Volcano Watch — Pu`ukapukapu: understandable but still mysterious

Sources/Usage: Public Domain. View Media Details

What is so mysterious is not the cliff of Pu`ukapukapu. That cliff is like so many other pali in the Hilina fault system, a family of stair-step faults that move the south flank of Kilauea downward and seaward. Hilina Pali is the best known of these fault escarpments; Holei and Poliokeawe Pali are other examples. The last time these faults moved was on November 29, 1975, during the M7.2 Kalapana earthquake, when downward movements of 1-2 m took place on several of the faults.

The mystery about Pu`ukapukapu is its top. Why is its top a pu`u-a hill-that stands above its surroundings? The other cliffs have no such hill; they simply break off from flat or gently seaward-sloping ground. From the top of Pu`ukapukapu, however, one looks down in all directions, including back toward the rest of Kilauea. The hill is not high, only about 30 m (100 feet), yet affords a commanding view, perhaps giving rise to its name, which means a very sacred or regal hill.

The hill on Pu`ukapukapu results from the jostling of different fault blocks within the Hilina fault system. As some blocks move, they may crunch or rotate against another one and keep it from dropping as far as the surrounding blocks. The result is a high-standing block, technically termed a horst. All the blocks go down, but the horst drops less than its surroundings. Pu`ukapukapu has had a rough time of it, surrounded by faults and tilted inland several degrees. Despite these travails, it remains taller than its neighboring blocks.

A couple of reasons help explain why Pu`ukapukapu is a horst. One is that it is caught in pincers. During eruptions, Kilauea's southwest rift zone typically widens toward the southeast, in the direction of Pu`ukapukapu. This tends to pinch or push Pu`ukapukapu against the south-southeast-moving part of Kilauea's south flank. Pu`ukapukapu is in this zone of compression and so cannot drop down quite as freely as the rest of the south flank. This effect may be small but is unavoidable.

The second, perhaps more important, reason why Pu`ukapukapu appears anomalous is that it may simply be a survivor. There could have been other horsts at one time that were later buried by lava flows. Relatively few lava flows make it to Pu`ukapukapu, which is the farthest point downslope from an eruption site in the entire Hilina fault system. Simply on the basis of statistics, it is the least likely place to be inundated by lava.

Another factor ups the chances that the Pu`ukapukapu area will escape lava inundation. The Koa`e fault system crosses the pathway of any flow erupted from Kilauea's summit and headed toward Pu`ukapukapu. Kulanaokuaiki Pali is the best known of these fault scarps. Most of the pali in the Koa`e, such as Kulanaokuaiki, face toward the summit and so tend to trap lava flows and keep them from moving farther seaward. The Koa`e guards Pu`ukapukapu.

Recently obtained radiocarbon ages illustrate this point. South of the Koa`e fault system, the youngest flow headed for Pu`ukapukapu erupted about 1,200-1,300 years ago. In and north of the Koa`e, however, several flows are younger, about 600-900 years; they were stopped by the Koa`e pali from threatening Pu`ukapukapu.

The bight in Kilauea's coastline at Pu`ukapukapu is probably explained by the long distance from vents and protection of the Koa`e. Few flows travel far enough to build the coastline outward. Some readers will realize that another horst in the Hilina fault system played a deciding role in the lava inundation of Kalapana and Kaimu Bay in 1990. Hakuma horst is the name given to the high area, bounded by cliffs, that deflected the flows from Kupaianaha away from their direct course into the ocean. Hakuma horst stands high because of the jostling of fault blocks, just as does Pu`ukapukapu.

Volcano Activity Update

Eruptive activity of Kilauea Volcano continued unabated and effusively at the Pu`u `O`o vent during the past week. Lava extruded sporadically from several vents in the crater of Pu`u `O`o as well as from the Episode 55 pit and Puka Nui, two small craters at the southwest base of the cone. The perched lava ponds of the "rootless" shields are crusted over, and two long flows emanate from the base of the shields. One flow has traveled 2.3 km (1.4 mi) to the southeast, close to the boundary of Hawaii Volcanoes National Park. The other flow has gone 2.6 km (1.6 mi) to the east and is about 1.5 km (1 mi) above the Royal Gardens subdivision. There are no flows in the coastal flats and no ocean entries.

There were no earthquakes reported felt during the week ending on May 2.