HR: 1340h
AN: V43E-1456    [Abstracts]
TI: A.D. 1835 Eruption of Volc\'{a}n Cosig\"{u}ina, Nicaragua: New Data and Interpretations
AU: * Scott, W
EM: wescott@usgs.gov
AF: U.S. Geological Survey, Cascades Volcano Observatory 1300 SE Cardinal Court, Vancouver, WA 98683 United States
AU: Gardner, C
EM: cgardner@usgs.gov
AF: U.S. Geological Survey, Cascades Volcano Observatory 1300 SE Cardinal Court, Vancouver, WA 98683 United States
AU: Devoli, G
EM: gde@geohazards.no
AF: International Centre for Geohazards and University of Oslo, Sognsveien 72 P.B. 3930 Ullevaal Stadion, Oslo, N-0806 Norway
AU: Alvarez, A
EM: aalvarez@ineter.gob.ni
AF: Instituto Nicaraguense de Estudios Territoriales, Frente a Policlinica Oriental, Managua, 2110 Nicaragua
AB: The eruption of Volc\'{a}n Cosig\"{u}ina, Nicaragua, in January 1835, greatly impressed witnesses owing to its large area of ash fallout and great distance at which eruption noises were heard. Initial estimates of tephra volume of tens to more than 100 km$^{3}$ were lowered substantially by later reconnaissance studies to less than 10 km$^{3}$-more likely 3 to 6 km$^{3}$. But little has been done to map the distribution of eruptive deposits and to collect systematic stratigraphic observations around the volcano. We present the results of preliminary field and laboratory investigations that provide new details about proximal deposits. Our key conclusions are the following. Initial eruptions produced a tephra sequence of ash to fine lapilli, including a white dacitic pumice-fall deposit that is thickest and coarsest in a narrow sector to the northwest of the vent. The overlying main tephra-fall deposit is composed of dark-colored andesitic scoria that is thick and coarse in a broad sector extending clockwise from northwest to southeast, but appreciably thinner and finer-grained in south and southwest sectors, consistent with transport by high-altitude westerly winds. Pyroclastic surges and flows of dark-gray andesitic scoria and ash followed the scoria fall and formed fans and valley fills in northwest, northeast, and southeast sectors. Ash clouds, probably mostly related to pyroclastic flows, produced thick (locally $>$1 m) ashfall deposits, rich in accretionary lapilli, chiefly in the southwest sector, transported by prevailing low-altitude northeasterly winds. Bulk volume of 1835 eruptive products may lie at the low range of previous estimates, about 3 to 4 km$^{3}$.
DE: 8404 Ash deposits
DE: 8414 Eruption mechanisms
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting