J.C. Gavilanes

Overview of the 1997^2000 activity of Volcan de Colima, Mexico

Abstract This overview of the 1997–2000 activity of Volcan de Colima is designed to serve as an introduction to the Special Issue and a summary of the detailed studies that follow. New andesitic block lava was first sighted from a helicopter on the morning of 20 November 1998, forming a rapidly growing dome in the summit crater. Numerous antecedents to the appearance of the dome were recognized, starting more than a year in advance, including: (1) pronounced increases in S/Cl and δD values at summit fumaroles in mid-1997; (2) five earthquake swarms between November–December 1997 and October–November 1998, with hypocenters that ranged down to 8 km beneath the summit and became shallower as the eruption approached; (3) steady inflation of the volcano reflected in shortening of geodetic survey line lengths beginning in November–December 1997 and continuing until the start of the eruption; (4) air-borne correlation spectrometer measurements of SO2 that increased from the background values of

Emplacement of pyroclastic flows during the 1998–1999 eruption of Volcán de Colima, México

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 km 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 m 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/m3. 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 24×105 m3.

Chemical and isotopic composition of fumarolic gases and the SO2 flux from Volcán de Colima, México, between the 1994 and 1998 eruptions

Volcan de Colima is the most active volcano in Mexico, including a significant effusive eruption in 1991 and a weak explosive event in 1994. Geochemical monitoring of the volcano started in January 1996 with collection of fumarolic gases approximately every 3 months from two sites in the summit crater (Z3, ∼800°C, and Z2, ∼400°C). Chemical compositions of volcanic gases from the Z2 fumarole showed a peak in S/Cl in the middle of 1997, 14–15 months before the start of the 1998 lava eruption. High-temperature volcanic vapors from Z3 changed progressively in water isotopic composition and showed a weak trend of increasing HCl/HF and a peak in S/Cl at the same time. Starting from the summer of 1997, the volcanic water became gradually enriched in deuterium, which probably indicated an increasing contribution from deep, less degassed magma. A negative correlation between δD and Cl content for the Z3 fumarolic gases is explained by shallow degassing of the Colima hot lava plug. This hypothesis is consistent with published data on Cl contents and solubilities in magmas and supported by the observed positive trends of S/Cl and HCl/HF with δD for the Z3 fumarole. Until September 1998, the SO2 flux from the volcano was at a low level (<100 tons/day), and often below the detection limit of the instrument (∼30 tons/day). A sharp increase in the SO2 flux was recorded 1 month before the eruption started. It jumped from 400±50 tons/day to 1600 tons/day 2 days before new lava emerged on 20 November 1998. The SO2 flux correlated well with seismicity.