Late Holocene Pelean-style eruption at Tacana volcano, Mexico and Guatemala: Past, present, and future hazards

Tacana volcano, located on the border between Mexico and Guatemala, marks t he northern extent of the Central American volcanic chain. Composed of thre e volcanic structures, it is a volcanic complex that has had periodic explo sive eruptions for at least the past 40 k.y. The mast recent major eruption occurred at the San Antonio volcano, the youngest volcanic edifice forming the complex, about 1950 yr ago. The Pelean style eruption, issued from the southwest part of the dome, and swept a 30 degrees sector with a hot block and ash now that traveled about 14 km along the Cahoacan ravine. Deposits from this event are well exposed around the town of Mixcun and were therefo re given the name of that town, the Mixcun pow deposit. The Mixcun flow dep osit is, in the channel facies, alight gray, massive, thick (>10 m), matrix -supported unit with dispersed lithic clasts of gravel to boulder size, div isible in some sections into a variable number of flow units. The overbank facies is represented by a thin (<1 m), massive, matrix-supported unit, In both of these facies the deposit has disseminated charcoal, fumarolic pipes , and juvenile lithics with cooling joints, The Mixcun flow deposit contain s clasts of (1) light gray, dense andesite, (2) dark gray, glassy and bande d andesite, and (3) minor altered red andesite from the edifice, set in a m atrix of sand and silt. The Mixcun flow deposit covers an area of at least 25 km(2) and has a minimum estimated volume of 0.12 km(3). Basaltic-andesit e inclusions (54% SiO2) and various signs of disequilibrium in the mineral assemblage of the two-pyroxene andesitic products (60%-63% SiO2) suggest th at magma mixing may have triggered the eruption. Following deposition of th e Mixcun flow deposit andesitic to dacitic (62%-64% SiO2) lava flows were e xtruded and a dacitic dome (64.4% SiO2) at the San Antonio summit formed. S yneruptive and posteruptive lahars flooded the main drainages of the Cahoac an and Izapa-Mixcun valleys in the area of the present city of Tapachula (p opulation 250 000) and the pre-Hispanic center of Izapa. Three radiocarbon ages date this event between A.D. 25 and 72 (range +/-1 sigma, 38 B.C-A.D. 216), which correlates with a halt in construction at Izapa (Hato phase of ca. 50 B.C.-A.D. 100), probably due to temporary abandonment of the city ca used by lahars. Another similar event would produce extensive damage to the towns (papulation of about 68 000 people) now built upon the Mixcun flow d eposit, The main summit of Tacana volcano continues to show signs of fumaro lic activity; the most recent period of activity in 1985-1986 culminated in a minor phreatic explosion.