HR: 0800h
AN: V31E-0716    [Abstracts]
TI: Changes in Explosive Eruptive Styles From Morne Trois Pitons, Dominica, Lesser Antilles
AU: * Tinnin, B
EM: bt68@nau.edu
AF: Department of Geological Sciences, California State University, Northridge, 18111 Nordhoff Street, Northridge, CA 91330-8266, United States
AU: Smith, A
EM: alsmith@csusb.edu
AF: Department of Geological Sciences, California State University, San Bernardino, 5500 University Parkway, San Bernardino, CA 92407-2318, United States
AU: Vazquez, J
EM: jvazquez@csun.edu
AF: Department of Geological Sciences, California State University, Northridge, 18111 Nordhoff Street, Northridge, CA 91330-8266, United States
AB: The island of Dominica, which is located in the central part of the Lesser Antilles island arc, has witnessed three large-volume Plinian eruptions within the last 100,000 years. One of these eruptions issued from the Morne Trois Pitons caldera and produced the widespread (>62 km2) Layou ignimbrite deposits. The Layou ignimbrite is exposed in two pyroclastic fans that extend NE and NW from the Morne Trois Pitons caldera (now infilled by a Pelean dome complex). Field observations of the NW fan show that the ignimbrite sequence can be subdivided into an unlithified and a lithified faces. The former occurs in distal exposures along the margins of the ignimbrite, whereas the latter is dominant in deposits that occupy central valley-fill locations. In its most distal exposure, the Layou ignimbrite overlies a thick sequence of fluviatile conglomerates. Here the basal unit is an approximately 18 cm thick bed of pumiceous lapilli fall. Lithic clast sizes in this basal unit are consistent with an eruption column height of circa 20-25 km. The basal fall deposit is overlain by 10 cm of ash fall that contains abundant accretionary lapilli (up to 8 mm in diameter), suggesting magma-water interaction and a phreatoplinian style of eruption. SEM imaging of these accretionary lapilli reveal two lapilli types: 1) an aggregate variety comprised entirely of ash (heterogeneous lapilli) and 2) an agglomerated variety composed of ash rimming a core of pumice (homogeneous lapilli). The heterogeneous accretionary lapilli contain concentric stratification of fine and coarse ash laminae suggesting pyroclastic cycling within the moist phreatoplinian column. Overlying these basal fall deposits are at least 3 unlithified pyroclastic flow units with a maximum thickness of over 20 m. Within the Layou Valley west of the caldera the flow units increase in thickness and reach a maximum of 180 m in the upper Layou Valley. The Layou sequence records transitions from plinian fall to phreatoplinian fall to ignimbrite during a single explosive event from Morne Trois Piton caldera. These transitions are likely related to changes in column buoyancy and degree of magma-water interaction.
DE: 8404 Volcanoclastic deposits
DE: 8428 Explosive volcanism
DE: 8486 Field relationships (1090, 3690)
DE: 8499 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting