OahuKauaiNiihau - The Geology of O‘ahu Kaua‘i and...

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Unformatted text preview: The Geology of O‘ahu, Kaua‘i, and Ni‘ihau A Quick Review of Island Formation • Islands form over a “hot spot” in the mantle then move away with the motion of the Pacific Plate – Results in a chain of islands with an age progression along its length – Responsible for different magma types in different life stages , or whether there is any magma available at all • When magma supply is low, erosion dominates the shaping of the landscape • Islands experience sea level changes – global sea level change from ice ages – local sea level change from plate flexure • islands sink under their own weight • islands can be lifted as a larger neighboring island sinks – results in different zones of coastal erosion and coral growth on the flanks of the volcano Location of the various islands over the hot spot (red circle) at different times O‘ahu Kaua‘i Ni‘ihau Geology of O‘ahu • Composed of two separate volcanoes – Wai‘anae Volcano, > 4.0 to 2.8 Ma • Shield-building and postshield alkalic cap stages • No rejuvenation stage! – Ko‘olau Volcano, 2.5 to 1.5 Ma (shield stage) • Shield-building and rejuvenation stages • No postshield alkalic cap! • Both volcanoes have also passed through erosion and coral reef growing stages – amphitheater-headed valleys – Abundant raised reefs • Both volcanoes have experienced giant submarine landslides Wai‘anae Volcano Ko‘olau Volcano Wai‘anae Volcano, Shield Stage • Lualualei Member – >4.0 to 3.55 Ma – normal tholeiitic basalt, ~50% silica • Kamaile‘unu Member – 3.55 to ~3.1 Ma – transitional caldera-filling sequence • wider variety of minerals and rock types • some are very silica-rich, up to 66% • dying magma chamber? Waianae Volcano, Postshield Stage...
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OahuKauaiNiihau - The Geology of O‘ahu Kaua‘i and...

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