HR: 1340h
AN: V53B-1546    [Abstracts]
TI: A Textural Exploration of the Physical Attributes of Pumice Clasts From the 23,000 Y.B.P. Eruption of Popocatepetl Volcano in Central Mexico
AU: * Myers, N R
EM: NRMyers@mail.utexas.edu
AF: University of Texas, Austin, The University of Texas at Austin Geol Science Dept 1 University Station C1140, Austin, TX 78712-0254 United States
AU: Gardner, J E
EM: gardner@mail.utexas.edu
AF: University of Texas, Austin, The University of Texas at Austin Geol Science Dept 1 University Station C1100, Austin, TX 78712-0254 United States
AU: Siebe, C
EM: csiebe@geofisica.unam.mx
AF: Universidad Nacional Autonoma de Mexico, Departamento de Vulcanologia Instituto de Geofisica, Ciudad Universitaria Coyoacan, Ciudad de Mexico, DF 04510 Mexico
AB: The eruption of Popocatepetl Volcano 23,000 Y.B.P. produced the youngest in a series of debris avalanches. This 9 km³ debris avalanche was followed by a large blast deposit and then the White Pumice fall, on which we focus. We are investigating whether the large decompression that resulted from the debris avalanche may have caused the vesiculation process to change, thereby altering the physical properties of the deposit. The White Pumice is the thickest fall deposit at Popocatepetl and has an estimated minimum volume of 1-2 km³ DRE. Its relative reverse grading in lithic sizes suggests mass flux increased throughout the eruption, assuming no significant change in winds during deposition. While the mass flux appears to increase, the average vesicularity of pumice clasts decreases through the deposit. Bulk vesicularity of the pumices ranges from 55.3% to 88.3%, with a mean value of 75.4% and a mode of 77%, based on density measurements of 900 pumice clasts. We are also characterizing the textural attributes of pumice from the deposit, which we are qualitatively analyzing through ESEM pictures of the surface of low, medium, and high vesicularity pumice clasts. Preliminary results suggest that individual pumice clasts throughout the vesicularity range share the same textural qualities in different proportions. A variable degree of vesiculation on the surface of individual pumices occurs in sections that transition into one another. Some areas are smooth with little to no vesiculation and an abundance of cracks. In low vesicularity sections of individual pumice, there are predominately rounded to slightly elongate vesicles with irregular bubble shapes. These areas often have a rough surface texture due to pieces of glass welded to the smooth, glassy bubble walls. In more vesicular regions of the same pumice, the rough texture due to welding is less common and the vesicles become more elongate with smoother bubble walls and a more regular shape. Areas with tube vesicles are the most vesicular, and have strings of smooth glass representing the diminishing bubble walls. Pumice with low bulk vesicularity consist of mostly low to moderately vesicular sections with few highly vesicular areas and a larger proportion of smooth areas. Highly vesicular pumice consist of mainly of highly vesicular and tubular vesicle sections, and a low percentage of low to moderately vesicular sections and smooth areas. The medium vesicularity pumice tend to incorporate characteristics of both extremes. Our results preliminarily show that the vesicularity is similar to that of other eruptions that were not preceded by a large debris avalanche, suggesting that the sudden decompression may not affect the process of vesiculation. Future use of X-Ray Computed Tomography will allow us to clarify bubble interactions both internally and on the surface of the clasts, thereby characterizing the differing spatial vesicularity.
DE: 8404 Volcanoclastic deposits
DE: 8428 Explosive volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005