Roche magmatique | EPFL Graph Search

Les roches magmatiques ou roches ignées (anciennement, roches éruptives), se forment quand un magma se refroidit et se solidifie, avec ou sans cristallisation complète des minéraux le composant. Cette solidification peut se produire :

lentement en profondeur, cas des roches magmatiques plutoniques (dites « intrusives ») ;
rapidement à la surface, cas des roches magmatiques volcaniques (dites « extrusives » ou « effusives »).

Dans tous les cas, les roches magmatiques sont qualifiées d'endogènes car formées en profondeur, par opposition aux roches exogènes (telles les roches sédimentaires et les roches métamorphiques). Elles sont formées par solidification de matériaux à la surface du globe. Les roches volcaniques ne sont que trempées à la surface, la cristallisation s'effectue bien en profondeur. Les roches magmatiques les plus courantes sont le granite et le basalte : la famille des granites représente 95 % des roches plutoniques et les basaltes 90 % des roches volcaniques. De

Influence of Microplastics on Microbial Structure, Function, and Mechanical Properties of Stream Periphyton

Renata Behra, Kristin Schirmer, Ahmed Tlili

Periphyton is a freshwater biofilm composed of prokaryotic and eukaryotic communities that occupy rocks and sediments, forming the base of the food web and playing a key role in nutrient cycling. Given the large surface that periphyton comprises, it may also act as a sink for a diverse range of man-made pollutants, including microplastics (MP). Here we investigated the effect of 1-4 mu m and 63-75 mu m sized, spherical polyethylene MP with native and ultraviolet (UV)-weathered surface on developing natural stream periphyton communities over 28 days. In order to ensure proper particle exposure, we first tested MP suspension in water or in water containing either Tween 80, extracellular polymeric substances - EPS, fulvic acids, or protein. We found the extract of EPS from natural periphyton to be most suitable to create MP suspensions in preparation of exposure. Upon exposure, all tested types of MP were found to be associated with the periphyton, independent of their size and other properties. While biomass accrual and phenotypic community structure of the photoautotrophs remained unchanged, the prokaryotic and eukaryotic communities experienced a significant change in composition and relative abundances. Moreover, alpha diversity was affected in eukaryotes, but not in prokaryotes. The observed changes were more prominent in periphyton exposed to UV-treated as compared with native surface MP. Mechanical properties, as assessed by compression rheology, showed that MP-exposed periphyton had longer filamentous streamers, higher stiffness, lower force recovery and a higher viscoelasticity than control periphyton. Despite the observed structural and mechanical changes of periphyton, functional parameters (i.e., photosynthetic yield, respiration and nutrient uptake efficiencies) were not altered by MP, indicating the absence of MP toxicity, and suggesting functional redundancy in the communities. Together, our results provide further proof that periphyton is a sink for MP and demonstrate that MP can impact local microbial community composition and mechanical properties of the biofilms. Consequences of these findings might be a change in dislodgement behavior of periphyton, a propagation through the food chains and impacts on nutrient cycling and energy transfer. Hence, taking the omnipresence, high persistence and material and size diversity of MP in the aquatic environment into account, their ecological consequences need further investigation.

FRONTIERS MEDIA SA2022

A deep crust-mantle boundary in the asteroid 4 Vesta

Harold Gabriel Eugène Justin Clenet, Philippe Gillet

The asteroid 4 Vesta was recently found to have two large impact craters near its south pole, exposing subsurface material. Modelling suggested that surface material in the northern hemisphere of Vesta came from a depth of about 20 kilometres, whereas the exposed southern material comes from a depth of 60 to 100 kilometres. Large amounts of olivine from the mantle were not seen, suggesting that the outer 100 kilometres or so is mainly igneous crust. Here we analyse the data on Vesta and conclude that the crust-mantle boundary (or Moho) is deeper than 80 kilometres.

