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Volcano Watch: A geologic tour of the Hawaiian Islands: Oahu

January 18, 2016 - 9:51am

January is Volcano Awareness Month, during which our “Volcano Watch” articles are exploring the geology of the Hawaiian Islands. The series continues this week with a look at Oahu.

The island of Oahu is composed of two volcanoes. Waianae, the western and older of the two, started growing from the sea floor around four million years ago. The birth of Koolau, which forms the east side of the island, probably occurred a few hundred thousand years later.

At Waianae, shield-building volcanism — the most active time of a Hawaiian volcano’s life — ended by about three million years ago and was followed by a brief, about 200,000-year-long, period of post-shield volcanism. Since then, erosion has carved steep cliffs and valleys on the volcano. While some of this erosion has been caused by water, catastrophic collapses of the island have also occurred, some of which shed debris north onto the ocean floor across a distance of 70 miles.

Koolau, on the other hand, was vigorously active until about 1.8 million years ago. Like Waianae, Koolau has also been extensively eroded and has been the source of multiple huge landslides. In fact, Nuuanu Pali may be the erosional trace of the scarp that marks one of the largest landslides in the world. The gigantic Nuuanu slide extends across the ocean floor about 120 miles northeast from Oahu, and one of the blocks of debris in the submarine landslide measures over 18 miles long and 1-mile thick.

The extensive erosion of Koolau has afforded geologists an unparalleled opportunity to view the interior of a Hawaiian volcano. For example, investigations of ancient lava flows reveal that Koolau’s caldera is located in the Kailua, Oahu, area, and that the caldera continued to subside even after eruptions had ended (perhaps pulled downward by the weight of the solidified magma chamber).

Even more spectacular are the swarms of dikes — solidified sheet-like or tabular intrusions of magma — that define the volcano’s rift zones, parallel to the crest of the current Koolau Range. In some places, over half the rock is made up of these dikes. The exposures of Koolau’s rift zone provide a sense of what Kilauea’s current East Rift Zone must look like about 0.6 mile beneath the surface. Studying Koolau therefore provides insights into how active Hawaiian volcanoes work.

Koolau also hosts extensive rejuvenated volcanism, which marks the final stage of a Hawaiian volcano’s eruptive life — essentially a weak sputtering before the volcano goes extinct.

Rejuvenated eruptions on Koolau followed a pause of about one million years after shield-building ended. About 40 discrete eruption sites are preserved across the east side of Oahu, including several extinct cinder cones that are now famous landmarks: Punchbowl, Tantalus, and Leahi (Diamond Head). The youngest of these rejuvenated eruption sites include Koko Crater, Hanauma Bay, and Tantalus, which formed about 70,000–100,000 years ago. Leahi and Punchbowl are about 400,000–500,000 years old.

As on Kauai (the focus of last week’s “Volcano Watch”), future rejuvenated eruptions on Oahu are a possibility. Such eruptions could be hazardous, given that they would probably be at least mildly explosive, especially if they occur near the coast where magma and seawater can interact. The odds are remote, however, of such eruptions occurring during our lifetimes, or even those of many future generations.

Next week we’ll explore Maui, Lanai, Molokai and Kahoolawe, all of which once stood above sea level as a single island with several volcanoes.

In the meantime, we hope to see you at one or more Volcano Awareness Month talks offered by USGS Hawaiian Volcano Observatory scientists around Hawaii Island. This week’s schedule includes presentations at Lyman Museum on Monday, Hawaii Volcanoes National Park on Tuesday, Makaeo Event Pavilion (Old Kona Airport Park) on Wednesday, and Pahoa on Thursday. Details are posted at You can also email or call 967-8844 for more information about upcoming HVO talks.

Volcano activity updates

Kilauea continues to erupt at its summit and East Rift Zone. During the past week, the summit lava lake level varied between about 95-135 feet below the vent rim within Halemaumau Crater. On the East Rift Zone, scattered lava flow activity remained within about 4 miles of Puu Oo.

Mauna Loa is not erupting. Seismicity remains elevated above long-term background levels. In the last week, earthquakes occurred mostly beneath Mauna Loa’s upper Southwest Rift Zone at depths less than 3 miles. GPS measurements continue to show deformation related to inflation of magma reservoirs beneath the summit and upper Southwest Rift Zone of Mauna Loa.

One earthquake was reported felt on Hawaii Island this past week. On Wednesday, at 1:27 p.m., a magnitude-3.3 earthquake occurred 6.7 miles west of Kalapana at a depth of 4.9 miles.

Visit the HVO website ( for past Volcano Watch articles, Kilauea daily eruption updates, Mauna Loa weekly updates, volcano photos, recent earthquakes info, and more; call 967-8862 for summary updates at Kilauea or 967-8866 for updates at Mauna Loa. Questions can be emailed to

Volcano Watch is a weekly article and activity update written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.

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