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Rock avalanches display a distinct and complicated behavior that is not yet completely understood. Analyzing the deposit characteristics is crucial for studying the comminution mechanism during its emplacement. Combining field investigation, image processing, and numerical modeling, we explore the deposit characteristics and comminution process of the Wangjiapo rock avalanche triggered by the Mw6.2 Ludian earthquake. First, we use UAV photogrammetry to obtain a three-dimensional point cloud model and a digital orthophoto, from which we semi-automatically extract the geometry and position of 80,602 blocks by adopting a new block photo-analytical procedure, in addition to a further 4388 coarse blocks that were mapped from manual procedure. We analyze the longitudinal and transversal variation of the block distribution within the deposit. Large boulder content was more abundant in the middle part, while the fine blocks were distributed more evenly. Blocks were prevalently equant-sphere (ES) shaped with no dependence on volume and location, but the roundness increased significantly with runout. Additionally, we find fractal dimensions ranging from 1.42 to 1.83, lower with respect to other rock avalanches, probably due to the high weakness of materials. Furthermore, we propose two comminution models based on the significant differences observed between the eastern deposit II and western deposit III, which were caused by variably-cemented breccias. Finally, numerical modeling indicates that the large amount of fine material damped the grain-grain velocity during runout, leading to a limited spread of the rock avalanche and so explaining the short displacement of the front observed.

The process and dynamics of rock fragmentation during the collapse of rockfalls and rock avalanches is a poorly developed topic. The most severe fragmentation often leads to the formation of a rock dust that rises to form a cloud suspended in the air. The understanding of fragmentation processes is hampered by the environmental disturbances that alter the dust cloud deposit shortly after deposition. Here, we study the fragmentation of the October 2017 Pousset rockfall, detached from a NNE facing steep bedrock wall in the permafrost zone, that involved 8,300m3 of metamorphic rock and fell about 800 m. The collapse generated large boulders which rolled downslope and a thick and large dust cloud. The source and deposit were investigated, and dust cloud material was sampled at different locations to reconstruct an exponential thickness distribution and perform grain size characterization. The fragmentation energy was estimated by integrating the spectrum of the grains assuming that the fragmentation energy is proportional to the generated area. The fragmentation energy was found to be about 0.4% of the initial potential energy. Most probable fragmentation points and block deposition areas were evaluated and positioned by means of the HyStone 3D rockfall simulator. Furthermore, we calculated the flow rate of the suspended powder generated by the fragmentation process and compared the results with observations available for the evolution of the phenomenon and the collected samples. The Pousset event, in its relatively simple dynamics, may be a good testing ground to address the current theories of rockfall and rock avalanche fragmentation and dust cloud behavior.

Rock avalanches are extremely rapid, flow-like movements of dry debris mass with extreme destructive potential. Among the dynamical aspects of rock avalanches still inadequately understood, the flow along an erodible substrate stands out as a particularly significant one. We carried out a series of flume experiments to investigate the mobility and the erosion of granular flows on an erodible substrate. It was found that granular flows with a large total mass could evolve into a stable multi-layer flow when suffering the enhanced basal resistance (e.g., loose and dry erodible substrate). Without the variation of the properties of erodible material, the increase of total mass (from 5 kg to 15 kg) as well as the reduction of grain size (from 4 to 8 mm to 1–2 mm) has a positive influence on the mobility of the granular flows (max. 12% and max. 3.4% growths in runout distance, respectively), emphasizing the important roles of both the event magnitude and the internal friction. Total mass and the grain size have prominent influences also on the erosion effect of granular flow. An increase in flow mass causes an enhancement of the eroded depth and of the expansion in both backward and forward directions, while grain size enhanced only forward expansion in addition to the eroded depth. Altogether, the experiments indicate that an erodible substrate may have dramatic effects on the dynamics of an experimental granular flow. When applied to the field, they point to the importance of the nature of the basal material onto which a natural landslide collapses.

The exploration of the solar system has shown that mass movement is a common process on the surface of the terrestrial planets (Mercury, Venus, Mars) and on many moons, including our own. However, it is on Mars that mass movements represent a major geomorphologic force both in terms of frequency, volume (often greater than 10km3, with a record volume of 106 km3), and runout, normally longer than several tens of kilometers. The study of landslides and more in general of mass movements on Mars has important implications for assessing the rock and solid properties and is a tool for understanding the geologic and climatic history of the planet. In contrast to our planet, where a landslide deposit is erased or covered after few thousands of years, mass movements on Mars are still perfectly preserved after times that may be greater than 1 billion years, making of such impulsive events a key for understanding the conditions of the planetary surfaces deep in time. In particular, landslides and their associated primary and secondary deposits may shed light on the possible presence of water or ice on the planet at the instant of flow, which is of great astrobiological importance. Firstly, we describe the different types of landslides on Mars. Many types are similar to those on Earth, albeit often at a larger scales. We document the characteristics of slumps and rock avalanches, most of which occur in Valles Marineris, a 4000km-long system of 6-8km deep gorges. Landslides in Valles Marineris exhibit a variety of morphologies on their surfaces, such as Toreva blocks, longitudinal grooves, pressure ridges, run-ups on pre-existing mounds indicating high speed of emplacement. Other forms of mass movements are dubious of at least not as common as they are on Earth. Rockfalls have been documented in some cases, but their smaller size can be spotted only with high-resolution cameras. The presence of slow-expanding lateral spreads may indicate deep clay layers. Some landforms may be i

