Volcano Watch — What made the Ninole hills?

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A number of hills, elongate in an upslope-downslope (mauka-makai) direction, rise 30-425 m (100-1,400 feet) above the surrounding gentle slopes of Mauna Loa, inland from Punalu`u. Ipu`u Ridge, which forms the exceptionally steep southwestern side of Wood Valley above Pahala, is a similar elongate hill.

The landmarks, collectively known as the Ninole hills, don't resemble cinder or spatter cones, such as are common on the flanks of Mauna Kea and Hualalai and in lower Puna. They also don't have the gentle flanks of shields, such as Mauna Ulu and Mauna Iki on Kilauea.

The hills have steep, even precipitous, sides. Their tops are relatively flat or slope gently seaward. Most hills are several times longer than wide. For example, Enuhe Ridge, the northeast side of the Ninole-Wailau Homesteads, is 3.2 km (1.9 miles) long in a downslope direction, ending at Pu`u Enuhe, and only 200-800 m (650-2,600 feet) wide.

The hills occur as high as 1,650 m (5,400 feet) at Pu`u Iki, though all of the larger hills, and most of the smaller, are below 1,200 m (4,000 feet). They reach as low as 335 m (1,100 feet), the base of Pu`u Enuhe.

Rocks forming the hills are deeply weathered and appear rather old. Early workers were struck by how different they are from the fresh lava flows of Mauna Loa. This led to the earliest notion about the hills-they are all that remains of an eroded volcano older than Mauna Loa and even Hualalai.

This idea remained unchallenged until the concepts of plate tectonics and hot spots were developed in the late 1960s. Those concepts worked quite well in Hawai`i, but the Ninole hills didn't conform. Why was an older volcano farther down the chain than some others if the Pacific plate were moving northwest over the hot spot?

At about this time, the mobile nature of Kilauea's south flank was recognized, as were huge submarine landslides north of Moloka`i and O`ahu. Quickly more seafloor slides were discovered, and geologists began to relate the motions of volcano flanks to the generation of slides. Could evidence related to large slides be found on any of the islands?

Indeed it can, and the Ninole hills are one example. The best work on the Ninole, done by Pete Lipman (former HVO staff member), concludes that the hills are the dissected headwall of a large slide known offshore as the Punalu`u landslide. The sequence of events is as follows:

The flank of Mauna Loa, including what are now Punalu`u and Pahala, began to slide into the sea. The upper end of the slide was at the base of what are now the Ninole hills. Once the slide was over, the headwall area, where the slide pulled away from the flank, was very steep. Over thousands of years, storm runoff eroded deep canyons through the steep headwall, dissecting the once continuous flank into a cluster of hills. Later eruptions of Mauna Loa sent flows down the canyons, eventually surrounding the hills with younger rocks.

Lipman's geochemical work found that the rocks in the hills were erupted from Mauna Loa probably between 100,000 and 300,000 years ago. The ages are clearly rather old, although the rocks are too weathered for precise analysis. They are much older than those on the rest of the surface of Mauna Loa, mostly less than 10,000 years old. The slide is younger than the rocks in the hills, and other evidence shows that it is probably older than 100,000 years.

The geochemical results indicate Mauna Loa affinity and argue against the Ninole hills being part of a south rift zone of Hualalai buried under Mauna Loa--another possible origin once suggested for them.

The hills thus provide a window into the past of Mauna Loa. This window may eventually close, if flows from Mauna Loa's southwest rift zone cover the hills. But for now, the hills remind us that Mauna Loa is older than it superficially looks and that large landslides have reached fairly high on Mauna Loa in the distant past.

Volcano Activity Update

Eruptive activity of Kilauea Volcano continued unabated at the Pu`u `O`o vent during the past week. Bright glow persists over the "rootless" shield area. The status of lava flows remains the same as last week with flows on Pulama pali mainly crusted over and occasional breakouts occurring from the fan at the base of the pali. There are no surface flows in the coastal flats and no ocean entries.

Two earthquakes from the Kilauea summit area were reported felt during the week ending on March 7. A resident of the Volcano Golf Course subdivision felt an earthquake at 4:59 p.m. on Sunday, March 3. The magnitude-2.2 earthquake was located 3 km (1.8 mi) southwest of the summit of Kilauea at a depth of 3 km (1.8 mi). On the following day at 3:08 p.m., people in Hawaii Volcanoes National Park were shaken by a magnitude-2.5 earthquake. The temblor was located 1 km (0.6 mi) northwest of the summit of Kilauea at a depth of 2 km (1.2 mi).