CALDERA WALL FORMATION IN THE EAST DIAMANTE SEAMOUNT VOLCANO OF THE MARIANAS ARC

This study centers around the petrographic analyses of lavas sampled from the East Diamante Seamount Volcano in the Marianas Arc. The collection of these samples took place in June 2009 and was a joint United States and Japanese research effort targeting several volcanoes in the region. From the East Diamante Seamount Volcano three ROV dives were conducted: 1012, 1013, and 1014. Dive 1014 uses the interpretation of crystallization history and evolution of magma to understand the formation of the western portion of the southern caldera wall. The samples collected from Dive 1014 were diverse, even though they are all volcanic. During observation no systematic variations in size or petrography were noticed. The general compositions of these volcanic rocks are moderately vesicular hornblende dacite in a glassy matrix, pyroxene, andesite, basaltic andesites, and volcaniclastics. This problem was analyzed using petrographic examination of fourteen collected samples, which were then compared to a similar dive (Dive 1013). The petrographic analyses led to the documentation of the minerals present, irregular boundaries, crystal size and shape, and matrix composition. The results conclude that the western portion of the southern caldera wall was formed through an explosive event. Further analysis is needed to determine whether the caldera wall formation was the result of multiple lava compositions ejected during the eruption, or if the dacite/andesite samples began to crystallize in the vent prior to eruption and then were ejected along with pumice.