Mars has held large amounts of running and standing water throughout its history, as evidenced by numerous morphologies attributed to rivers, outflow channels, lakes, and possibly an ocean. This work examines the crater Antoniadi located in the Syrtis Major quadrangle. Some parts of the central area of the crater exhibit giant polygonal mud cracks, typical of endured lake bottom, on top of which a dark, tens of kilometers-long network of dendritic (i.e., arborescent) morphologies emerges, at first resembling the remnant of river networks. The network, which is composed of tabular sub-units, is in relief overlying hardened mud, a puzzling feature that, in principle, could be explained as landscape inversion resulting from stronger erosion of the lake bottom compared to the endured crust of the riverine sediments. However, the polygonal mud cracks have pristine boundaries, which indicate limited erosion. Furthermore, the orientation of part of the network is the opposite of what the flow of water would entail. Further analyses indicate the similarity of the dendrites with controlled diffusion processes rather than with the river network, and the presence of morphologies incompatible with river, alluvial, or underground sapping processes, such as overlapping of branches belonging to different dendrites or growth along fault lines. An alternative explanation worth exploring due to its potential astrobiological importance is that the network is the product of ancient reef-building microbialites on the shallow Antoniadi lake, which enjoyed the fortunate presence of a heat source supplied by the Syrtis Major volcano. The comparison with the terrestrial examples and the dating of the bottom of the crater (formed at 3.8 Ga and subjected to a resurfacing event at 3.6 Ga attributed to the lacustrine drape) contribute to reinforcing (but cannot definitely prove) the scenario of microbialitic origin for dendrites. Thus, the present analysis based on the images available from the

The recurrent failures of the 8 to 10-km high basal scarp of the Olympus Mons volcano on Mars has resulted in enormous landslides merging into a composite deposit known as the aureole. The largest of these landslides, the western (W) aureole, appears as a hummocky and extensive tongue reaching the distance of 700 ​km from the present Olympus Mons edge. Totalizing a volume of 106 Km3, the W aureole was created by a single, 100°-wide sector collapse of the Olympus Mons flank between north and west directions. Whereas the western segment of the landslide fell onto the flat area of Amazonis Planitia, the northern section came across the gently sloping ridge of Acheron Fossae after 550 ​km of travel, and subsequently rose against slope for some additional 50–80 ​km. Analysis of imagery on Acheron Fossae ridge beyond the front of the landslide shows a peculiar deposit and associated morphologies. Termed here Frontal Aureole Deposits (FAD), it occurs in two units: a smooth, uniform deposit immediately beyond the aureole terminus, followed in the distal part by distinct lobes directed against slope. These deposits are interpreted as the remnants of debris-rich sandy and muddy waves thrust by the W aureole landslide that inundated the Acheron Fossae ridge. Numerical and analytical calculations set up to study the dynamics of inundation show that the length of the deposits was in the reach of water thrust by a fast-travelling landslide. A possible explanation for the presence of water is that the front was travelling across an outlet of the ancient Martian ocean which, consequently, must have persisted when the landslide event took place. According to this hypothesis, FAD represents the remnant of a giant tsunami deposit similar, albeit at a much larger scale, to those that occur on Earth.

The mobility of landslides on Mars is studied based on a database of 3,118 events. To establish the volume of the landslides for the whole data set based on the deposit area, a new volume-area relationship based on a representative data set of 222 landslides is used. By plotting the H/L ratio between fall height H and runout L versus volume, the landslide mobility is analyzed and compared with existing empirical relationships for Martian and terrestrial landslides. By analyzing the mobility in terms of normalized residuals, that is, the relative deviation of the H/L ratio from the data set best-fit line, mobility is found to depend on both the landslide location on Mars and the landslide typology. This allows us to identify four different types of high-mobility (hypermobile) landslides. Three classes of high-mobility landslides are associated respectively to meteoroid impact, the Olympus Mons aureoles, and landslides with Toreva-block failure style, and their mobility can be explained by the peculiar flow mechanics. The fourth class includes landslides associated with isolated craters, those in the regions wetted by the putative Oceanus Borealis, and the ones at high latitudes. We suggest that the common factor behind all the hypermobile landslides of this fourth kind is the presence of ice. This is confirmed by our data showing that landslides increase in mobility with latitude. The latitudinal trend mirrors the distribution of ice as detected by radar, neutron probes, and the presence of glacial and layered ejecta morphologies. Because the overall landslide distribution supports the presence of ice-lubricated conditions, two ice lubrication models are presented showing how ice melting within or underneath the landslides could enhance mobility. By proper analysis in terms of apparent friction residuals, we find that the mobility of landslides in Valles Marineris with the largest landslide concentration is lower than average. We explain this circumstance partly from