Geology of Island of O‘ahu

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Geology of Island of Oahu

• Composed of two different shield volcanoes
• Waianae Volcano, gt 4.0 to 2.8 Ma
• Shield-building and postshield alkalic cap stages
• No rejuvenation stage!
• Koolau 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

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Kaena Ridge
Kaena Slump
Oahu
Koolau
Waianae
Waianae Slump
Penguin Bank
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Island of Oahu
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Island of Oahu Physical features
Ulupau crater
Pacific Ocean
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Formation of Waianae Volcano 4 million years (MY)
ago
Waianae shield building, and Koolaus emergence
2.5 MY
Making up of Oahu
Oahu, 2 MY ago
Present day Oahu
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Waianae Volcano, Shield Stage

• Lualualei Member
• gt4.0 to 3.55 Ma
• normal tholeiitic basalt, 50 silica
• Kamaileunu 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?

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Waianae Volcano, Postshield Stage

• 3.1 to 2.8 Ma
• Mostly composed of hawaiite, an intermediate,
  alkalic igneous rock
• Broken into two members
• Palehua (older) and Kolekole (younger)
• separated by an erosional surface
• Kolekole lavas are found on top of mudflow
  deposits
• Kolekole is more mafic (has less silica) than
  Palehua
• may be related to the Waianae Slump, a giant
  submarine landslide
• removal of mass would lower pressure within the
  volcano, promoting decompression melting and
  higher degrees of partial melting

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Koolau Volcano, Shield Stage

• Tholeiitic basalt, but unusually high Si, Al, Na,
  and Fe
• old idea chemical variations within the hot spot
  plume
• New data from drilling shows that unusual
  chemistry disappears with depth and that deeper
  rocks are more like other shield-stage lavas in
  Hawaii
• therefore, unusual chemistry is only in the top
  layer of Koolau
• may represent decline of melting in the volcano
• was on its way toward a postshield stage that
  never happened

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The Koolau erupted along vertical cracks that
extended from beyond Kahuku in the north to past
Makapuu in the south. A central vent and summit
depression, or caldera, formed near the present
region of Kaneohe and Kailua. The major volcanic
activity stopped here about three million years
ago. The Koolau volcano was also partially built
by lava rising to the surface through thousands
of cracks in preexisting flows. Lava cooled in
these cracks and formed nearly vertical sheets of
dense basaltic rock called dikes. Many such
dikes, each about one meter (3 ft) in width, can
be seen in the high cuts made for the Pali
Highway near Castle Junction and along the
Kaneohe - Kailua section of the H-3 highway near
Mokapu Blvd. These dikes retained infiltrating
rainwater and created a storage area as well as
the artesian springs at lower elevations. The top
of the Koolau volcano was about 3.8 km (2 mile)
above sea level when it ceased building. The old
crater region covers the area from Waimanalo to
Kaneohe. Also located in the site of the old
crater is Olomana Peak, an isolated mountain that
sweeps up into a sharp 500 m (1,643 ft) peak.
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Nuuanu Landslide

• Longest in the Hawaiian Islands, 230 km long
• Traveled 80 km past axis of the Hawaiian Deep
• in other words, moved uphill
• Moved as a giant debris avalanche
• a more catastrophic type of landslide
• moved as isolated blocks
• the Waianae Slump, by comparison, moved as a
  single unit (similar to Kilauea)
• Age?
• Paleomagnetism gives an ambiguous age, gt 2.58 or
  1.8 Ma, but well within the shield stage of
  Koolau

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Geologic History of Koolaus Volcano A.
Formation about 2.5 million years ago
B. Giant Landslide about a year ago Collapse
of Koolaus Caldera due to a giant slump into the
ocean, Nuuanu Landslide
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Koolau Volcano, Rejuvenation Stage

• Called the Honolulu Volcanic Series
• Very low silica rocks, 36-42 SiO2
• very low degrees of partial melting
• come directly from mantle, no magma chambers
• Ages range from 800-600 Ka (poorly constrained)
  at the start to as young as 30 Ka
• Sometimes erupt simultaneously along fissures
• Koko Fissure
• Sugarloaf-Roundtop-Tantalus
• Both wet and dry eruptions, even in same
  fissure
• Vents do not lie on shields rift zone, but
  rather their alignments are perpendicular to it

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true alignmentsor wishful thinking?
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Map of a very large lava flow field (magenta)
recently discovered on the Hawaiian arch north of
O'ahu and Kaua'i. Note the size of this flow
compared to the islands (adapted from Clague et
al. 1990). Age may be recent.
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