Nature Publishing Group2014

Geological characterization of cataclastic rock samples using medical X-ray computerized tomography

Pierre Guillaume Christe

Technical difficulties associated with tunnelling operations in tectonized geological settings are frequently encountered. They may include instantaneous and delayed cavity convergence, sudden collapse of walls or roof of a gallery, outpouring of fault-filling materials and water inflows. These phenomena may negatively affect economical and safety aspects of construction sites. The present study refers to two previous research projects conducted at the EPFL addressing the problem of an improved geological and geomechanical characterization of weak cataclastic rocks in underground excavation works (Cataclastic fault rocks in underground excavations, a geological perspective, Bürgi, 1999; Caractérisation géomécanique de roches cataclastiques rencontrées dans des ouvrages souterrains alpins, Habimana, 1999). Cataclasis is an evolutionary rock degradation process in time and magnitude related to fault zone activity in the Earth's crust. A conceptual model describing the occurrence and variability of cataclastic rocks at shallow crustal conditions is proposed in this research. It describes environment of fault zones based on both theoretical considerations and field experiences collected during self-realized short-drilling operations performed in contrasted petrological settings. The model considers cataclasis from a regional perspective (macroscopic scale) down to its microscopic manifestations on rock materials. Cataclastic rock characteristics are strongly influenced by the degree of tectonic solicitation and the initial rock composition. Guidelines about specific damage microstructures diagnostic of the cataclastic intensity and function of the initial rock protolith can meaningfully assist the interpretation of rock specimens recovered from reconnaissance drilling operations. Despite the poor mechanical quality often observed for such samples and their readiness to collapse, this study demonstrates that the in situ extraction of cataclastic specimens can be largely improved with the use of a high-quality drilling equipment. Accordingly, with the availability of representative materials for subsequent studies, the uncertainties affecting the geological prognosis can be strongly limited at a preliminary stage of geotechnical studies. It is shown that the geotechnical characterisation of cataclastic rock cores by means of laboratory investigations requires combining information about rock microstructure and mineralogy with strain-stress relationship derived from triaxial compression tests. Such a framework makes it possible to interpret and explain the specific mechanical behavior of heterogeneous-anisotropic rock materials in a much more coherent manner. With this regard, the approach followed by Bürgi (1999) with a mineralo-structural index (MSI) determined on 2D thin sections and proposed to predict cataclastic rock strength based on geological evidences, is found to have potential for geotechnical studies. However, Bürgi's approach still faces heavy criticism in the light of the practical application. Its limitations are detailed and discussed in this study. Accounting for both aspects of the rock characterization procedure, an operational methodology is therefore proposed aiming at its application in geotechnical studies. The medical X-ray computerized tomography (XRCT) has been specifically implemented in this context for the analysis of geotechnical rock cores, as a non-destructive technique which allows a rapid indirect imaging of the rock core structure before any further analytical manipulations are conducted. The technique is found facilitating to a great extent the study of cataclastic materials. An irradiation protocol has been defined and an acquisition strategy minimalizing inherent artifacts of the XRCT technique has been tested. Obtained results are highly encouraging. Moreover, an image analysis tool has been developed in order to improve the segmentation of geological damage features from indirectly recorded X-ray models. As discussed in details within this study, a critical analysis of 3D segmented models corresponding to sample structures before and after mechanical tests is thought to be capable to improve the triaxial test interpretation. The XRCT is therefore envisaged as a bridge technique permitting an improved correlation between specific properties of cataclastic rocks based on indirect evidences. With it, the geological and mechanical characterizations can be performed on the exactly same samples, reducing uncertainty in the characterization procedure. This step goes towards the development of a precise geotechnical classification for cataclastic rocks. Based on the XRCT technique, it is also discussed how Bürgi's approach could be translated in a three-dimensional form to reach a wider agreement. Based on a database of carbonatic cataclasites, it is evidenced that cataclastic strength and deformation properties are strongly influenced by textural and structural characteristics of pre-existing damage microstructures. Strength data obtained in this research show that in the same fault zone environment, different mode of deformations affect cataclastic rocks and that peak triaxial strength can vary significantly. Heterogeneity and material anisotropy are prime factors affecting specific behaviors of cataclastic rocks. Therefore, it is suggested to incorporate geological features into constitutive frameworks which describe mechanical behavior of such materials. This last aspect constitutes a very stimulating research field where further work is needed today.

EPFL2009