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Title: Volume and petrologic characteristics of the Koloa volcanics, Kauaʻi, Hawaiʻi 
Author: Gandy, Christian Erick
Date: 2006
Abstract: The Koloa Volcanics on the island of Kaua'i represent the most voluminous example of the rejuvenated-stage in Hawai'i. They provide an excellent opportunity to evaluate the cause of rejuvenated-stage eruptions through the study of their volume and petrologic characteristics. We have conducted an extensive field study of the Koloa Volcanics, collecting 164 samples and measuring sections from most regions of the island where Koloa rocks are exposed. New field and geochronological work on Koloa lavas show they range in age from 2.6 to 0.15 Ma. These weakly to strongly olivine-phyric lavas are picrobasalts, alkalic basalts, basanites, foidites, and melilite foidites. There is no temporal correlation in rock types (e.g., alkalic basalts and basanites were erupted during the entire duration of Koloa volcanism). Most olivine compositions are forsterite 83-86, which is too low to be in equilibrium with the whole rock compositions (Mg#), indicating that the rocks have variable amounts of olivine accumulation and/or resorption of ultramafic xenoliths. Olivine compositions indicate Koloa parental magmas had Mg#s of 62.5-67.5. Majoroxide and trace element variation diagrams show trends indicating low but variable degrees of partial melting from a somewhat heterogeneous source. Data from 41 measured sections, 80 water well logs, and previous geologic studies of Kaua'i were synthesized using ARC/GIS to create four subsurface images of the Koloa Volcanics for volume calculations. A conservative estimate for the subaerial volume of Koloa Volcanics is -58 km3. This is the first quantitative volume estimate for Hawaiian rejuvenation volcanism. When compared with new volume estimates for Kaua'i (~57,000 km3 ), the Koloa Volcanics represent -0.1 % of the total volume of the island, consistent with a previous guesstimate (<<1%). Current models for rejuvenated stage volcanism provide predictable consequences for the volumes and duration of volcanism. The flexure-induced decompressional melting and convective mantle plume upwelling models predict a hiatus of 0.85-2 m.y. between shield and rejuvenated volcanism, and an eruptive duration for rejuvenation volcanism close to those observed for Kaua'i (1.75-3 m.y. vs. 2.45 m.y.). Both of these models suggest a plume-derived source for rejuvenated lavas, consistent with recent seismic evidence for a thinned lithosphere and Nd, Sr, Pb, Lu, Hf, isotopic data that advocates a plume source. A third model, lithospheric melting by conductive heating, proposes a lithospheric source, a much larger volume of magma produced over a longer period than observed, and little or no volcanic hiatus. These predictions are inconsistent with new observations. The trends of eruptive duration and surface areas of rejuvenated lava flows on Hawaiian Islands south of Kaua'i are suggestive that the islands of Oahu, Molokai, and Maui may experience additional rejuvenated-stage eruptions.
Description: Thesis (M.S.)--University of Hawaii at Manoa, 2006. Includes bibliographical references (leaves 103-107). ix, 107 leaves, bound ill. (some col.), col. map 29 cm
URI: http://hdl.handle.net/10125/20619
Rights: All UHM dissertations and theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission from the copyright owner.
Keywords: Volcanism -- Hawaii -- Kauai, Petrology -- Hawaii -- Kauai

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