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Unformatted text preview: Life Stages of Hawaiian Life Stages of Hawaiian Volcanoes Why do Hawaiian Volcanoes Change With Time?
• Plate tectonics and hot spots
– Source of magma is fixed in the mantle – Pacifc Plate is moving over magma source
• The types of volcanic activity are dependent on how close the volcano is to hot spot determining:
– different magma types from varying degrees of partial melting – different frequencies of eruptions • The ocean
– different kinds of rocks form above, below, and at sea level – pronounced erosion above sea level (Changed to “post-shield” stage) When it first starts forming, not yet directly over hot spot so the volcano is first formed from alkalic basalt as the result of low degrees of partial melt As the submarine volcano moves over the hot spot, normal tholeiitic basalt starts to form from higher degrees of partial melt Caldera and rift zone formation? Probably!! Yes!! Lava erupts as pillows with low vesicularity (few bubbles) due to high pressure in deep ocean environment Example: Lo‘ihi (“Long One”) Seamount Lo‘ihi Seamount
rift zone pit craters; (site of future caldera) rift zone Tholeiitic basalt Characterized by hydromagmatic eruptions Submarine pyroclasts are called hyaloclastite No volcano in Hawaii is currently in this stage Evidence for this stage found by drilling through volcanoes, and by observing other volcanoes Surtsey Island, Iceland Tholeiitic basalt Well developed caldera and rift zones mostly “dry” pyroclasts ‘a‘a and pahoehoe flows Bulk of volcano growth happens in this stage caldera indicates a shallow magma chamber in geologic time, almost constant eruptions Examples: Kilauea and Mauna Loa Mauna Loa, a shield volcano As volcano starts to move away from hot spot, magmas are again the result of low degrees of partial melt and are made of alkalic basalt Later on, alkalic basalts evolve into more felsicbut still alkaline rocks by crystal fractionation alkalic basalts have more volatiles, thus cause more explosive eruptions Eruptions less frequent, deeper magma chamber Examples: Mauna Kea, East Maui Volcano (Haleakala) lower viscocity, thicker and shorter lava flows, and steeper slopes vents follow rift zones Eruptions have ceased Therefore, erosive processes now dominate as the action of wind and water wear the islands down to sea level Landslides also reduce the size of volcanoes Coral reefs grow on shallow submarine slopes Example: West Maui Volcano NaPali Coast, Kaua‘i Moloka‘i Also called Rejuvenation Stage Magma produced by even lower degrees of partial melt than in the postcaldera alkalic cap Nephalanites very low silica, very fluid lava monogenitic eruptionseach vent erupts only once
long periods of time can pass between each eruption vents do not lie on the rift zones of the old shield Example: on Ko‘olau Volcano, O‘ahu can have explosive hydromagmatic eruptions Diamond Head, Kaimuki, Koko Head, Tantalus, Punchbowl Rejuvenation eruptions have mostly ceased Erosion has worn island down to sea level Construction by coral reef growth only Islands also sink due to: Examples: Northwest Hawaiian Islands Laysan, Midway, French Frigate Shoals, etc. Their own weight/Aging of the oceanic lithosphere Ultimately island becomes a guyot, a flattopped seamount Laysan Island, an Atoll Detailed Cross Section of a Hawaiian Shield Volcano
hyaloclastite km Hawaiian Swell Hawaiian Swell Weight of the Hawaiian Islands bend the Pacific Plate The islands form the Hawaiian Ridge At the islands, plate bends DOWN, forming the Hawaiian Deep or Moat But plate bends UP away from islands, forming the Hawaiian Arch ar ch ar ch ...
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- Summer '10