| || || Fepuleai, Aleni|
| || || Establishing a holocene tephrochronology for Western Samoa and its implication for the re-evaluation of volcanic hazards|
Institution: University of the South Pacific.
Subject: Tephrochronology -- Samoa, Volcanic eruptions -- Samoa
Call No.: pac QE 527 .56 .F47 2016
Copyright:Over 80% of this thesis may be copied without the authors written permission
Abstract: Samoan volcanism is tectonically controlled and is generated by tension-stress activities associated with the sharp bend in the Pacific Plate (Northern Terminus) at the Tonga Trench. The Samoan island chain dominated by a mixture of shield and post-erosional volcanism activities. The closed basin structures of volcanoes such as the Crater Lake Lanoto enable the entrapment and retention of a near-complete sedimentary record, itself recording its eruptive history. Crater Lanoto is characterised as a compound monogenetic and short-term volcano. A high proportion of primary tephra components were found in a core extracted from Crater Lake Lanoto show that Crater Lanoto erupted four times (tephra bed-1, 2, 3, and 4). The four major episodes generated via the western slide motion mechanism (WSMM), correspond with the western movement of the Pacific Plate. The WSMM also triggered simultaneous activities along the easternmost part of Upolu, as shown by the presence of contaminant tephra components throughout the core and the westward shifts in the locus of volcanism. In addition, the WSMM influenced the westward progression in volcanic age of the Samoan hotspot. This suggests that it occurred before the development of Savai’i and Upolu. The similarity in radiometric ages of shield volcanoes and post-erosional activities along the islands chain indicates that the hotspot volcanism and the WSMM process activated simultaneously. Vesicularities in tephra sand indicate explosive activities during the four major episodes. The new radiometric ages of lava and tephra indicate that the Crater Lanoto volcano was activated between 200 ka (corresponding to the Salani Formation, Pleistocene) and 3.4 ka (Lefaga Formation, Holocene). In addition, the new radiometric age viewed with respect to the distance from the current hotspot (Vailuluu) show that Crater Lanoto volcano is a part of more widerspread post-erosional volcanism. There is no long term consistency in the eruption interval among the six volcanic formations in Samoa; due to the lack of radiometric dating and the fact that stratigraphic sampling needs to be higher resolution. The high content of organic material associated with the primary tephra deposit of the tephra bed- 2 episode suggests a cone collapse event (CCE). This CCE is a part of Salani volcanism cut-off, at least 22.3 kyrs ago, associated with the Fagaloa-Falealili Fault and Manase-Gataivai Fault on Upolu and Savai’i, respectively. The similarities of the compound monogenetic features of the Crater Lanoto volcano with other cones along the main fissure enable, to re-evaluate the potential vent scenario on the main islands, for future volcanic prediction.