********************************************************************* Arenal Volcano: PFs, deposits and impact on forest. From:Eliecer Duarte <eduarte@xxxxxxxxx> ********************************************************************* On July 7th and 8th, 2009 a probable lava front collapse took place to the south side of Arenal Volcano, Costa Rica. For several months, prior to this event, minor explosive activity and rolling blocks were low. Most of the fragmented blocks were associated to the emplacement of summit material from activity triggered during the mid months of 2008. (details: http://www.ovsicori.una.ac.cr/informes_prensa/2008/indice_2008.htm# ) Around noon on Tuesday and 10pm on Wednesday, collapses started from the summit transporting fresh and pre-emplaced blocky lava down the flank, leaving a wide channel (80-100m wide, 20-30m deep, 800m long) and some other topographical changes. Witnesses reports and photographic evidence shows dust clouds developing from the pyroclastic flows as they descended the flanks and moving N and NW up to some 5 km. from the source. Although this activity impacted part of the forest in that flank no tourist or parkrangers were directly affected. The National Park shut down twice and tourists evacuated for safety reasons. A visit on Friday 10th gave the opportunity to document videographically and photographically several minor flows and rolling blocks. Deposits from hot avalanches were assessed at the end of the wide fan generated at ~900asl. A mixture of angular, fresh lava blocks were found deposited along with pre-existent angular and subangular fragments from the eroded walls. Deposits were rich both in blocks and ash. Some of the largest blocks (2x2.5x2m) rolled down to the distal deposits lying almost intact at the site of emplacement. Evidence in the area showed that some incandescent blocks had ignited several patches of forest and buried tall trees along their path. Deposits were lobate and several generations of flow lobes from 7 to 8m in thickness were present. A field report (Spanish) is available on request for those interested in details and photos. Just send an email to ovsicori@xxxxxxxxx ============================================================== 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. ==============================================================
