Construction of granulometric profiles of pyroclastic deposits by means of optical methods

Abstract

Granulometric analysis of pyroclastic deposits provides important information about the transport and sedimentation mechanisms of the flows that generated them. In this paper, we introduce the alternative method of Rosiwal’s intercepts to study this type of deposits. The method consists in taking photographs of the outcrop from a known position. By means of an image-analysis software, the images are calibrated and digitally processed in order to improve the resolution. Successively, a set of line probes are established at different heights in the deposit and parallel to the stratification surfaces. The clasts that intercept these lines are measured. By using the Rosiwal’s method, we obtain the granulometric distribution at different levels of the deposit. A comparison of this method, with the more widely used and largely tested point-counting method, shows that de Rosiwal’s intercept method is more reliable. Because all the photographs taken from an outcrop are altered by the perspective of the photograph, we propose an empirical, practical, and yet rigorous method to correct the obtained data. The method was applied to two outcrops of an apparent single block- and ash-flow deposit at Colima volcano. The study of the vertical variations of granulometry by means of the Rosiwal’s intercept method allowed us to recognize structures, not evident to the naked eye. The observed variations are probably related to the presence of two different flow units in the block-and-ash flow deposit that might be due to pulsating flow waves at the time of deposition. This technique is very useful to study pyroclastic deposits and the results suggest that it could be also applied to other kind of granular deposits.