LAS CUMBRES AVALANCHE AND DEBRIS-FLOW DEPOSIT, MEXICO

Las Cumbres Volcano is located in the Trans-Mexican Volcanic Belt (TMVB), a chain of active volcanoes that runs east west across central Mexico. At the eastern end of the TMVB a north-trending series of volcanoes that includes Las Cumbres has products of various compositions ranging from basalt to rhyolite. Las Cumbres is the only stratovolcano in this area with lavas that contain abundant hornblende, a useful index mineral for identifying its clastic deposits.

Las Cumbres collapsed more than 40,000 years ago leaving a horseshoe shaped caldera. A large deposit consisting of avalanche and debris flow products related to this collapse extends eastward 110 km reaching the Gulf of Mexico. A volume estimate based on morphologic analysis of the remnant cone as well as on thickness and planimetric area of the deposit yields a volume in excess of 10¹⁰ m³. This clastic deposit appears to be a single continuous stratigraphic unit. If this proves to be true, this horizon would be the largest sub-aerial volcanic debris deposit in the world.

Samples taken from various lavas exposed in Las Cumbres caldera were compared with boulders in the debris deposit to test the hypothesis that Las Cumbres is its source. Samples were size-sorted, and particles of selected grain-sizes were analyzed using a scanning electron microscope (SEM) to examine morphology and energy dispersive X-rays (EDX) to determine chemical composition. Thin sections also were examined optically to establish the mineralogical content. X-ray diffraction (XRD) techniques were used to resolve the type and degree of alteration of the fine fraction material in the debris flow matrix. The presence of hornblende in samples collected from various locations in the debris deposit proves that Las Cumbres is the source for this very large avalanche.