islets. The geometrty of each basin is discussed in relation to the volcanic and tectonic structure of the graben between Kos and Tilos.The intensity of the active geodynamic acrtivity is demonstrated by the creation of a volcanic relief of 1400m in the Nisyros volcanic field.