Journal of Volcanology and Geothermal Research, 23 (1984) 169--191 169 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands CRATER LAKE AND POST-ERUPTION HYDROTHERMAL ACTIVITY, EL CHICHON VOLCANO, MEXICO THOMAS J. CASADEVALL 1, SERVANDO DE LA CRUZ-REYNA :, WILLIAM I. ROSE Jr. 3, SUSAN BAGLEY 3, DAVID L. FINNEGAN 4 and WILLIAM H. ZOLLER 4 U.S. Geological Survey, Vancouver, WA 98661 (U.S.A.) 2 Instituto de Geofisica, U.N.A.M., Mexico 20, D.F. (Mexico) 3 Michigan Technological University, Houghton, MI 49931 (U.S.A.) 4 University of Maryland, College Park, MD 20742 (U.S.A.) (Received April 26, 1984) ABSTRACT Casadevall, T.J., de la Cruz-Reyna, S., Rose, W.I., Bagley, S., Finnegan, D.L. and Zoller, W.H., 1984. Crater lake and post-eruption hydrothermal activity, E1 ChichSn Volcano, Mexico. In: J.F. Luhr and J.C. Varekamp (Editors), E1 ChichSn Volcano, Chiapas, Mexico. J. Volcanol. Geotherm. Res., 23: 169--191. Explosive eruptions of Volc~in E1 ChichSn in Chiapas, Mexico on March 28 and April 3--4, 1982 removed 0.2 km 3 of rock to form a 1-km-wide 300-m-deep summit crater. By late April 1982 a lake had begun to form on the crater floor, and by November 1982 it attained a maximum surface area of 1.4 × l0 s m: and a volume of 5 × 104 m 3. Accumu- lation of 4--5 m of rainfall between July and October 1982 largely formed the lake. In January 1983, temperatures of fumaroles on the crater floor and lower crater walls ranged from 98 to 115°C; by October 1983 the maximum temperature of fumarole emissions was 99°C. In January 1983 fumarole gas emissions were greater than 99 vol. % H20 with traces of COs, SO2, and H2S. The water of the lake was a hot (T = 52--58°C), acidic (pH = 0.5), dilute solution (34,046 mg L -1 dissolved solids; C1/S = 20.5). Sediment from the lake contains the same silicate minerals as the rocks of the 1982 pyroclastic deposits, together with less than 1% of elemental sulfur. The composition and tempera- ture of the lake water is attributed to: (1)solution of fumarole emissions; (2) reaction of lake water with hot rocks beneath the lake level; (3) sediments washed into the lake from the crater walls; (4) hydrothermal fluids leaching sediments and formational waters in sedimentary rocks of the basement; (5) evaporation; and (6) precipitation. INTRODUCTION Volcan E1 ChichSn in Chiapas, Mexico erupted violently on March 28 and April 3--4, 1982, forming a new 1-kin-wide summit crater and sending erup- tion columns of gas and ash into the stratosphere (Hoffer et al., 1982; Krueger, 1983). These stratospheric injections of volcanic material were among the most voluminous of this century (DeLuisi et al., 1983; Hirono and Shibata, 1983). The volume and global dispersion of this material, 0377-0273/84/$03.00 © 1984 Elsevier Science Publishers B.V.