This work presents the first exhaustive study of the entire surface of the Reunion Island volcanic system. The focus is on the submarine part, for which a compilation of all multibeam data collected ...during the last 20 years has been made. Different types of submarine features have been identified: a coastal shelf, debris avalanches and sedimentary deposits, erosion canyons, volcanic constructions near the coast, and seamounts offshore. Criteria have been defined to differentiate the types of surfaces and to establish their relative chronology where possible. Debris avalanche deposits are by far the most extensive and voluminous formations in the submarine domain. They have built four huge Submarine Bulges to the east, north, west, and south of the island. They form fans 20–30 km wide at the coastline and 100–150 km wide at their ends, 70–80 km offshore. They were built gradually by the superimposition and/or juxtaposition of products moved during landslide episodes, involving up to several hundred cubic kilometers of material. About 50 individual events deposits can be recognized at the surface. The landslides have recurrently dismantled Piton des Neiges, Les Alizés, and Piton de La Fournaise volcanoes since 2 Ma. About one third are interpreted as secondary landslides, affecting previously emplaced debris avalanche deposits. On land, landslide deposits are observed in the extensively eroded central area of Piton des Neiges and in its coastal areas. Analysis of the present-day topography and of geology allows us to identify presumed faults and scars of previous large landslides. The Submarine Bulges are dissected and bound by canyons up to 200 m deep and 40 km long, filled with coarse-grained sediments, and generally connected to streams onshore. A large zone of sedimentary accumulation exists to the north–east of the island. It covers a zone 20 km in width, extending up to 15 km offshore. Volcanic constructions are observed near the coast on both Piton des Neiges and Piton de la Fournaise volcanoes and are continuations of subaerial structures. Individual seamounts are present on the submarine flanks and the surrounding ocean floor. A few seem to be young volcanoes, but the majority are probably old, eroded seamounts. This study suggests a larger scale and frequency of mass-wasting events on Reunion Island compared to similar islands. The virtual absence of downward flexure of the lithosphere beneath the island probably contributes to this feature. The increased number of known flank–failure events has to be taken into consideration when assessing hazards from future landslides, in particular, the probability of landslide-generated tsunamis.
•A drone acquired TIR and visible data on two harbour quays during daytime.•We detected thermal anomalies associated to surface subsidences.•The anomalies are not explained by surface properties nor ...topography.•The vertical permeability within the subsidences may control the surface temperatures.
This work presents for the first time associated thermal anomalies and deformations over subsurface cavities/voids located within two harbour quays in Le Havre Harbourg, Normandy, France. An U.A.V. (Unmanned Aerial Vehicle) was used to acquire visible and thermal images over a diurnal cycle (from 7 a.m. to 5 p.m.). The visible images were processed to realize an altimetric model of the platforms by mean of the photogrammetric method, while the thermal infrared (TIR) images were used to study the evolution of their surface temperatures. The obtained 3D model shows the location of five topographic depressions on both quays. The analysis of the evolution of the surface thermal field leads to the detection of cold thermal anomalies that are (1) not correlated to surface properties, and (2) spatially associated to the flanks of the five topographic depressions. Using a 2D conductive-radiative model, we show that these anomalies are not directly due to the thermally insulating effect of an air-filled area. Finally, we conclude that preferential infiltration and subsequent evaporation in the micro-fracturation present within the flanks of the depressions may be responsible for the strong cooling of these zones.
We present an effective method of determining the doping level in n-type III–V semiconductors at the nanoscale. Low-temperature and room-temperature cathodoluminescence (CL) measurements are carried ...out on single Si-doped GaAs nanowires. The spectral shift to higher energy (Burstein–Moss shift) and the broadening of luminescence spectra are signatures of increased electron densities. They are compared to the CL spectra of calibrated Si-doped GaAs layers, whose doping levels are determined by Hall measurements. We apply the generalized Planck’s law to fit the whole spectra, taking into account the electron occupation in the conduction band, the bandgap narrowing, and band tails. The electron Fermi levels are used to determine the free electron concentrations, and we infer nanowire doping of 6 × 1017 to 1 × 1018 cm–3. These results show that cathodoluminescence provides a robust way to probe carrier concentrations in semiconductors with the possibility of mapping spatial inhomogeneities at the nanoscale.
