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Hosted at SCHOOL OF OCEAN AND EARTH SCIENCE AND TECHNOLOGY • UNIVERSITY OF HAWAI‘I AT MANOA

Waianae Volcano

     The Waianae Range is one of two shield volcanoes on the island of Oahu. It rises to about 1.2km above sea level, making it taller than neighboring Koolau volcano. Waianae is the oldest of these two Oahu volcanoes. It is located in the Northwest quadrant of the island.

Eruptive History

     Eruptions of the main shield-building stage took place around 3.8-2.95 Ma. The well-known Mauna Kuwale Rhyodacite flow errupted at about 3.2 Ma. Alkalic Post-shield activity occured around 2.95-2.4 Ma. There has possibly been post erosional activity as recently as the Pliestocene, but the volcano is now extinct

Geology

     The Waianae Volcanic Series is divided into lower, middle, and upper members. The lower member is made up of the lava flows and pyroclastics that built the main mass of the Waianae shield; The middle member is mainly rocks that accumulated in the caldera, gradually filling it; The upper member is a thin cap that has covered much of the shield late in its history. The volcano is now extensively eroded, bearing large ampitheater valleys on its western slopes. These valleys (such as Lualualei) are some of the largest in Hawaii, and they are believed to represent the sources for large landslides now seen on the sea floor to the west of the island.
     Lavas of Waianae volcano span compositions ranging from Tholeiitic and alkalic basalt through to evolved compositions such as icelandite, rhyodacite, hawaiite, and mugearite

     The Waianae Range is on the west coast of Oahu. It is largely shielded from the rains brought into the islands by the Northeasterly bearing trade winds by its neighbor, Koolau Volcano. This makes Waianae Volcano much drier than the Koolaus, particularly on its westward (leeward slopes). This dryness keeps runoff to a minimum and thus, some of the clearest water in Hawaii is found at the beeches of the Waianae coast.

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