Mapping and sampling of the interior of El Chichón Volcano was made possible by the exposures of the 1982 explosion crater. The 1-km-diameter, 200-m-deep crater exposes rocks produced during several eruptive episodes: volcanic domes, talus breccias, plinian airfall deposits and pyroclastic-flow deposits of compositionally similar alkalic hornblende-trachyandesites with phenocrysts of anhydrite. All of the rocks, except those leached of anhydrite, have extremely high S contents. El Chichón consists of a volcanic dome complex, airfall debris, and pyroclastic outflow sheets. Prior to 1982, its last major activity was about 650 ± 100 yr B.P.