Reappraisal of the 1982 eruptions of El Chichón Volcano, Chiapas, Mexico: new data from proximal deposits

 Additional data from proximal areas enable a reconstruction of the stratigraphy and the eruptive chronology of phases III and IV of the 1982 eruption of El Chichón Volcano. Phase III began on 4 April at 0135 GMT with a powerful hydromagmatic explosion that generated radially fast-moving (∼100 ms^–1) pyroclastic clouds that produced a surge deposit (S1). Due to the sudden reduction in the confining pressure the process continued by tapping of magma from a deeper source, causing a new explosion. The ejected juvenile material mixed with large amounts of fragmented dome and wall rock, which were dispersed laterally in several pulses as lithic-rich block-and-ash flow (F1). Partial evacuation of juvenile material from the magmatic system prompted the entrance of external water to generate a series of hydromagmatic explosions that dispersed moisture-rich surge clouds and small-volume block-and-ash flows (IU) up to distances of 3 km from the crater. The eruption continued by further decompression of the magmatic system, with the ensuing emission of smaller amounts of gas-rich magma which, with the strong erosion of the volcanic conduit, formed a lithic-rich Plinian column that deposited fallout layer B. Associated with the widening of the vent, an increase in the effective density of the uprising column took place, causing its collapse. Block-and-ash flows arising from the column collapse traveled along valleys as a dense laminar flow (F2). In some places, flow regime changes due to topographic obstacles promoted transformation into a turbulent surge (S2) which attained minimum velocities of approximately 77 ms^–1 near the volcano. The process continued with the formation of a new column on 4 April at 1135 GMT (phase IV) that emplaced fall deposit C and was followed by hydromagmatic explosions which produced pyroclastic surges (S3). Received: 13 May 1996 / Accepted: 12 November 1996