HR: 1330h
AN: V12A-0566    [PDF]
TI: Physical Parameters of Vulcanian Eruptions at Pichincha Volcano, Ecuador: Bomb Morphologies and Textures
AU: * Wright, H M
EM: hwright1@darkwing.uoregon.edu
AF: Department of Geological Sciences, 1272 University of Oregon, Eugene, OR 97403 United States
AU: Cashman, K
EM: cashman@oregon.uoregon.edu
AF: Department of Geological Sciences, 1272 University of Oregon, Eugene, OR 97403 United States
AU: Rosi, M
AF: Earth Science Department, Via S. Maria, 53, Pisa, I-56126 Italy
AU: Cioni, R
AF: Earth Science Department, Via Trentino, 51, Cagliari, I-09124 Italy
AB: Vulcanian eruptions are common at many volcanoes around the world. These eruptions occur in energetic pulses and eject relatively small amounts of material. Each blast event (vulcanian eruption) has been inferred to represent a "throat-clearing" process that ejects a conduit plug. As such, we can examine the ejected material to reconstruct the conduit stratigraphy. The recent sequence of vulcanian eruptions at Guagua Pichincha volcano provides an opportunity to learn more about the dynamics of and pressurization conditions preceding vulcanian eruptions. From late 1999 - mid 2000, Pichincha experienced a series of vulcanian eruptions that ejected ballistic bombs, which now cover the surface of the crater. Bomb types range from dense to highly vesicular, with many exhibiting the breadcrusting that is ubiquitous in vulcanian deposits. Clast morphology varies with clast density, with slightly vesicular bombs having thick, glassy crusts and widely spaced cracks, whereas more vesicular bombs have thinner crusts and more closely spaced, regular crack patterns. The wide range of clast types appears to represent the stratigraphy of the conduit prior to each eruptive event, with denser blocks formed from more degassed magma near the top of the pre-eruptive conduit plug and more vesicular blocks representing deeper, less degassed levels in the conduit. This study uses the ballistic bombs, including the abundant breadcrust bombs, to learn more about conduit processes during a typical vulcanian eruption. In particular, we use the rapidly quenched crusts of breadcrust bombs, which preserve pre-eruptive conduit material, to determine gradients in volatile and crystal content in the conduit. The volatile content (both H$_2$O and CO$_2$) of the pre-eruptive melt was determined from FTIR spectroscopic analysis of bomb rind matrix glass. These values reach up to 1.2 wt% water and 10 ppm CO$_2$, equivalent to 15 MPa maximum recorded presusure, or approximately 600 meters maximum depth. Coincident with the volatile gradient, microlite populations in bombs with dense, glassy crusts have uniform tabular shapes, whereas microlites in bombs with vesicular rinds have more variable crystal shapes. Insight into these degassing and crystallization conditions may help us understand pressurization mechanisms for the eruptions. The differences between the ballistic bombs will provide a picture of the conduit prior to eruption.
DE: 8400 VOLCANOLOGY
DE: 8414 Eruption mechanisms
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting