Morphology of the collapse scar of the Chiles Volcano (Colombia-Ecuador border) and geomechanical characterization of rock masses

Abstract

Chiles Volcano is an active stratovolcano of the Cordillera Occidental in a dormant state built by accumulation of andesitic-dacitic lava flows, which was affected by a large lateral collapse on its northern flank that subsequently generated one or multiple volcanic debris avalanches (VDAs). As a result of the morphological analysis of the collapse scar, it is proposed that this lateral collapse may have been caused by factors unrelated to the volcano's eruptive activity, which impacted the stability of the volcanic structure. This geomorphological study marks a significant step forward in understanding lateral collapses in an area where sedimentological data about the VDAs is extremely limited due not only to the complexity of the topography, but also to the glacial and fluvial processes of the area. The descriptive and geometric analysis of the scar resulting from the collapse indicates that this morphological expression is U-shaped and deep-seated and has the following dimensions: length = 3.0 km, width = 1.6 km, height = 0.8 km, and aperture angle = 31°. Respectively, the area and volume of material involved in the collapse are estimated at 4.0 km2 and 0.5 km3. The geomechanical characterization of the Chiles Volcano rocks was done in five outcrops located on the southern flank applying either the Rock Mass Rating (RMR) or the Geological Strength Index (GSI); additionally, these values obtained in the field were complemented with results from other studies in the area, which encompass the eastern and northern flanks of the volcano. GSI values for columnar, brecciated, and blocky lava flows range between 47–72; 15–65 and 37–57, respectively. Using triaxial tests by the multiple failure state method, the compressive strength of the rock mass was 24.92 MPa; respectively, cohesion and internal friction angle values were 4.21 MPa and 51.95° for peak strength, and 1.52 MPa and 52.4° for residual strength.

Keywords

• Collapse scar
• rock quality
• volcanic edifice
• debris avalanche
• flank stability
• volcanic hazard