Kaho`olawe was once a volcano, too

The island of Kaho`olawe, once called by some “the target isle,” has been in the news lately. After suffering years of degradation, first by feral goats, then by U.S. Naval gunnery practice, this uninhabited island has finally been returned to the Hawaiian people. Substantial unexploded ordnance dating back to the 1940s has been removed, an expensive and time-consuming process. Although much of Kaho`olawe remains contaminated by potentially dangerous warheads, scientists have already been getting a closer look at what the island is made of.

Of particular interest to geologists is the age of Kaho`olawe’s youngest volcanic rocks. Surprisingly, although several radiometric ages exist from older beds, none have been obtained from the youngest volcanic deposits.

Kaho`olawe is the exposed top of a shield volcano. Most volcanic activity ended prior to 1.1 million years ago, when a thin mantle of postshield lava flows coated much of the upper surface. The volcano was a part of the much larger Maui Nui, a big island that included the volcanoes of Lana`i, Moloka`i, and Maui. The effects of island subsidence and rising sea level have flooded the land bridges between some volcanoes, so that Kaho`olawe became its own separate island sometime between about 200,000 and 150,000 years ago.

From an altitude of 452 m (1,483 ft) at the volcano’s summit, Moa`ulanui, the land surface slopes away gently to the sea in most directions. The island’s east side, however, is embayed by the cliffs of Kanapou Bay, which probably resulted from a submarine landslide that took a bite off the east flank.

The north and south limits of the landslide coincide with faults that bounded what was once probably a circular caldera 4-5 km (2.5-3.1 mi) in diameter. This size is comparable to the caldera at Kilauea, in Hawaii Volcanoes National Park. The Kaho`olawe caldera was filled by late-shield lava, an interpretation that stems from the thickness of lava flows visible in the walls of Kanapou Bay.

Enter the latest stage of volcanism at Kaho`olawe: Geologist Harold Stearns, who mapped Kaho`olawe in 1939, recognized four cinder deposits and sparse, thin lava flows that drape the east-facing cliffs of Kanapou Bay. Feeder dikes associated with some of these deposits cut up through well-cemented alluvium that mantles those slopes. From these relations, Stearns correctly surmised that extensive erosion occurred before the eruption of the cinders. Hence, those latest eruptions were considered to be products of a rejuvenated stage of volcanism that occurred long after the volcano had ceased its main episodes of growth 1.1 million years ago.

To determine how young those youngest products might be, HVO scientists, in conjunction with colleagues at Kyoto University in Japan, recently sampled two of the dikes and a lava flow for dating. Although the results won’t be available for several months, we’ve already learned something of their age. The dikes and lava flow have magnetic characteristics that indicate they are younger than 780,000 years, the interval during which the Earth’s magnetic field has been oriented more or less as it is today.

How young could these deposits be? The youngest lava flow is deeply weathered along its fractures, producing a rounded form known as spheroidal weathering. By comparing it with dated lava flows in a similar arid setting on the south flank of Maui, we can suggest confidently that the sampled Kaho`olawe flow will be older than 100,000 years in age. The precise answer awaits laboratory analysis.

Our recent visit to the island was made possible by permission of the Kaho`olawe Island Reserve Commission. We are indebted to Paul Higashino, who facilitated our trip and accompanied us to the collecting sites. Also along was Kalei Tsuha, who ensured that our work posed no threat to cultural features on the island. John Wells, an expert in the detection of unexploded ordnance, always went first to guarantee that no harm befell us. To all these professionals we owe our deepest thanks.

Activity Update

Eruptive activity at Pu`u `O`o continues. Most lava flows are breaking out from the Mother’s Day lava tube downslope from the rootless shield complex south of Pu`u `O`o. The flows reached the top of Pulama pali on Tuesday morning, and lava has been visible high on the pali from the end of the Chain of Craters Road since then. No eruptive activity has taken place in the crater of Pu`u `O`o during the past week.

Two earthquakes were reported felt on the island during the week ending April 15. One, felt at the Red Hill cabin on the trail to the summit of Mauna Loa, took place at 2 a.m. on April 10. It had a magnitude of 2.5 and was centered 17 km (10 miles) east of the summit of Mauna Loa at a depth of 3 km (2 miles). The second felt earthquake, of magnitude 3.3, shook Kahuku Ranch at 1:37 a.m. on Sunday, April 11. It was located 18 km (11 miles) northwest of Na`alehu at a depth of 10 km (6 miles).

Mauna Loa is not erupting. The summit region continues to inflate slowly. Seismic activity remains very low, with 1 earthquake located in the summit area during the past week.

This article was written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory and is republished by HawaiiNews.com with permission.

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