************************************************** Fieldwork provides interpretation -- Poas Volcano ************************************************** From: Eliecer Duarte Gonzalez <eduarte@xxxxxxxxx> Field report -- Poas Volcano, Costa Rica Fieldwork confirms crater lake wall collapse and widening of active crater. Last Saturday 1st of april, staff from OVSICORI-UNA climbed down to Poas hot lake and surroundings. It is the first visit after the phreatic eruptions registered from Friday 24 to Sunday 26 of march, due to severe weather conditions. During this visit the widening of the lake perimeter was confirmed as well as the emplacement of sediments and preexistent blocks from both the bottom of the lake and its walls. Eventhough there were several eruptions, registered seismically, only in 3 cases witnesses` accounts refer to the ejection of rocks, sediments and water out of the lake area. The first eruption occurred noon march 24 and expelled preexistent material from the lake to the south flat area along the dome. Water and fine wet material covered most of the flat area while hundreds of diverse size blocks (ranging from few cm to 60cm), confined to an area of some 150m beyond and south of the dome. Most of those blocks were lithic although some are pieces of the dome mainly formed by sulphur which melted during the episode, suggesting temperatures above 116 ºC. The second eruption occurred the very first day that activity resumed, around 8:30pm. At least two components conformed this event; a vertical ejection that reached above the rim of the caldera (above 300m) and dispersed fine sediments to the SW in an area mostly covered by natural vegetation, traveling as far as 5km beyond its point of origin. A second component sent material, laterally towards the east, up to a distance of 700 m provocking impact craters and fragmentation of flying blocks. Yet a third eruption charged with sediments, water and some centimetric blocks was ejected towards W, just few minutes past 9pm, on Sunday 2nd. Most of the lithic blocks produced impact craters beyond the outer caldera rim and some of them showed melted sulphur. A thick layer of wet sediments colored the outer flanks to a dark gray in a radius of some 300m towards the W. During this visit, a greater fracturing of the dome`s N wall was confirmed as well as the disappearance of a 40x7x8 segment. A chunk of some 40x4x6m from the SE wall of the lake was also incorporated during one of the eruptions and pieces of material matches well with those found beyond 300m, above the middle terrace, to the E. Metric, lithic, angular blocks (ranging from 0.4 to 1.4m) were emplaced near the source of explosion, on the bottom of the caldera near the lake. Lake temperature registered 54ºC, and a pH of 0.63. The water turned light gray due to the great quantity of suspended sediments. The lake level descended from 3 to 5m compared with the previous data before the eruptions. Derived from this fieldwork OVSICORI holds samples of sediments and blocks as well as a set of digital photographs. ============================================================== To unsubscribe from the volcano list, send the message: signoff volcano to: listserv@xxxxxxx, or write to: volcano-request@xxxxxxxx To contribute to the volcano list, send your message to: volcano@xxxxxxxx Please do not send attachments. ==============================================================