New detailed swath bathymetry and backscatter data corroborate the existence of four large bulges on the submarine flanks of Reunion Island. These fan-shaped promontories are 20-25 km wide at the ...coastline and 70-150 km across the seafloor 40-50 km offshore. Their surfaces are characterized by a speckle sonar pattern, indicating the presence of large blocks up to several hundred meters across. Each bulge results from the superposition of multiple landslide deposits whose older ones are dissected and delimited by erosive channels as much as 200 m deep and 20 km long. The submarine flanks of Reunion Island are thus mostly built by accumulation of debris avalanche fans. Morphologic and geologic evidence define large subaerial source areas for these mass-wasting events. In particular, inferred headwalls of most landslides having affected the Piton des Neiges massif generally coincide with the boundaries of its "cirques" (Mafate, Salazie, and Cilaos), whereas recurrent landslides have resulted in the formation of large concentric amphitheatre structures through the Piton de la Fournaise massif. Thus, about 15 slide events accompanied growth of the Reunion Island shield since 2 Ma.PUBLICATION ABSTRACT
Two-dimensional (2D) ferroelectric (FE) materials are promising compounds for next-generation nonvolatile memories due to their low energy consumption and high endurance. Among them, α-In2Se3 has ...drawn particular attention due to its in- and out-of-plane ferroelectricity, whose robustness has been demonstrated down to the monolayer limit. This is a relatively uncommon behavior since most bulk FE materials lose their ferroelectric character at the 2D limit due to the depolarization field. Using angle resolved photoemission spectroscopy (ARPES), we unveil another unusual 2D phenomenon appearing in 2H α-In2Se3 single crystals, the occurrence of a highly metallic two-dimensional electron gas (2DEG) at the surface of vacuum-cleaved crystals. This 2DEG exhibits two confined states, which correspond to an electron density of approximately 1013 electrons/cm2, also confirmed by thermoelectric measurements. Combination of ARPES and density functional theory (DFT) calculations reveals a direct band gap of energy equal to 1.3 ± 0.1 eV, with the bottom of the conduction band localized at the center of the Brillouin zone, just below the Fermi level. Such strong n-type doping further supports the quantum confinement of electrons and the formation of the 2DEG.
The 26 October–23 November 2010 eruption is Merapi’s largest event (VEI 4) over the past 140 years. We used high-spatial-resolution (HSR) imagery from GeoEye, Pléiades, IKONOS, and SPOT5 satellites ...to assess the extent and effects of the pyroclastic density currents (PDCs) and subsequent lahars. We have tracked the geomorphic and structural (fracturing) changes of Merapi’s summit crater and dome between 2008 and 2012. The 4 September 2011 GeoEye image shows that due to the explosive eruption, the summit area lost about 10 × 10
6
m
3
. The eruption enlarged the SSE-trending Gendol breach to be 1.3 × 0.3 × 0.2 km. The 2010 tephra and PDC deposits covered about 26 km
2
in the two catchments of Gendol and Opak Rivers on Merapi’s south flank, i.e., 60–75 % of the total PDC deposit area, with a total bulk volume of 45 × 10
6
m
3
. The tephra-fall deposit covered an area of about 1300 km
2
with a range in volume of 18–21 × 10
6
m
3
. Supervised and object-oriented classification on HSR imagery enables us to map in detail the PDC deposits across the Gendol-Opak catchment. We delineated 16 spectrally and/or texturally distinct units of PDC deposits and compared them with previously published results. They encompass high-energy surge deposits within ca. 8 km of the summit, valley-confined PDC deposits channeled as far as 16.5 km in the Gendol River, and widespread overbank PDC with ash-cloud surge deposits on valley margins.
Additional high-resolution data are provided to map and analyze flooded areas due to lahar activity in 2011–2012 on the south and west flanks. Subsequent overbank lahars impacted selective small areas in the populated ring plain, devastating villages along the Putih River on the southeast flank and the Gendol River. We have analyzed the morphometric parameters (channel cross-sectional capacity, longitudinal rate of channel confinement, and channel sinuosity) of the Gendol-Opak River channels that govern overbank and avulsion of lahars in the ring plain. The paper demonstrates the potential of HSR satellite imagery to outline tephra, PDC, and lahar deposits; map the geomorphic and structural evolution of the summit area of persistently active composite cones; and thus improve hazard assessment for Merapi, a volcano whose summit, slopes, and drainages have changed more since October 2010 than at any other time since 1931.
Many models describing the Earth’s magnetic field exist and are maintained by international research organizations. However, no clear general overview of the cross-compared quality of these models is ...available. An evaluation of the internal component and of the anomaly field as estimated by a selection of nine geomagnetic models with global coverage is presented here. Anomaly field models are also compared with an external, independently processed marine dataset extracted from Shom’s (the French Hydrographic and Oceanographic Service) geophysical database. The comparative study shows that internal geomagnetic fields computed outside the provisional domain, up to degrees 12 or 13 of spherical harmonics (SH), can be considered as equivalent. Relative amplitude differences with the World Magnetic Model 2015 (WMM2015) and the 12th generation of the International Geomagnetic Reference Field (IGRF-12) taken as references are lower than 10 nT. For an extent reduced to the north-eastern Atlantic Ocean and western Mediterranean Sea, anomaly field models up to SH degree 720 and the 3rd version of the Earth Magnetic Anomaly Grid (EMAG2v3) at the equivalent 0.5° resolution are the closest models to the filtered marine data. However, they are significant spatial discrepancies and standard deviations of differences between models and data are about 65 nT. The 3-arc-minute World Digital Magnetic Anomaly Map (WDMAM) best reflects the unfiltered marine data, but with significant standard deviations of about 70 nT. Results demonstrate that any model can be used with insignificant errors to compute raw anomalies from total field measurements by removing the internal component of the Earth’s magnetic field. In addition, they suggest that the priority should be given to WDMAM for levelling and evaluating marine datasets and for computing enhanced high-resolution regional magnetic maps.
