Crater Collapse Research Articles

Fast droplets impacting liquid layer under a small angle: a numerical analysis of experimental observations

In presence of a strong wind above a liquid surface, liquid droplets get accelerated and eventually hit the interface with a high speed under a small angle. Experimental studies demonstrate that such droplets may escape the initial crater and either get broken into secondary droplets in the air, or protrude along the interface, leaving behind agitated surface with entrapped bubbles. Here we aim at reproducing the observed phenomena in a numerical model based on Volume of Fluid technique using Basilisk package. On a flat interface, the impacting droplets partially escape the crater and get broken in front of it within the range of impact angles from 15° to 45°, in agreement to the experiments and simple theoretical considerations. In presence of waves, the escaping droplets fly into the air but, being adhered to the crater, get stretched and broken into secondary droplets. For smaller angles, the droplets escape the crater and glide along the interface. The gliding droplet creates a chain of small craters in its wake. The rims between the craters may grow and get scattered into secondary droplets. Large liquid viscosity and small thickness of the liquid layer suppress the growth of the craters and secondary entrainment. Bubble entrapment during such impacts most likely occurs due to crater collapse with trapping air inside. However, such air pockets are not formed systematically in the present model. This discrepancy requires further investigation.

Twenty years of explosive-effusive activity at El Reventador volcano (Ecuador) recorded in its geomorphology

Shifts in activity at long-active, open-vent volcanoes are difficult to forecast because precursory signals are enigmatic and can be lost in and amongst daily activity. Here, we propose that crater and vent morphologies, along with summit height, can help us bring some insights into future activity at one of Ecuador’s most active volcanoes El Reventador. On 3 November 2002, El Reventador volcano experienced the largest eruption in Ecuador in the last 140 years and has been continuously active ever since with transitions between and coexistence of explosive and effusive activity, characterized by Strombolian and Vulcanian behavior. Based on the analysis of a large dataset of thermal and visual images, we determined that in the last 20 years of activity, the volcano faced three destructive events: A. Destruction of the upper part of the summit leaving a north-south breached crater (3 November 2002), B. NE border crater collapse (2017), and C. NW flank collapse (2018), with two periods of reconstruction of the edifice: Period 1. Refill of the crater (2002-early 2018) and Period 2. Refill of the 2018 scar (April 2018–December 2022). Through photogrammetric analysis of visual and thermal images acquired in 11 overflights of the volcano, we created a time-series of digital elevation models (DEMs) to determine the maximum height of the volcano at each date, quantify the volume changes between successive dates, and characterize the morphological changes in the summit region. We estimate that approximately 34.1x106 m3 of volcanic material was removed from the volcano due to destructive events, whereas 64.1x106 m3 was added by constructive processes. The pre-2002 summit height was 3,560 m and due to the 2002 eruption it decreased to 3,527 m; it regained its previous height between 2014 and 2015 and the summit crater was completely filled by early April 2018. Event A resulted from an intrusion of magma that erupted violently; we proposed that Events B and C could be a result of an intrusion as well but may also be due to a lack of stability of the volcano summit which occurs when it reaches its maximum height of approximately 3,590 and 3,600 m.

Water striders are impervious to raindrop collision forces and submerged by collapsing craters

Water striders are abundant in areas with high humidity and rainfall. Raindrops can weigh more than 40 times the adult water strider and some pelagic species spend their entire lives at sea, never contacting ground. Until now, researchers have not systematically investigated the survival of water striders when impacted by raindrops. In this experimental study, we use high-speed videography to film drop impacts on water striders. Drops force the insects subsurface upon direct contact. As the ensuing crater rebounds upward, the water strider is propelled airborne by a Worthington jet, herein called the first jet. We show the water strider's locomotive responses, low density, resistance to wetting when briefly submerged, and ability to regain a super-surface rest state, rendering it impervious to the initial impact. When pulled subsurface during a second crater formation caused by the collapsing first jet, water striders face the possibility of ejection above the surface or submersion below the surface, a fate determined by their position in the second crater. We identify a critical crater collapse acceleration threshold ∼ 5.7 gravities for the collapsing second crater which determines the ejection and submersion of passive water striders. Entrapment by submersion makes the water strider poised to penetrate the air-water interface from below, which appears impossible without the aid of a plastron and proper locomotive techniques. Our study is likely the first to consider second crater dynamics and our results translate to the submersion dynamics of other passively floating particles such as millimetric microplastics atop the world's oceans.

