A mile-high offshore segment of Kahuku pali reveals clues about Mauna Loa

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One of the more spectacular geologic features on the Island of Hawai‘i can easily be seen from Highway 11 near South Point. It is a cliff, informally known as the Kahuku pali (after the Kahuku Ranch), that has two Hawaiian names: Pali o Mamalu, for its mauka (inland) section, and Pali‘okūlani, for its makai (seaward) section.

The Kahuku pali is formed by a geologic fault. Its average height is 400 ft (120 m), but its maximum height, which is equal to the amount of offset (movement) on the fault, is approximately 560 ft (170 m).

The Kahuku pali also continues offshore, where the submarine segment of the pali is more than 5,000 ft (1,600 m) high. This offshore mile-high pali dwarfs two picturesque landmarks in the Hawaiian islands: the north coast of Moloka‘i, which, at 3,000–4,000 ft (915–1,220 m), is the highest sea cliff in the world, and the windward pali of O‘ahu, a cliff that is 3,100 ft (945 m) high.

Several research expeditions have studied the submarine extension of the Kahuku pali. The most recent study sent a remotely operated vehicle (ROV), called Jason II, to the base of the mile-high pali. The ROV, which has mechanical arms that allowed scientists to collect samples, was equipped with cameras that provided real-time video and still images. For two days, the vehicle photographed and sampled ancient flows exposed in the pali, from bottom to top. At the base of the pali is talus, a slope of loose rocks. Above the talus is layer upon layer of submarine-erupted pillow lavas, interspersed with dikes (A tabular igneous intrusion, typically much longer than it is wide), layers of silt, and rubble.

From this expedition we learned that the major-element chemical compositions of the rocks are surprisingly consistent. This means that lava erupted from Mauna Loa stayed nearly the same for about 400,000 years. How can this happen? One possibility is that the magma supplied to the volcano remained unchanged for about 400,000 years. This would imply that the material being melted is relatively homogeneous.

An alternate explanation is that the magma reservoir within Mauna Loa is large enough to allow different batches of magma to mix, thereby developing an average composition. In other words, each individual batch of magma could have a unique composition, but mixing them results in a blend of all the batches.

Ages were determined for some of the rocks collected from the mile-high Kahuku pali by the ROV. The ages range from 59,000 years for rocks near the top of the pali to 470,000 years for rocks near the base. However, the base of the pali does not expose the oldest rocks in Mauna Loa. Another rock sample collected from the distal end of the volcano's Southwest Rift Zone has an age of 657,000 years, plus or minus 175,000 years.

These ages span a long period of time, indicating that Mauna Loa has been active for over 650,000 years. In fact, the volcano must be older, because the measured ages do not reflect the earliest growth of Mauna Loa, when it was in a stage of volcanism similar to that of Lō‘ihi seamount, south of the island, today.

The origin of the Kahuku pali is still debated. One hypothesis is that the pali is a scissors fault, with zero offset above Highway 11 and increasing amounts of offset toward the south—similar to the way the two cutting surfaces of scissors get farther apart as you move from hinge to tip. On the Kahuku pali, the west side of the fault dropped down relative to the east side.

An alternate hypothesis is that the submarine portion of the pali is the headwall of a catastrophic landslide, with the west side falling away and leaving the mile-high scarp. Both hypotheses have pros and cons, and today, there is no definitive answer as to the origin of the pali.

By studying the submarine Kahuku pali, scientists have documented how this sector of Mauna Loa grew, the time scale over which the growth occurred, the types of lava that were produced, and how the chemical composition of the magma evolved.

The next time you drive around the southern tip of the Island of Hawai‘i, think about this—just offshore, there's a mile-high pali with lava flows that are nearly half a million years old!

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Volcano Activity Update

The June 27th lava flow from Pu‘u ‘Ō‘ō remained active on Kīlauea Volcano's East Rift Zone. As of October 16, the flow front had stalled approximately 1.1 km (0.7 mi) upslope from AP‘A‘ā Street/Cemetery Road, as measured along a straight line. The flow remained active, however, with scattered breakouts upslope of the leading edge of the flow. Within the Pu‘u ‘Ō‘ō crater, glow was visible above several small outgassing openings in the crater floor.

The summit lava lake within Halema‘uma‘u Crater produced nighttime glow that was visible via HVO's webcam over the past week. The lava level dropped slowly through much of the past week, and was roughly 60 m (200 ft) below the rim of the Overlook crater as of this writing (Thurs., Oct. 16).

Two earthquakes were reported felt on the Island of Hawai‘i during the past week. On Sunday, October 12, at 6:43 p.m., HST, a magnitude-4.0 earthquake occurred 10.8 km (6.7 mi) north of Nā‘ālehu at a depth of 5.4 km (3.4 mi). On Monday, October 13, at 12:28 p.m., a magnitude-2.7 earthquake occurred 7.8 km (4.8 mi) southwest of Hōlualoa at a depth of 11.4 km (7.1 mi).