LA <i>LAVINA</i> DEL VALLE CENTRAL (COSTA RICA): ¿LAHAR O <i>DEBRIS AVALANCHE</i>?

Abstract

The denominated lavina of the Central Valley, constituted by lava blocks floating in a volcanicmud matrix, had been interpreted genetically as a lahar or debris flow deposit. This work tries to discuss somecharacteristics that suggest that the deposit originated in a debris avalanche event, that transformed into a debrisflow. Avalanche amphitheaters were identified in extinct stratovolcanoes in the western sector of the Irazú volcano, where the most probable source of origin is the sector of collapse in the west flank of the Cabeza de Vacavolcano. This event could have occurred around 150 ka -250 ka, possibly during or at the end of glacial periodsin the northern hemisphere. Wells in most of the Metropolitan area allowed the morphometric definition of thestratigraphic surfaces, and determined a volume = 2.0 km3 in an approximated area of 130 km2.