Thin Ice Lithospheres and High Heat Flows on Europa From Large Impact Structure Ring‐Graben

AbstractCraters are probes of planetary surface and interior properties. Here we measure depths, widths, and spacing of circumferential ring‐graben surrounding the two largest multiring impact structures on Europa, Tyre and Callanish. We estimate formation conditions including the ice shell structure. The radial extension necessary to form these graben is thought to be caused by asthenospheric drag of warmer, more ductile ice and/or water flowing toward the excavated center of the crater, under a brittle‐elastic lithospheric lid. Measurements of graben depths from stereo‐photoclinometric digital elevation models result in estimates of displacement, strain, and stress experienced by the ice shell. Graben widths are used to estimate the intersection depth of the bounding normal faults, a quantity related to the brittle‐ductile transition depth that approximates elastic shell thickness during crater collapse. Heat flows at the time of crater formation as well as ice lithosphere and total shell thickness are thus also constrained. Average widths and depths tend to decrease with increasing distance from the structure center, while inter‐graben spacing generally increases. Varied assumptions yield plausible total conductive ice shell thickness estimates between 4–8 and 2.5–5 km for Tyre and Callanish, respectively, and heat flows of ∼70–115 (±30) mW m−2 for realistic thermal conductivities, consistent with other geophysical estimates for Europa. Higher heat flows are consistent with thin (≲10 km), conductive ice shells and impact breaching, or penetration of the stagnant lid for a convecting ice shell. Callanish, geologically younger, formed in a time or region of greater heat flow than Tyre.

Nyiragongo Crater Collapses Measured by Multi‐Sensor SAR Amplitude Time Series

AbstractCrater morphology undergoes rapid changes at active volcanoes, and quantifying these changes during volcanic unrest episodes is crucial for assessing volcanic activity levels. However, various limitations, including restricted crater access, cloud cover, haze, and intra‐crater eruptive activity, often impede regular optical or on‐site crater monitoring. To overcome these challenges, we utilize multi‐sensor satellite Synthetic Aperture Radar (SAR) imagery to generate dense time series of quantitative indicators for monitoring crater morphological changes. By combining images from diverse satellites and acquisition modes, we achieve high temporal resolution. Nevertheless, due to variations in acquisition geometries, direct image comparisons become impractical. To address this, we develop PickCraterSAR, an open‐access Python tool that employs basic trigonometry assumptions to measure crater radius and depth from SAR amplitude images in radar geometry. We apply our methodology to study the crater collapse associated with the May 2021 and January 2002 eruptions of Nyiragongo volcano. Following the 2021 collapse, we estimate the maximum depth of the crater to be 850 m below the rim, with a total volume of 84 ± 10 Mm3. Notably, the post‐2021 eruption crater was 270 m deeper but only 15%–20% more voluminous compared to the post‐2002 eruption crater. Additionally, we demonstrate that the 2021 crater collapse occurred progressively while a dike intrusion migrated southward as a consequence of the drainage of the lava lake system. Overall, our study showcases the utility of multi‐sensor SAR imagery and introduces PickCraterSAR as a valuable tool for monitoring and analyzing crater morphological changes, providing insights into the dynamics of volcanic activity.

Crustal Block and Muted Ring Development During the Formation of Mercury's Caloris Megabasin

AbstractIn contrast to multiring basins, megabasins lack clear ring structures and display crustal profiles characterized by a thin layer of crust at their center that gradually increases in thickness to the surrounding region. The 1,550‐km‐diameter Caloris basin on Mercury is often speculated to be a multiring basin; however, its crustal structure is indicative of a megabasin, perhaps explaining the lack of discernible rings. Here, we model the formation of Caloris basin using iSALE‐2D under a range of preimpact thermal conditions to determine whether the processes responsible for its megabasin crustal structure could lead to the formation of basin rings. We find that thermal gradients between 22 and 30 K/km—hotter than previously inferred—facilitate the reproduction of Caloris basin's crustal structure through crustal flowback, though such preimpact thermal structures preclude its formation as a multiring basin. Instead, repeated thrusting and necking events during transient crater collapse induce instances of fault reactivation, which diminish initial fault offsets and produce a series of discrete crustal blocks. Models assuming cooler thermal gradients lead to overly thin or non‐existent crust at the basin center, in contrast to observations. Even if crustal deficits were made up via a differentiating melt pool, the crust elsewhere would be overly thick, supporting the idea that the crustal structure of megabasins is primarily associated with transient crater collapse. We conclude that the transition to a megabasin morphology on terrestrial planetary surfaces, like the multiring transition, is dependent upon the size of the basin compared to the lithospheric thickness.

