HR: 1340h
AN: V33B-1464    [Abstracts]
TI: Eruptive History of Arenal Volcano, Costa Rica
AU: * Soto, G J
EM: katomirodriguez@yahoo.com
AF: Consultant, Apdo. 360 San Francisco 2 Rios, San Jose, 2350 Costa Rica
AU: Alvarado, G E
EM: galvaradoi@ice.go.cr
AF: Area de Amenazas y Auscultacion S¡smica y Volcanica, ICE, Apdo. 10032, San Jose, 1000 Costa Rica
AB: Tephra-stratigraphy, volcanic history, eruption types, eruptive dynamics and area distribution of tephra from the most important eruptions of Arenal volcano, are reviewed and updated. Deposits of explosive eruptions are named AR-1 to AR-22 (from older to younger). All previous and new Arenal and neighbouring Chato volcanoes' radiocarbon dates were calibrated: the last Chato eruption occurred 3720$\pm$150 B.P and the first known eruption of Arenal (AR-1), 7010+170-130 B.P. Isopachs and characteristics of the most relevant and recognized airfall deposits are presented. Area distributions of the key layers are oriented toward W, WSW and SW, except the lowest layer of AR-17, which is distributed northward. According to lithic isopleths, an eruption column height of 23 km (subplinian) was calculated for AR-20. Most fall layers in Arenal show associated pyroclastic flow deposits up to a distance of 6.5 km from the summit. They are restricted to river valley paths. Among Arenal eruptions, plinian-subplinian type events like AR-20, AR-15, AR-12 and AR-9, are the most violent and destructive, with tephra volumes between 0.2 and 1 km3. There are also violent strombolian eruptions which were sustained for a considerable time, resembling subplinian eruptions (the AR-19 case, total volume 0.45 km3), and pelean type eruptions, as AR-22, which erupted 0.026 km3 of tephra. A temporal correlation of explosive events after AR-8 shows that the four most important dacitic eruptions (AR-20, 15, 12 and 9), are separated by an average period of 800 years ("long term"). Three of them (AR-20, 15 and 9) have been preceded by two important explosive events with a more basic chemistry, which occurred around 300 years or less previously to the dacitic eruption ("short term" periods). Nevertheless, between cycles AR-13/14/15 and AR-18/19/20, there are two events (AR-16 and 17) that do not belong to any cycle. In fact, between the major eruptions AR-15 and 20, the highest eruptive frequency of all volcano history occurred, which notes that "cyclicity" is relative. The longest periods of explosive inactivity follow the dacitic events AR-9, 12 and 20. The most recent lava cycles have been temporally well determined, erupting volumes of 0.5 - 1 km3, following the most destructive plinian-subplinian eruptions. Lava cycles older than 1300 years B.P. have not been detailed because of their poor exposures. The tephra sequence previous to AR-9 ($\sim$3300 B.P. - 7010 B.P.) has no dark soils, while the overlying sequence ($\sim$3200 B.P. - present) does. It is presumed to be a consequence of regional climatic changes from a dry environment to a rainy tropical one around 3300 years B.P.
DE: 8400 VOLCANOLOGY
DE: 8404 Ash deposits
DE: 1035 Geochronology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting