Modal optical granulometry: An easy-to-use methodology to quantitatively determine particle size in consolidated and inaccessible deposits.

This work presents a methodology called Modal Optical Granulometry (MOG), designed to quantitatively analyze particle size in consolidated or inaccessible deposits. This optical, non-invasive, low-cost method enables the generation of grain size profiles from digital photographs without the need for physical sampling. MOG is based on stereological principles, which involves measuring the intersections between superimposed horizontal lines and clasts in calibrated images. The technique consists of three stages: (1) photographing or filming the outcrop with a visible scale in the field, (2) image analysis using software such as Image Pro Plus or free alternatives, and (3) data processing using a custom-designed Excel spreadsheet. In addition to general granulometric analysis, the method enables the construction of Vertical Granulometric Profiles (VGP) and the analysis of Longitudinal Grain Size Evolution (LGE) along the deposit, providing valuable information on the dynamics of pyroclastic flows, lahars, and other sedimentary processes. The methodology is validated through a case study at Colima Volcano (Mexico), demonstrating its effectiveness in identifying depositional units and subtle textural variations. The technique also shows potential applications in other fields, such as, economic geology, and hydrological connectivity studies, due to its ability to deliver accurate and representative data in an accessible manner. Bullet points: Optical methodology for quantitative particle size analysis in volcaniclastic deposits Non-invasive technique suitable for inaccessible or consolidated outcrops Easy-to-use, robust, low-cost optical granulometry method.