The formation and evolution of Pluto's Sputnik basin prior to nitrogen ice fill

We simulate Sputnik Planitia (SP), Pluto's largest impact basin, from formation through cooling and relaxation prior to loading of the basin with N2 ice. To assess potential conditions for Pluto's interior that permit the formation of an SP-like basin in terms of depth and diameter, we consider impacts into targets that possess a subsurface ocean with variable ice shell thicknesses and thermal structures. We use a shock physics code to model excavation and transient crater collapse. Then, using the final temperature and density structure from the shock physics code as initial conditions, we simulate the subsequent cooling and viscoelastic relaxation of basin topography using a finite element model (FEM). We show that a thin ice shell (on the order of 100 km) overlying a thick ocean (on the order of 228 km) produces an SP-like basin in terms of diameter and pre-N2 depth. A basin formed in a much thicker ice shell (200 km) overlying a thin ocean (128 km) can reproduce SP if the ice shell is relatively warm and possesses a strong rheology. Our results suggest that SP was not a mascon basin prior to subsequent loading by N2 ice, but rather close to isostatically compensated. We find that SP could have developed into a mascon basin following N2 loading if the ice shell is thin, conductive and possesses a lithosphere capable of supporting the N2 load.

Fluid‐Like Behavior of Crushed Rock Flows

AbstractThe puzzle of the unexpectedly high mobility of large geophysical flows has been reported as “solved” many times since Albert Heim drew attention to it after a disastrous landslide at Elm, Switzerland. However, the mechanism of high mobility remains mysterious. A series of high‐speed rotary shear experiments on particles of different minerals was conducted to explore the high mobility flow‐like behavior of crushable dense granular flow. We found that the flow behavior was more explicable when the shear resistance was considered as a viscous resistance rather than as a frictional resistance. We also found that for the crushable material, the viscosity dramatically decreased and reached a constant and relatively low value, which controlled the high fluidity and hypermobility of large geophysical flows such as rock avalanches. More precisely, there are two phases of the flow behavior, that are separated at a weakening point in the accumulating strain for crushable material. The first phase was a simple Newtonian or non‐Newtonian‐like flow. The second phase was more complex, in which the flow viscosity decreased profoundly and reached a constant viscous resistance at a considerable strain. This finding is important for explaining the hypermobility of many large geophysical processes, such as rock avalanche motion, natural faulting and crater collapse. In particular, we demonstrate that the behavior of rock avalanches is similar to that of complicated fluids with extensive weakening and that the viscosity of this special “liquid” is as low as 500 Pa·s. This finding can also help improve the accuracy and reliability of the numerical simulation of rock avalanches by using the viscous model obtained from the experiments.

Olivine and glass chemistry record cycles of plumbing system recovery after summit collapse events at Kīlauea Volcano, Hawai‘i

