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October 29, 1998

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.

Flowing Lava Falls Hard to Observe

Every day, tourists and locals visit Kolekole stream to see 70 cubic meters of water per second (18,000 gallons per minute or gpm) cascade 135 m (442 feet) over Akaka Falls to the plunge pool below. The sight can be both dramatic and serene.

Even Big Island residents who visit the falls repeatedly throughout their lives may not notice that the head of the falls is slowly getting farther away from the viewing area. Although nobody is measuring it, we can guess that this is happening because it happens at other waterfalls. The famous American Niagara Falls used to cut back at a rate of 1 m (3 feet) per year until half of its supply of 5,700 cubic m/sec (90 million gpm) was diverted for hydroelectric power generation.

Lava inside tubes flows in ways that are similar to the way in which water flows in streams and rivers. For example, a lava stream can branch and rejoin like a braided stream. Lava streams typically flow at 5 cubic m/s (1320 gpm) and can downcut into their base at rates up to 10 cm (four inches) per day.

The lava stream can also form lava falls. Hawaiian Volcano Observatory scientists often see lava falls when looking into skylights, openings in the tube where the roof has fallen in. In fact, the formation of skylights seems to be linked in an unknown way to the presence of lava falls. When we see lava falls, they are always immediately below or just under the edge of the skylight. We assume, however, that not all lava falls have skylights over them, but nobody has volunteered to go in and see how many lava falls occur in the currently active tube!

Fortunately there are many lava tubes from past eruptions on our island that folks can walk through and study in relative comfort. Cavers (people who explore caves) report that in some of these tubes, lava falls account for between one-third and one-half of the total elevation drop by the lava from the vent to the end of the lava flow. While it is not the only falls on Kolekole stream, Akaka Falls accounts for 35% of the total elevation drop of the stream from the top of the falls to the ocean.

Cavers also report evidence that lava falls cut back like their water cousins. From patterns preserved in drained falls, they conclude that these falls must have cut back 10 m (30 ft) or more. Since lava tubes remain active for only a few months to a few years, lava falls must cut back at a pretty healthy rate. This appears to be sound geologic reasoning relying on the previously studied behavior of waterfalls.

The only problem is that it does not match what HVO scientists see through the skylights. We have observed at least half a dozen lava falls in the past several years. Some have been visited at least weekly for nearly a year before the skylights above them crusted over. In that time, no lava falls have been observed to cut back. We know this with some certainty, because we obtain lava samples through these same skylights by dangling an iron hammerhead on a stainless steel cable into the lava fall itself. If the falls were receding, we should have to dangle our sampler lower and lower every week to get a sample.

How, then do lava falls form? At two locations, we have observed lava falls forming, not by cutting back of the falls, but by deepening of the plunge pool. Some of them get so deep that the lava at the surface of the plunge pool flows toward the falls! Lava plunge pools, like waterfall plunge pools, are much wider than the stream feeding the falls, and lava flows through the plunge pool more slowly. For water, that means less erosion of the plunge pool than the falls. For lava, that apparently means more downcutting, because the slower fluid in the plunge pool can melt the lava tube bottom more efficiently than the narrower and faster stream at the top of the falls.

While lava falls are not the same magnitude of tourist attraction that Niagara or Akaka Falls are, they are an important scientific window into the processes by which lava tubes and channels form. And they also are both dramatic and serene at the same time.

Eruption and Earthquake Update

A red glow reflecting off the steam cloud above the Pu`u `O`o vent was visible on several occasions during the past week. This is the result of the clear weather we have been experiencing and not from a change in the eruptive activity. Lava continues to erupt from Pu`u `O`o and flow through a network of tubes from the vent to the seacoast. Breakouts from the tube system continue to produce surface flows, mainly near the base of the pali and on the coastal flats. Lava is entering the ocean at two localities west of Kamokuna. It has stopped flowing into the ocean at the site east of Kamokuna. The public is reminded that the ocean entry areas are extremely hazardous, with explosions accompanying frequent collapses of the lava delta. The steam clouds are highly acidic and laced with glass particles.

At 4:56 a.m. on October 28, residents of Pahala were awakened by a magnitude-3.3 earthquake. The earthquake was located 1.5 km (1 mi) northwest of Pahala at a depth of 6 km (3.6 mi). Later that night, at 10:25 p.m., residents of Kihei, Pukalani, and Kula on the island of Maui were jolted by a magnitude-4.0 earthquake. The earthquake was located 15 km (9 mi) west of Makena at a depth of 17 km (10.2 mi).

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Updated: 09 Nov 1998