During their growth and evolution, oceanic hot-spot and arc shield volcanoes are destabilized and often dismantled by landslides. The origin of these phenomena can be found in the structure of ...volcanic islands marked by several low strength horizons such as a marine substratum, hyaloclastites, hydrothermally altered rocks, or volcaniclastic deltas, that weaken the edifice. We have tested the role of these Low Strength Layers (LSLs) with analogue modeling. We conclude that destabilization on these levels can lead to landslides or gravitational spreading. In our experiments, slides generated are only slump-type events, but they can evolve, in nature, into debris avalanches through mechanisms that could not be modeled in the laboratory. The role of fan-shaped volcaniclastic deltas included in the edifice structure is critical to slump formation. This process is more efficient if deltas are associated with another upper LSL, such as weak hyaloclastites or hydrothermally altered rocks. In this case, recurrent events generating collapses with increasing width are observed. Gravitational spreading of a volcanic island occurs when it is underlain by a ductile marine substratum, or includes an intra-edifice unconstrained hyaloclastic layer, 1/3 as thick as the height of the subaerial edifice. Spreading is characterized by edifice stretching and subsidence and does not generate major landslides. We also show that gravitational spreading on a marine substratum tends to inhibit slide generation, especially when the thickness of the sedimentary layer is larger than 1/10 of the edifice total height. However, if this value is lower, spreading occurs later and slumps can develop initially. We describe natural examples of oceanic hot-spot or arc shield volcanoes affected by both spreading and landslides, such as Reunion Island, Hawaii Island, James Ross Island (Antarctica) and Martinique Island (Antilles). The main structural features on these volcanoes can be explained solely by deformation on LSLs. We therefore propose a general model for the destabilization of volcanic islands that is applicable to all shield-like edifices.
Marine magnetic data extracted from the geophysical database of the SHOM (the French Hydrographic and Oceanographic Service) offer a first overview of the magnetic offshore environment in the Gulf of ...Saint-Malo (Brittany, France). Their cross-interpretation with available geological and geophysical knowledge provides a new land and sea model of the western part of the Late Proterozoic North Armorican Cadomian belt. In particular, marine data exhibit relatively intense and heterogeneous magnetic signatures, mainly interpreted as the offshore prolongation of plutonic, volcanic, and metamorphic geologic formations recognized onshore. Imprints of major faults well known on land can be inferred at sea from discontinuities and shifting of magnetic anomalies or followed by high-resolution bathymetry. An impressive and dense dolerite dyke swarm propagated on more than 50km from the coast seaward. Dykes are characterized by both 1-km-deep and less than 50-m-deep superficial magnetic responses, suggesting the existence of deeply rooted super dykes to which individual small-scale structures are connected.
Erosion processes on active volcanoes in humid climates result in some of the highest sediment yields on Earth. Episodic sediment yields after large eruptions have been evaluated, but not the ...long-term and continuous patterns on persistently active volcanoes. We have used high-spatial resolution satellite imagery and DEMs/DSMs along with field-based geologic mapping to assess accurately sediment budgets for the active Semeru Volcano in Java, Indonesia. Patterns of aggradation and degradation on Semeru differ from that of other active volcanoes because (1) both episodic pyroclastic density currents (PDC) and continuous supplies of tephra generate pulses of sediment, (2) sediment is transferred via cycles of aggradation and degradation that continue for >15 years in river channels after each PDC-producing eruption, and (3) rain-triggered lahars remove much greater material than fluvial transport during long, intense rainfall events. The geomorphic response of two of Semeru’s rivers to volcanic sediment migration indicates that (1) each river experiences alternating aggradation and degradation cycles following PDC-producing eruptions and (2) spatial patterns of sediment transfer are governed by geomorphic characteristics of the river reaches. Usually high degradation in the steep source reach is followed by a long bypassing middle reach. Aggradation predominates in the depositional reaches further down valley on the ring plain. Average sediment yields (10
3
–10
5
t/km
2
/year) at persistently active volcanoes are two to three orders of magnitude lower than sediment yields after large and infrequent eruptions, but the continuous and steady sediment transfer in rivers removes more sediment on a mid-term (10 years) to long-term (30 years) basis. In contrast to the trend observed on composite cones after large and infrequent eruptions, decay of sediment yields is not exponential and river channels do not fully recover at steadily active volcanoes as episodic inputs from BAF eruptions, superimposed on the background remobilization of daily tephra, have a greater cumulative effect.
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