The eruptive activity of Kīlauea Volcano (Hawai‘i) in the past 2500 years has alternated between centuries-long periods dominated either by explosive or effusive eruptions. The onset of explosive periods appears to be marked by caldera collapse events at the volcano's summit accompanied by draining of Kīlauea's magmatic plumbing system. Here we leverage >1800 olivine forsterite (Fo) contents, >900 glass MgO contents, and estimated magma supply rates from the past six centuries to describe the relationships between summit collapse and the composition of erupted material. On a first order basis, the major element chemistry of the centuries-long eruptive periods largely originates from fundamental differences between fractional crystallization of shallowly stored magmas during high-supply effusive-dominated periods versus little evolution of mafic recharge magmas during low-supply explosive-dominated periods. The modern effusive period (1820s-present) is dominated by relatively evolved olivine forsterite contents (Fo81–82) for Kīlauea, which is interpreted to reflect a buffered crustal reservoir system in which shallow storage and fractional crystallization control the composition of magmas. In contrast, olivine crystals from the explosive Keanakāko‘i Tephra (1500 - early 1800s C.E.) are dominated by higher olivine forsterite contents (Fo89) which are interpreted to reflect more primitive compositions, are correlated with glass MgO compositions extending to high values (e.g.,11.0 wt%), and show signs of magma mixing (zoned olivine, bimodal Fo populations). These signatures reflect a disrupted reservoir system in which high-MgO recharge melts mix with melts left over from draining of the shallow (<5 km) magma plumbing.Superimposed on these explosive-effusive periods are three decades- to centuries long periods of progressively evolving olivine and glass compositions. Eruptions that occur after caldera collapse in ~1500C.E. and smaller scale crater collapse events in 1790 (inferred) and 1924 have heterogeneous olivine populations dominated by ≥Fo88 and typically high MgO glasses. These compositions reflect inefficient mixing of stored and primitive recharge magmas after the disruption of the shallow plumbing system. After these collapses, olivine Fo and glass MgO subsequently evolve to <Fo82 and <7.0 wt% compositions, reflecting the recovery of the crustal plumbing system to an end-member system state characterized by efficient mixing of recharge and stored magmas that serve to buffer the shallow magma reservoirs. These evolved signatures suggest that a mature and buffered reservoir system may be a key condition for significant disruptions of volcanic plumbing systems. Plumbing system recovery is slower following large-scale caldera collapse (hundreds of years) compared to recovery following smaller crater collapse (tens of years), which may be modulated by differences in magma supply rates. Following the 2018 crater collapse olivine populations have high-Fo but glasses are low MgO, suggesting that this collapse might have disrupted shallow magma pathways but not strongly impacted the reservoir(s). Ultimately, olivine and glass major element chemistry record the impacts of caldera and smaller but significant summit crater collapse events at Kīlauea and could be used to provide a framework for better characterizing long-term volcano evolution in Hawai‘i and shield volcanoes elsewhere.

Orientations of planar cataclasite zones in the Chicxulub peak ring as a ground truth for peak ring formation models

Hypervelocity impact cratering is an important geologic process but the rarity of large terrestrial impact craters on Earth and the limited technical options to study cratering processes in the laboratory hinders our understanding of large-scale impact processes. Drill core recovered from the peak ring of the Chicxulub impact crater during International Ocean Discovery Program (IODP)/International Continental scientific Drilling Program (ICDP) Expedition 364 provides an opportunity to examine target rock deformation and thus, to assess cratering models in this regard. Using oriented computer tomography (CT) scans and line scan images of the core, we present the orientations of mm-to-cm-scale planar cataclasite and ultracataclasite zones in the deformed granitoid target rock of the peak ring. In the upper 470 m of the target rock, the cataclasite zones dip towards the crater center, whereas the dip directions for the ultracataclasite zones are approximately tangential to the peak ring. These two orientations are consistent with deformation expected from hydrocode-modeled principal stress directions for the outward movement of rocks as the transient crater develops, and the inward movement of rocks associated with collapse of the transient crater. Near the base of the core is a 96 m-thick interval of highly-deformed target rock with impact melt rock and rock fragments not observed elsewhere in the core; this interval has previously been interpreted as an imbricate thrust zone within the peak ring. The cataclasite zones below this thrust zone have different orientations than those in the 470 m-thick block above. This observation implies a differential rotation from the overlying block during the final stages of peak-ring formation. Our results support an acoustic fluidization process, wherein blocks that vibrate or slide relative to each other allow the target rock to flow during transient crater collapse, and that the size of coherent rock blocks increases over the course of crater modification as the target rock regains its cohesive strength and acoustic fluidization decreases.

