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Learn about the two volcanoes that formed Oahu
Waianae and Koolau

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Waianae Mountains
Waianae Mountains Photo

The Waianae Range is about 22 miles long and comprises the western or leeward half of Oahu. Mount Kaala is the highest point on Oahu at 4025 feet. The oldest lava from this volcano is estimated to be about 3.9 million years old. The last eruption is estimated at 2.5 million years ago.







Koolau Mountains
Koolau Mountains Photo

The Koolau Range is about 37 miles long and comprises the eastern or windward half of Oahu. Most of Honolulu's residential neighborhoods are located on its leeward slopes and valleys. Koolau is estimated to have first erupted more than 2.5 million years ago. All that is left of Koolau is the western half of the volcano. The entire eastern half, including much of the summit caldera, was destroyed in prehistoric times when it slid into the Pacific Ocean. The volcano became dormant about 1.7 million years ago but became active again hundreds of thousands of years later. There have been around 30 eruptions in the last 500,000 years creating some of Oahu's famous landmarks such as Koko Head, Hanauma Bay, Punchbowl Crater and Diamondhead. Geologists believe there is a remote possibility that Koolau will erupt again.

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