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The island of Oahu is made of two volcanoes: Waianae and Koolau (map). Waianae, the older of the two volcanoes, (21.4N, 158.2W) makes up the west part of the island. The shield of Waianae volcano formed between 3.8 and 2.95 million years ago. A caldera is located near the center of the Waianae Range and rift zones extend to the northwest and southeast. Lava flows of the capping-stage are about 1.8 million years ago. Rocks from the renewed volcanism stage are also present but have not been dated. This volcano has been extensively modified by erosion.


Koolau is a large basaltic volcano that makes the east side of the island. The main shield is about 2.7 to 1.8 million years old. A caldera is located just south of Kaneohe Bay, the prominent bay on the north side of the island. Numerous dikes mark the location of a rift zone that trends to the northwest. About half a mile (1 km) of the volcano was eroded to expose these dikes. No rocks from the capping stage have been found on Koolau volcano. Some of Hawaii's most famous landmarks (e.g., Diamond Head, Hanauma Bay, and Koko Crater), are tuff rings and cinder and spatter cones from the renewed volcanism stage. These features formed between one million and, possibly, 10,000 years ago. Much of Koolau volcano was also removed by a giant landslide .

Hanauma Bay Tuff Ring, HI


Source of information:

Volcanoes of North America by Wood and Kienle.