Onset and evolution of Kīlauea's 2018 flank eruption and summit collapse from continuous gravity

Prior to the 2018 lower East Rift Zone (ERZ) eruption and summit collapse of Kīlauea Volcano, Hawai‘i, continuous gravimeters operated on the vent rims of ongoing eruptions at both the summit and Pu‘u ‘Ō‘ō. These instruments captured the onset of the 2018 lower ERZ eruption and the effects of lava withdrawal from both locales, providing constraints on the timing and style of activity and the physical properties of the lava lakes at both locations. At the summit, combining gravity, lava level, and a three-dimensional model of the vent indicates that the upper ∼200 m of the lava lake had a density of about 1700 kgm−3, slightly greater than estimates from 2011–2015 and possibly indicating a gradual densification over time. At Pu‘u ‘Ō‘ō, gravity and vent geometry were used to model both the density and the rate of crater collapse, which was unknown owing to a lack of visual observations. Results suggest the withdrawal of at least 11×106 m3 of lava over the course of two hours, and a material density of 1800–1900 kgm−3. In addition, gravity data at Pu‘u ‘Ō‘ō captured a transient decrease and increase about an hour prior to crater collapse and that was probably related to a small, short-lived fissure eruption on the west flank of the cone and possibly to dike intrusion beneath Pu‘u ‘Ō‘ō. The fissure was the first event in the subsequent cascade that ultimately led to the extrusion of over 1 km3 of lava from lower ERZ vents, collapse of the summit caldera floor by more than 500 m, and the destruction of over 700 homes and other structures. These results emphasize the importance of continuous gravity in operational monitoring of active volcanoes.

Investigating the Influences of Crustal Thickness and Temperature on the Uplift of Mantle Materials Beneath Large Impact Craters on the Moon

AbstractIn this work, we examine variations in the mantle uplift associated with large lunar impact craters and basins between major terranes. This study is based on Bouguer gravity anomalies of 100–650‐km diameter impact craters using Gravity Recovery and Interior Laboratory (GRAIL) observations and the Lunar Orbiter Laser Altimeter (LOLA) crater database. The Bouguer gravity anomalies of 324 large impact craters analyzed herein are primarily controlled by the uplifted crust‐mantle (Moho) interface in the central region of these impact craters, while postimpact mare deposits that represent ∼25%–35% of the postimpact deposit column also contribute to the gravity anomalies of mare craters. The central uplift of the Moho interface is primarily controlled by impact energy and increases to ∼ 30 km for a 650‐km diameter crater. Analyses of nonmare craters in the Feldspathic Highlands Terrane (FHT) with varied crustal thickness (Tc) reveal that the onset crater diameter (Dmin) with an uplifted Moho interface is dependent on the local Tc: Dmin ∼ 150 + 1.3Tc (in a unit of km). This equation also provides a quantification of the depth‐dependent attenuation of impact‐induced structural uplift, using the Moho uplift as a proxy for the structural uplift. The Moho uplift of large craters in the South Pole‐Aitken (SPA) Terrane is not statistically different from the FHT craters, consistent with limited overall thermal difference between these two terranes and compensational thermal effects at crater collapse and viscoelastic relaxation stages.

Multitude of dimple shapes can produce singular jets during the collapse of immiscible drop-impact craters

Abstract

Why the lunar South Pole-Aitken Basin is not a mascon

Lunar mascon basins exist for only a range of observed diameters. We modeled the full formation of South-Pole Aitken basin using a sequential two-code (hydrocode and Finite Element Model) approach to understand why this range does not extend to the largest lunar basin. Similar to previous work, we found that the best-fit hydrocode had impact parameters of a 170km in diameter dunite projectile striking at 10km/s, with a pre-impact lunar thermal gradient of 50K/km (until a depth of 28 km at which an adiabat is reached), and pre-impact crustal thickness value of 40 km. Unlike previous work, we matched the crustal distribution of the inner basin by utilizing a weaker melt rheology in our model. The crust in the models with a weaker melt rheology flowed inward after crater collapse to cover the basin center; therefore, not requiring an additional step of melt differentiation. Given the large diameter of South-Pole Aitken and the high geothermal gradient at impact, the steady state configuration after relaxation and thermal cooling was driven by isostatic forces without a significant contribution from lithospheric rigidity. Without lithospheric rigidity, South-Pole Aitken relaxed to a slightly negative free-air gravity signature, not forming a mascon, in agreement with GRAIL observations.

Composition, Stratigraphy, and Geological History of the Noachian Basement Surrounding the Isidis Impact Basin.

