C-14 AGES AND ACTIVITY FOR THE PAST 50-KA AT VOLCAN-GALERAS, COLOMBIA

Volcan Galeras is the southernmost Colombian volcano with well-recorde d historic activity. The volcano is part of a large and complex volcan ic center upon which 400,000 people live. Historic activity has center ed on a small-volume cone inside the youngest of several large amphith eaters that breach the west flank of the volcano, away from the city o f Paste (population 300,000). Lava flows (SiO2 between 54.6 and 64.7 w t.%) have dominated activity for more than 1 Ma, but explosive events have also occurred. Joint studies by volcanologists from Colombia, Ecu ador, Peru, Bolivia, Argentina, and the United States produced 24 new C-14 ages and more than 100 stratigraphic sections to interpret the pa st 50 ka of activity at Galeras, including sector collapse events. The youngest collapse event truncated 12.8 ka lava flows and may have occ urred as recently as 8 to 10 ka. Tephra-fall material rapidly thins an d becomes finer away from the vent area. The only widespread marker in the <10 ka section is a biotite-bearing tephra deposited between 4.1 and 4.5 ka from a source south of Galeras. It separates cryoturbated f rom largely undisturbed layers on Galeras, and thus dates a stratigrap hic horizon which is useful in the interpretation of other volcanoes a nd geotectonics in the equatorial Andes. Pyroclastic flows during the past 50 ka have been small to moderate in volume, but they have left n umerous thin deposits on the north and east flanks where lava flows ha ve been impeded by crater and amphitheater walls. Many of the pyroclas tic-flow deposits are lithic rich, with fines and clasts so strongly a ltered by hydrothermal action before eruption that they, as well as th e sector collapse deposits, resemble waste dumps of leached cappings f rom disseminated sulfide deposits more than volcanogenic deposits. Thi s evidence of a long-lived hydrothermal system indicates susceptibilit y to mass failure and explosive events higher than expected for a volc ano built largely by lava flows and modest Vulcanian eruptions. Photog raphs, written accounts, and our study document historic north and eas t flank pyroclastic flows as far as 10 km from the summit; however, no ne have left recognizable deposits in Paste for more than 40 ka.