Reconstructing the eruption history of El Chichón volcano from river terraces (Chiapas, Mexico)

Abstract

El Chichón volcano (Chiapas, Mexico) is known for its devastating eruption in 1982. This event instigated more research to improve our knowledge on its eruption history. Although 12 eruptions have been recognized over the last 8000 years, the data are incomplete and leave uncertainties concerning frequency and impacts of the eruptive events, specifically for the early history of the volcano. Because El Chichón eruptions often have an explosive character, much of the activity record is represented by tephra deposits. A distal archive has recently become available from tephra-fall deposits collected in drill core from a swampy area in the Usumacinta-Grijalva delta. Based on the chemical signature of glass shards, the distinguished eruptive events could generally be attributed to activity of El Chichón, except for relatively silica-rich tephra which has a deviating composition not recognized in the volcano so far. In order to further explore the eruption history of El Chichón with proximal deposits, river terraces of the Platanar and Magdalena rivers were investigated, assuming that they formed during or shortly after an eruption of El Chichón and thus are “event related”. This thesis addresses the following research questions: 1. Are river terraces that largely consist of pyroclastic material suitable for tephrochronology? 2. Does each terrace level of the Magdalena and Platanar rivers represent a single eruption? Are they so-called ‘event terraces’? 3. Does the SiO2-rich distal tephra from the delta plain area have an equivalent in the river terraces and could it therefore be linked to El Chichón or not? To answer these questions, fieldwork was carried out to collect pumice and matrix samples from terraces in the valleys of the Magdalena and Platanar rivers, in close proximity to El Chichón. Electron microprobe analysis of glass shards, mineral phases and microliths of pumices and matrix material was used to obtain geochemical fingerprint of each terrace. A total of eight eruption-related terraces were distinguished using evidence from Lidar images, reports on hyper-concentrated and debris flows in the river valleys, existing maps of eruption deposits, and C-14 dating of charcoal. Morphological features in the terrace stratigraphy, inverse grading, and intervals of coarse-grained pumice-rich layers were useful as additional indicators. The river terraces of the Magdalena and Platanar rivers appeared suitable for tephrochronology and geochemical fingerprinting, although the limited compositional variability of the volcanic products of El Chichón through time make it difficult to distinguish the different eruptions. Four terraces represented the 1982 eruption and three had developed during older eruptions with unknown ages. All of the analysed glasses from pumices and shards in terrace matrix showed a fairly uniform geochemistry. Nonetheless, the results of this thesis demonstrate that geochemical fingerprinting of river terraces around active, tephra-producing volcanoes can be used as an additional tool to reconstruct eruption histories in cases where primary deposits are lacking or inaccessible. The source of the high SiO2 cluster found in the delta plain of Tabasco, Mexico is most likely the 84 ka Los Chocoyos eruption of the Guatemalan Atitlan volcano. Major and trace-element compositions of glasses from this cluster are similar to air fall tephra of this eruption, and are distinct from the signatures of El Chichón products.