The western part of the Isidis basin structure hosts a well-characterized Early Noachian to Amazonian stratigraphy. The Noachian Basement comprises its oldest exposed rocks (Early to Mid-Noachian) and was previously considered a single low-Ca pyroxenes (LCP)- and Fe/Mg-smectite-bearing unit. Here, we divide the Noachian Basement Group into five distinct geological units (Stratified Basement Unit, Blue Fractured Unit, Mixed Lithology Plains Unit, LCP-bearing Plateaus Unit, and Fe/Mg-smectite-bearing Mounds Unit), two geomorphological features (megabreccia and ridges), and a mineral deposit (kaolinite-bearing bright materials), based on geomorphology, spectral characteristics, and stratigraphic relationships. Megabreccia contain four different pre-Isidis lithologies, possibly including deeper crust or mantle materials, formed through mass wasting associated with transient crater collapse during Isidis basin formation. The Fe/Mg-smectite-bearing Stratified Basement Unit and LCP-bearing Blue Fractured Unit likewise represent pre-Isidis units within the Noachian Basement Group. Multiple Fe/Mg-smectite-bearing geological units with different stratigraphic positions and younger kaolinite-bearing bright materials indicate several aqueous alteration episodes of different ages and styles. Units with slight changes in pyroxene spectral properties suggest a transition from low-Ca pyroxene-containing materials to those with higher proportions of pyroxenes higher in Ca and/or glass that could be related to different impact and/or igneous processes, or provenance. This long history of Noachian and potentially Pre-Noachian geological processes, including impact basin formation, aqueous alteration, and multiple igneous and sedimentary petrogeneses, records changing ancient Mars environmental conditions. All units defined by this study are available 20 km outside of Jezero crater for in situ analysis and sampling during a potential extended mission scenario for the Mars 2020 rover.

Seismic attribute analysis of Chicxulub impact crater

Chicxulub crater formed ~ 66 Ma ago by an asteroid impact on the Yucatan platform in the southern Gulf of Mexico. The crater has a ~ 200 km rim diameter and has been covered by carbonate sediments up to ~ 1.1 km thick in the central zone. Previous studies have identified the structure and major crater units through geophysical models from seismic reflection and potential field data, classified as the central uplift, terrace zone, outer and inner ring fault zones and impactite deposits. Impact produced a deep excavation cavity, with fragmentation and ejection of large volumes of crustal target rocks. Understanding the pre-existing structures, impact-induced deformation and post-impact processes requires high-resolution images of the crater and target zone. For this study, we use complex trace attributes of instantaneous phase, frequency, envelope amplitude and similarity, in an E-W seismic reflection profile crossing the crater in the marine sector. Geophysical logs and borehole lithological columns from the on-land drilling projects are used to constrain the petrophysical analysis. Seismic attributes aid to characterize the radial fault zones and physical property contrasts, revealing asymmetries in the crater structure. The reflector packages in the post-impact sediments and target Cretaceous sequence are identified in the frequency and phase attributes. The bottom crater reflectors, with the basal sediments filling the crater floor topography, are enhanced with the envelope amplitude attribute. A set of high-amplitude reflectors is shown in the similarity attribute, in which the reflector geometry is delineated on the target carbonate sequence. The offsets in the high-amplitude reflectors between the eastern and western sectors are possibly associated to target pre-impact asymmetries, impact deformation and effects of central crater collapse.

Modeling the Formation of the Lunar Upper Megaregolith Layer

Abstract In this work, we divide the classic “lunar megaregolith” layer into three distinct regions: (1) a Surficial Regolith layer, about 5–20 m in depth, consisting of loose, unconsolidated fines and breccia, and characterized by frequent overturn and comminution caused by small impacts; (2) an Upper Megaregolith layer, about 1–3 km in depth, consisting of depositional layers of brecciated and/or melted material, and characterized by the transport and deposition of material via either transient crater gravitational collapse or impact ejecta ballistic sedimentation; and (3) a Lower Megaregolith layer, about 20–25 km in depth, consisting of bedrock that has been fractured in place, and characterized by a fracture-density and fragment-size distribution that decreases rapidly with increasing depth. The objective of this study is to model the formation of the lunar Upper Megaregolith layer, the least well characterized of these three layers, using modern scaling relationships and a three-dimensional terrain, Monte Carlo cratering model. We first developed a model impactor population that accurately reproduces the Lunar Highlands crater population, which is assumed to originate in the Main Asteroid Belt. We then applied this impactor population in multiple full-scale lunar surface simulations, producing an Upper Megaregolith depth of 1.4 ± 1.0 km at the point of best χ 2 fit between model and actual crater counts. This Upper Megaregolith layer consists of ∼60% crater collapse deposits and ∼40% impact ejecta deposits. We find that a total delivered impactor mass of 3.72 ± 1.14 × 1019 kg, or 0.0506 ± 0.0156 lunar weight percent (wt%), is required to reproduce the observed Lunar Highlands cratering record.

