Analyse morphologique quantitative des cendres des dépôts pyroclastiques d'origine hydrovolcanique et magmatique

Abstract : Volcanic ash particles from different fragmentation mechanisms have different surface textures and morphologies. Every single ash particle may convey information about its own formation environment and conditions. Certain features may give a hint about the fragmentation regime, the intensity of fragmentation and quantity of water that partakes in the fragmentation process. However, volcanic ash was characterized widely by qualitative set phrases up to date and the success in the discrimination of ash types depended on the experience of the person who dealt with characterization of particles. Furthermore, the complexity and variability of volcanic particle shapes make quantitative characterization difficult and numerical characterization of volcanic ash surface texture, because of its complexity, still remains largely unresolved. Simple and well-defined surface descriptors were presented in this study. It is based on assuming that the gray level variation is related to the roughness variation of the surface. Here, gray level values were used in a form corresponding to vertical z-values on the surface. The disadvantages and advantages of the simplicity of method have been discussed. Seven roughness descriptors (Rq, Ra, Rsk, Rku, Rv, Rp, Rt), three quadtree parameters (nQT, mQT, sQT) and fractal dimensions (FDs) on ash surfaces were measured. A gradient analysis based on Sobel operators was performed and polar plots were generated based on frequency of gradients. Different shape descriptors such as form factor, compactness, roundness, aspect ratio, solidity, convexity and the fractal dimensions (FD_PP) were calculated on polar plots. Samples from five volcanoes with different eruption styles were selected for 2D analysis (Nemrut, Erciyes (Turkey), Galunggung (Indonesia), Villarica (Chile) and Miyakejima (Japan)). However, threedimensional images allow different morphological features to be correctly interpreted and definitively measured. Therefore, optically sectioned particles, stereoscopy and controlled grinding were used to acquire 3D datasets for reconstruction. Surface descriptors were calculated on 3D surfaces of ash particles from Villarica volcano (Chile). The surface descriptors were used to cluster different type of particles according to their fragmentation mechanisms and designated the phase changes in eruptions associated with interacting water/magma ratios. The optimum distinctive surface descriptors were determined with the assistance of correlations between qualitative surface descriptions, vesicularity, xenolith abundance, calculated water/magma mass ratios (R) and quantitative surface descriptors. It is agreed that roughness parameters Ra and Rq are sensible to micro-roughness (eg. fine dust) while other roughness parameters Rsk and Rku measure the macro-roughness associated with cavities on the surface (vesicles). Furthermore, quadtree parameter nQT, fractal dimension of surfaces and solidity of polar plots also have the same response to microroughness similar to Ra and Rq. Because of increasing interest in surface properties of natural materials, surface descriptors proposed in this study can be applied to surfaces of various natural materials.
Document type :
Volcanology. Université Blaise Pascal - Clermont-Ferrand II, 2007. French. <NNT : 2007CLF21769>
Contributor : Camille Meyer <>
Submitted on : Monday, July 16, 2012 - 2:55:40 PM
Last modification on : Monday, July 16, 2012 - 3:11:46 PM


  • HAL Id : tel-00718234, version 1


Ersoy Orkun. Analyse morphologique quantitative des cendres des dépôts pyroclastiques d'origine hydrovolcanique et magmatique. Volcanology. Université Blaise Pascal - Clermont-Ferrand II, 2007. French. <NNT : 2007CLF21769>. <tel-00718234>




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