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 SO 2 that increased from the background values of 3 /s) on 20 November, and was spilling over the SW rim of the crater by the morning of 21 November to feed block-and-ash flows (pyroclastic flows) ahead of an advancing lobe of andesitic block lava. The pyroclastic flows were initially generated at intervals of 3–5 min, reached speeds of 80–90 km/h, and extended out to 4.5 km from the crater. The block lava flow was already ∼150 m long by the afternoon of 21 November. It ultimately split into three lobes that flowed down the three branches of Barranca el Cordoban on the SSW flank of Volcan de Colima; the lava advanced atop previously emplaced pyroclastic-flow deposits from the same eruptive event, whose total volume is estimated as 24×10 5 m 3 . The three lava lobes ultimately reached 2.8–3.8 km from the crater, had flow fronts ∼30 m high, and an estimated total volume of 39×10 6 m 3 . By early February 1999 the lava flows were no longer being fed from the summit crater, but the flow fronts continued their slow advance driven by gravitational draining of their partially molten interiors. The 1998–1999 andesites continued a compositional trend toward relatively higher SiO 2 and lower MgO that began with the 1991 lava eruption, completing the reversal of an excursion to more mafic compositions (lower SiO 2 and higher MgO) that occurred during 1976–1982. Accordingly, the 1998–1999 andesites show no signs of a transition toward the more mafic magmas that have characterized the major explosive eruptions of Volcan de Colima, such as those of 1818 and 1913. A large explosion on 10 February 1999 blasted a crater through the 1998–1999 lava dome and marked the beginning of a new explosive stage of activity at Volcan de Colima. Incandescent blocks showered the flanks out to 5 km distance, forming impact craters and triggering numerous forest fires. Similar large explosions occurred on 10 May and 17 July 1999, interspersed with numerous smaller explosions of white steam or darker ash-bearing steam. Intermittent minor explosive activity continued through the year 2000, and another large explosion took place on 22 February, 2001.