HR: 1340h
AN: V33B-1468    [Abstracts]
TI: Grainsize, Mass per day and Morphological Characteristics of Tephra from Arenal Volcano, Costa Rica: 1987-2000.
AU: * Duarte, E A
EM: eduarte@una.ac.cr
AF: OVSICORI-UNA, Universidad Nacional, Heredia, Her x Costa Rica
AU: Cole, P
EM: p.cole@cov.ac.uk
AF: Dpt of Geography, University of Coventry, Coventry, Cov xxxxx United Kingdom
AU: Fernandez, E A
EM: efernan@una.ac.cr
AF: OVSICORI-UNA, Universidad Nacional, Heredia, Her x Costa Rica
AU: Saenz, W
EM: wsaenz@una.ac.cr
AF: OVSICORI-UNA, Universidad Nacional, Heredia, Her x Costa Rica
AU: Malavassi, E
AF: OVSICORI-UNA, Universidad Nacional, Heredia, Her x Costa Rica
AB: We studied tephra samples from collectors located at various locations on the flanks of Arenal Volcano. They correspond to monthly samples collected between 1987-2000. In addition, we studied samples collected at two sites from the fallout events that occurred July 9, 1987 and February 3, 1996. Glass fragments of the most abundant fractions were examined with a scanning electron microscope to determine the range in clast morphologies and allow discussion on the nature of the fragmentation mechanisms involved in their generation. Tephra fall from single explosions, cumulative fallout was subject to grainsize analyses. Explosive activity clearly diminished in 1997 and remained low between 1998 and 2000. Isomass maps were calculated for a number of months across the reporting period. Assuming eruption frequencies of 20 - 40 explosions day -1 for the majority of the reporting period, a mean explosion volume (fraction < 2 mm only) can be estimated. This data yield total tephra masses of between 1 and 10 m3 per explosion. These volumes do not, however, consider the coarser tephra fraction, including ballistics. The grain size of the cumulative monthly tephra varies relative to the location within the dispersal axis. Generally more distal and peripheral samples (those off-axis) are finer grained and more poorly sorted than those along axis. Samples of monthly tephra collected 2 km west of the vent (Caseta C) are consistently coarser grained and better sorted than those from 2.9 km west-south west (Salida del Bosque) of the vent. In addition 85% of the samples from the Salida del Bosque site show a subtle bimodality with one strong mode at +2 phi and another much weaker mode at +6phi. Less than 30% of the samples from Caseta C show such bimodality. Tephra, both from cumulative monthly samples and single explosions, was separated into crystals, lithics (altered grains) and glassy fractions and fragments which were composed of approximately equal proportions of crystals and glass. The scanning electron microscopy allowed subdivision of the glassy fraction into one of four types: vesicular, fluidal, blocky and clasts with both blocky and fluidal portions. The results showed that the blocky type form the most abundant clast morphology forming between 50 and 71%. Part fluidal and part blocky types form between 21 and 40%. Wholly fluidal types form up to 6% and vesicular up to 5 %. Proportions of different clast types for single explosions were similar for cumulative monthly samples. The different clast morphologies are thought to represent the different nature of the state of the magma on fragmentation. Therefore vesicular clasts represent vesicular, degassing magma. Fluidal clasts represent wholly molten fragments of magma. Blocky clasts are interpreted as being cool, rigid parts of the magma that had degassed and part blocky- part fluidal represent fluidal, molten magma that was fragmented as the clast cooled during eruption. This would indicate that the fragmentation process was sequential occurring over tens of seconds and not instantaneous.
DE: 7280 Volcano seismology (8419)
DE: 3630 Experimental mineralogy and petrology
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
DE: 1299 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting