Emplacement of pyroclastic flows during the 1998-1999 eruption of Volcan de Colima, Mexico

After three years of quiescence, Volcan de Colima reawakened with increasing seismic and rock fall activity that reached its peak on November 20, 1998, when a new lava dome forced its way to the volcano's summit. The new lava rapidly reached the S-SW edge of the summit area, beginning the generation of Merapi-type pyroclastic flows that traveled down La Lumbre, and the El Cordoban Western and Eastern ravines, reaching distances of 3, 4.5, and 3 km, respectively. On December 1, 1998, the lava flow split into three fronts that in early 1999 had reached 2.8, 3.1, and 2.5 kin in length, advancing down the El Cordoban ravines. The lava flow fronts disaggregated into blocks forming pyroclastic flows. One of the best examples occurred on December 10, 1998. As the lava flow ceased moving in early 1999, activity became more explosive. Strong blasts were recorded on February 10, May 10, and July 17, 1999. The last event developed a 10-km-high eruptive column from which a pyroclastic flow developed from the base, traveling 3.3 km SW from the summit into the San Antonio-Montegrande ravines. Regardless of the mechanism of pyroclastic-flow generation, each flow immediately segregated into a basal avalanche that moved as a granular flow and an upper ash cloud in which particles were sustained in turbulent suspension. When the basal avalanche lost velocity and eventually stopped, the upper ash cloud continued to move independently as a dilute pyroclastic flow that produced a massive pyroclastic-flow deposit and an upper dune-bedded surge deposit. The dilute pyroclastic flow scorched and toppled maguey plants and trees, and sandblasted vegetation in the direction of the flow. At the end of the dilute pyroclastic-flow path, the suspended particles lifted off in a cloud from which a terminal ash fall was deposited. The basal avalanche emplaced block-and-ash flow deposits (up to 8 in thick) that filled the main ravines and consisted of several flow units. Each flow unit was massive, monolithologic, matrix-supported, and had a clast-supported steep front (ca. 1.5 to 2 m thick) composed of boulders up to 1.7 m in diameter. The juvenile lithic clasts had an average density of 1800 kg/ml. The dilute pyroclastic flow emplaced overbank deposits, found on valley margins or beyond the tip of block-and-ash flow deposits. They consist from bottom to top of a massive medium to coarse sand-size flow layer (2-4 cm thick), a dune-bedded surge layer (2-10 cm thick), and a massive silt-size layer (0.5 cm thick). The total estimated volume of the pyroclastic-flow deposits produced during the 1998-1999 eruption is 24 X 10(5) m(3). (C) 2002 Elsevier Science B.V. All rights reserved.