Chapter 8

Abstract

Cerro Machín Volcano is an active volcano (4° 29' N; 75° 23' W) located in the central part of the Colombian Andes (Tolima Department, Colombia). Lahars are one of the greatest hazards associated with Cerro Machín Volcano. Approximately 300 000 people live in lowlands around the volcano. This added to the explosive potential, chemical composition, magnitude of its eruptions, and distribution of its deposits make Cerro Machín one of the most dangerous volcanoes in Colombia. Volcanic risk management of Cerro Machín requires a balance of science, education, risk reduction, and disaster management, which are all critical to the safety, well-being, and quality of life of the population living around the volcano.The highly explosive eruptions that have occurred in the last 10 000 years have generated thick sequences of pyroclastic fall and pyroclastic density current deposits, which have filled the valleys of rivers and streams within a 15 km radius of the volcano. The interaction of pyroclastic material with water generates large lahars that flow for distances greater than 100 km along the Coello and Magdalena Rivers channels. The distribution and total extension of the lahar deposits allow us to identify three deposition areas: proximal, intermediate, and distal. I identified six lahar units between the towns of Carmen de Bulira and Nariño, located 42 and 113 km from the volcano, respectively. Four units were defined as being dominated by hyperconcentrated flow deposits and two by debris flow deposits. The former are associated with pumice pyroclastic flows, while the latter are associated with block and ash pyroclastic flows. The lahars from Cerro Machín had a key role in the configuration of the regional landscape of a particularly important sector of Tolima Department. The lahar deposits dammed and changed the course of the Coello and Magdalena Rivers, forming terraces and large coalescent fans, El Guamo and El Espinal. Dam breakouts originated lahars. Hyperconcentrated flow deposits (dated 9130-8540 y BP; 4360 ± 105 y BP; 3618-3186 y BP; 1200 ± 105 y BP) and debris flow deposits (dated ca. 2500 y BP and <1200 y BP) cover an area of approximately 1074 km 2 (equivalent to a volume of 22.5 km 3 ). Numerous towns, such as Payandé,