QUATERNARY ERUPTIVE HISTORY AND HAZARD-ZONE MODEL AT NEVADO-DEL-TOLIMA AND CERRO-MACHIN VOLCANOS, COLOMBIA

The ice-clad and fumarolic Nevado del Tolima volcano (4 degrees 39'N, 75 degrees 20'W) south of Nevado del Ruiz, is offset toward the southe ast from the axis of the volcanic Ruiz-Tolima massif with respect to t he major NE-trending strike-slip Palestina fault. It is composed of fo ur units: (1) a pre-Tolima plateau-like basement of basaltic andesite lava flows of early Quaternary age; (2) a dissected, ancestral Tolima stratovolcano, cut by a presumed collapse caldera of middle Pleistocen e age; (3) an older Tolima stratovolcano of late Pleistocene age, part ly destroyed by a summit caldera; and (4) composite domes of the cone- shaped young and present Tolima. Young Tolima volcano is an andesitic and dacitic composite cone formed over the past 40,000 years within a 3-km-wide caldera that opened around 0.14 Ma. Deposits of welded and n onwelded pumice- and scoria-flows were emplaced toward the southeast ( Rio Combeima) and northeast (Rio Totare). Repeated growth of lava dome s over the past 16,000 years is witnessed by thick block-lava flows on the southern and eastern flanks and by block-and-ash or scoria-rich p yroclastic-flow deposits. This activity occurred during at least six e ruptives stages as follows: El Placer, ca. 16,200-14,000 yr B.P.; Rome rales, ca. 13,000-12,300 yr B.P.; Canalones, ca. 11,500-9750 yr B.P.; Mesetas, ca. 7200-4600 yr. B.P.; Encanto, ca. 3600-1700 yr B.P., and N ieves, historical. Interactions with the ice cap probably triggered de bris flows that partly filled the Combeima and Totare valleys and form ed the Holocene terraces on the upper Pleistocene volcaniclastic fans of Ibague and Venadillo as much as 60 km from the source. The latest m ajor activity was a plinian eruption, which deposited a pumice-fall la yer ca. 3600 yr B.P. (0.5 km(3) actual volume) mainly toward the west and northwest. Minor tephra-falls and debris flows occurred during the historical period before the reported 1918 and 1943 small (phreatic ? ) events. A general hazard-zone map shows areas potentially affected b y future eruptions both at Nevado del Tolima and at active Cerro Machi n 12 km southward. The extent of areas likely to be affected by tephra -falls, debris flows, pyroclastic flows or surges, debris avalanches a nd lava flows is shown. Subplinian and plinian eruptions of Nevado del Tolima were used to represent the moderate and large events to be exp ected. 300,000 people live within a 35-km distance from those volcanoe s, which have exhibited a behaviour more explosive than Nevado del Rui z. Despite the small-sized ice cap, debris flows are the most probable hazard for even a minor eruption, because of the very steep slope gra dient, and because of probable interactions of hot eruptive products w ith ice and snow. Additionally, scoria flows and debris avalanches can be directed toward the southeast and could be transformed into debris flows that would devastate the Combeima valley and suburbs of Ibague city, where about 50,000 people live.