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Quaternary eruptive history and hazard-zone model at Nevado del Tolima and Cerro Machin volcanoes, Colombia

Abstract:

The ice-clad and fumarolic Nevado del Tourna volcano (4 ° 39′N, 75 ° 20′W) south of Nevado del Ruiz, is offset toward the southeast from the axis of the volcanic Ruiz-Tolima massif with respect to the major NE-trending strike-slip Palestina fault. It is composed of four units: (1) a pre-Tolima plateau-like basement of basaltic andesite lava flows of early Quaternary age; (2) a dissected, ancestral Tourna stratovolcano, cut by a presumed collapse caldera of middle Pleistocene age; (3) an older Tolima stratovolcano of late Pleistocene age, partly 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 nonwelded pumice- and scoria-flows were emplaced toward the southeast (Rio Combeima) and northeast (Rio Totare). Repeated growth of lava domes 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 pyroclastic-flow deposits. This activity occurred during at least six eruptives stages as follows: El Placer, ca. 16,200-14,000 yr B.P.; Romerales, 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 Nieves, historical. Interactions with the ice cap probably triggered debris flows that partly filled the Combeima and Totare valleys and formed the Holocene terraces on the upper Pleistocene volcaniclastic fans of Ibagué and Venadillo as much as 60 km from the source. The latest major activity was a plinian eruption, which deposited a pumice-fall layer ca. 3600 yr B.P. (0.5 km3 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 by future eruptions both at Nevado del Tolima and at active Cerro Machin 12 km southward. The extent of areas likely to be affected by tephra-falls, debris flows, pyroclastic flows or surges, debris avalanches and lava flows is shown. Subplinian and plinian eruptions of Nevado del Tolima were used to represent the moderate and large events to be expected. 300,000 people live within a 35-km distance from those volcanoes, which have exhibited a behaviour more explosive than Nevado del Ruiz. Despite the small-sized ice cap, debris flows are the most probable hazard for even a minor eruption, because of the very steep slope gradient, and because of probable interactions of hot eruptive products with 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 Ibagué city, where about 50,000 people live. © 1995.