HR: 14:10h
AN: V23D-03    [Abstracts]
TI: Vent , Voluminous Lava Emissions, Steep Slopes and Pyroclastic Flows at Arenal Volcano, Costa Rica
AU: * Malavassi, E
EM: emalava@una.ac.cr
AF: OVSICORI. Universidad Nacional, Universidad Nacional, Heredia, 86-3000 Costa Rica
AU: Fern ndez, E
EM: efernan@una.ac.cr
AF: OVSICORI. Universidad Nacional, Universidad Nacional, Heredia, 86-3000 Costa Rica
AU: Duarte, E
EM: eduarte@una.ac.cr
AF: OVSICORI. Universidad Nacional, Universidad Nacional, Heredia, 86-3000 Costa Rica
AU: Van der Laat, R
EM: rvanderl@una.ac.cr
AF: OVSICORI. Universidad Nacional, Universidad Nacional, Heredia, 86-3000 Costa Rica
AU: Berrocal, M
EM: milena03cr@yahoo.com.mx
AF: OVSICORI. Universidad Nacional, Universidad Nacional, Heredia, 86-3000 Costa Rica
AU: Barboza, V
EM: vbarboza@una.ac.cr
AF: OVSICORI. Universidad Nacional, Universidad Nacional, Heredia, 86-3000 Costa Rica
AU: Hern ndez, E
EM: ehernan@una.ac.cr
AF: OVSICORI. Universidad Nacional, Universidad Nacional, Heredia, 86-3000 Costa Rica
AU: Marino, T
EM: tmarino@una.ac.cr
AF: OVSICORI. Universidad Nacional, Universidad Nacional, Heredia, 86-3000 Costa Rica
AU: S enz, W
EM: wsaenz@una.ac.cr
AF: OVSICORI. Universidad Nacional, Universidad Nacional, Heredia, 86-3000 Costa Rica
AU: Hern ndez, L
EM: lauranha@yahoo.com
AF: OVSICORI. Universidad Nacional, Universidad Nacional, Heredia, 86-3000 Costa Rica
AB: The initial explosion of Arenal Volcano, occurred July 29-31, 1968, opened three new craters on the west flank of the volcano. The first lava flow was issued from crater A (1050 m) on September 20, 1968. This crater continued the emission of lava flows until November, 1973. In April, 1974 crater C (1460m) erupted its first lava flow. Between 1974 and 2004 crater C has been constructing a cone which is off set 500 m from the prehistoric summit Arenal cone (1633 m) and has reached an elevation of 1670 m. The slopes of the present day cone developed on crater C are very steep, specially north, northeast and northwest of the cone. Crater C lavas were issued from two vents: south and north vents. The location of the vents inside crater C controlled the direction of flow of the lavas issued from each vent. During the last 30 years both vents of crater C have been active at different times, and it was only in the late eighties during a period of intense strombolian activity, that both vents were active at the same time. During the highest intensity of strombolian activity (1987-1989) fall back ejecta from strombolian explosions produced pyroclastic flows. The shift of activity from the south vent to the north vent coincided with the generation of the two largest pyroclastic flows issued by Arenal volcano since 1968. During July, 1975 a very voluminous lava flow erupted from the north vent of crater C was descending the NW flank of Arenal when gravitational collapse of the flow produced the 1975 Tabac¢n River valley pyroclastic flow. This was the first time that a lava was flowing on the very steep, partially altered unconsolidated slopes of the NW flank of Arenal. This pyroclastic flow reached Arenal River located 4.3 km from the summit, this is the largest pyroclastic flow produced by gravitational collapse at Arenal volcano. A similar event occurred in August, 1993 when the migration of activity from the south vent to the north vent of crater C and the emission of a voluminous lava flow, preceded the collapse of its NW rim. This rim consisted mainly of loose bombs and blocks accumulated after the period of strombolian eruptions during the late eighties and early nineties. After 1999, the south vent of crater C deflated showing a crateric depression. In contrast, the north vent of crater C became inflated and filled with lava. New vents opened on the north vent rim of crater C. They formed cones located NE and SW of the vent. Both cones have been active, nevertheless the most active vent has been the NE vent after 1999. The nature of these cones have been discussed extensively as there are evidences that small lobes have been extruded from their flanks suggesting a dome-like behavior. In parallel, after 1999, lava flows were shorter, developing blocks that cascaded the slopes of the volcano at very short distances from the vent. Several small volume, gravitational collapse pyroclastic flow events originated from the NE cone of the north vent of crater C since 1999. They represent the most important volcanic hazard for visitors of the volcano that get too close to see the volcano. Plume heights, erupted volumes and the number of daily explosions have been decreasing after 1999, suggesting an important decrement in the activity of Arenal volcano.
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