Subject: Volcanic Eruptions on Oahu

When was the last time volcanoes on Oahu erupted?

    The island on Oahu is formed from two principle volcanoes: Waianae and Koolau. Waianae is about 2.2-3.8 million years old and Koolau is about 1.8-2.6 million years old. They are now "dead" volcanoes.
    The island is also riddled in spots by smaller volcanic features such as diamond head, punchbowl, salt lake crater, Koko head and Koko crater, etc..
    These smaller volcanoes formed in more recent times, although the age determiniations on them are not very good. In general, these features (sometimes called "post-eriosional" or "rejuvenation" volcanoes) are believed to be between 70,000 and 500,000 years old. Some believe that some flows in east honolulu are younger (5000-10000 years old) but the evidence is not very convincing.
    So, is there a threat of a volcano erupting here again? It is always possible, but the odds are actually slim. One way volcanologists predict eruptive activity at locales that erupt very infrequently is to look at the historical frequency of eruptions in an area in the past. Although the data is limited, the suggestion is that the post-erosional series on Oahu experienced eruptions every few tens of thousands of years. Since it has been longer than that since the last eruption, it may be that the activity is now finally over. One other bit of info regarding volcanic hazards is that these small volcanoes usually only erupt once. A new eruption in the area would create a new small volcano, instead of re-occupying an older one.

Dr. Ken Rubin, Assistant Professor
Department of Geology and Geophysics
University of Hawaii, Honolulu, HI 96822

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