Friction weakening by mechanical vibrations: A velocity-controlled process.

Frictional weakening by vibrations was first invoked in the 70s to explain unusual fault slips and earthquakes, low viscosity during the collapse of impact craters or the extraordinary mobility of sturzstroms, peculiar rock avalanches which travels large horizontal distances. This mechanism was further invoked to explain the remote triggering of earthquakes or the abnormally large runout of landslides or pyroclastic flows. Recent experimental and theoretical works pointed out that the key parameter which governs frictional weakening in sheared granular media is the characteristic velocity of the vibrations. Here we show that the mobility of the grains is not mandatory and that the vibration velocity governs the weakening of both granular and solid friction. The critical velocity leading to the transition from stick-slip motion to continuous sliding is in both cases of the same order of magnitude, namely a hundred microns per second. It is linked to the roughness of the surfaces in contact.

A modified gas hydrate-geomorphological model for a new discovery of enigmatic craters and seabed mounds in the Central Barents Sea, Norway

Hundreds of newly discovered gas seeps, craters, and mounds are presented from the central Barents Sea, Norway. Where they occur together, the crater-mound pairs have a preferred orientation towards the southeast. According to complex cross-cutting relationships with iceberg ploughmarks, both the craters and mounds formed during deglaciation (ca. 11–13 ka BP). Free gas, trapped beneath shallow gas hydrates in discrete areas, deformed or displaced overlying bedrock and sediment, forming mounds. Over-steepened slopes, weak layers, fractured bedrock, and unstable sediment resulted in small landslides towards the southeast: the direction of regional slope. In some cases, the highly buoyant, hydrate-cored sediment mounds may have instead, detached and floated with bottom currents before being re-deposited, either locally or distally. In this scenario, southeasterly flowing bottom currents would also explain the preferred orientation of the observed crater-mound pairs. Crater collapse would have occurred simultaneously or subsequently to landslide or detachment and floating due to the rapid dissociation of any remaining gas hydrates; a result of attendant regional changes in temperature and pressure following deglaciation and local removal of overlying sediment. The model of formation developed in this study explains the southeasterly trend in crater-mound orientation, but differs from other studies that interpret modern mounds, located elsewhere in the Barents Sea, as gas hydrate-cored.

Structure of Lō‘ihi Seamount, Hawai‘i and Lava Flow Morphology From High-Resolution Mapping

High-resolution bathymetric data were collected using AUV Sentry at the summit and at two hydrothermal vent fields on the deep south rift of L¯o‘ihi Seamount. The summit map records a nested series of caldera and pit crater collapse events, uplift of one resurgent block, and eruptions that formed at least five low lava shields that shaped the summit. The earliest and largest caldera, formed ~5900 years ago, bounds almost the entire summit plateau. The resurgent block was uplifted slightly more than 100 m and has a tilted surface with a dip of about 6.5° towards the SE. The resurgent block was then modified by collapse of a pit crater centered in the block that formed West Pit. The shallowest point on L¯o‘ihi’s summit is 986 m deep and is located on the northwest edge of the resurgent block. Several collapse events culminated in formation of East Pit, and the final collapse formed Pele’s Pit in 1996. The nine mapped collapse and resurgent structures indicate the presence of a shallow crustal magma chamber, ranging from depths of ~1 km to perhaps 2.5 km below the summit, and demonstrate that shallow sub-caldera magma reservoirs exist during the late pre-shield stage. On the deep south rift zone are young medium- to high-flux lava flows that likely erupted in 1996 and drained the shallow crustal magma chamber to trigger the collapse that formed Pele’s Pit. These low hummocky and channelized flows had molten cores and now host the FeMO hydrothermal field. The Shinkai Deep hydrothermal site is located among steep-sided hummocky flows that formed during low-flux eruptions. The Shinkai Ridge is most likely a coherent landslide block that originated on the east flank of L¯o‘ihi.