Movement and emplacement of lava flows at Volcan de Colima, Mexico:: November 1998-February 1999

The first lava eruption of Volcan de Colima since 1991 was anticipated beginning in mid-1997 when seismic activity began to exceed background levels. On 20 November 1998, growth of a new andesitic lava dome began inside a crater produced by an explosion through the summit dome in July 1994. The 1998 block-lava dome grew rapidly at a rate of 4.4 m(3)/s, attaining a volume of 3.8 X 10(5) m(3) in 24 h. About mid-day on 21 November the lava dome began to collapse, generating a series of block-and-ash flows that descended the SSW flank at speeds of 80-90 km/h, at intervals of 3-5 min. These pyroclastic flows were mostly deposited in the three branches of Barranca El Cordoban, reaching a maximum distance of 4.8 km from the summit at an altitude of 2040 in. The largest pyroclastic flows were emplaced on 25-26 November and their deposits had an estimated volume of 7.3 X 10(5) m(3). An andesitic block-lava flow (59 wt% SiO2) then followed down the three branches of Barranca El Cordoban, reaching a maximum distance of 3.8 kin in the central branch. The lava fronts of the western and central branches almost touched at 2300 m elevation. The eastern lava flow reached 2.8 km at 2560 m elevation. The total volume of the three lava flows is estimated as 39 X 10(6) m(3), assuming an average thickness of 20 m. The advance rate of the lava fronts in the early hours reached 36 m/h on steep slopes (up to 40degrees). These diminished to 0.2-1.0 m/h on flatter slopes (similar to14degrees) in early February, as the activity waned. The viscosity of the block-lava flows, estimated from field observations, was extremely high: 10(9)-10(10) Pa-s, similar to6 orders of magnitude greater than viscosities estimated for fluid basaltic magmas. The lava flows ceased movement in early February 1999, and the style of eruptive activity entered a more explosive phase, with a series of dome explosions that have continued into February 2001. (C) 2002 Elsevier Science B.V. All rights reserved.