Volcano Watch — Pu`u `O`o eruption is long but far from the longest

In 1983, the year the eruption began, Ronald Reagan was president, "Flashdance" and "Gandhi" were movie hits, "Dallas" and "Dynasty" topped the TV charts, the leading pop song was "Down Under" by Men at Work, and Miami ranked number one in college football. Now all that's left is Miami as number one...and the eruption.

Nineteen years. A long time. Time to go from crib to college. But for a volcano, 19 years is nothing. Kīlauea has been erupting off and on for more than 100,000 years. How many times has it erupted continuously for 19 years or more?

That question can't be answered, because the past is so dim, but we can make a good guess for the time that people have lived on the island.

The present eruption has almost certainly lasted longer than any other eruption on the east rift zone in the past 1000-1500 years. Geologic evidence shows that no rift eruption during that time even approaches the current one in terms of volume of lava erupted or area covered. The closest is the five-year Mauna Ulu eruption in 1969-71 and 1972-74, but its volume is less than one-third, and its area less than one-half, the figures for the Pu`u `O`o eruption.

Another large eruption formed Kane Nui o Hamo 600-1,000 years ago. The size of that edifice and the extent of its flow field likewise suggest an eruption duration less than that of Pu`u `O`o.

No Hawaiian oral history points to a long-lasting eruption from Kīlauea's east rift zone, either. Consequently, it is likely that the Pu`u `O`o eruption takes first prize for lasting longer than any other Kīlauea rift eruption during human occupation of the island.

But don't jump to conclusions. Kīlauea has had longer eruptions during Hawaiian history, two for sure and most likely several others. The catch? They weren't on the east rift zone.

Vents in Kīlauea's caldera, generally at Halemaumau, were active continuously throughout most of the 19th century, as documented by journal accounts and the Volcano House register. After a brief respite, Halemaumau resumed activity early in the 20th century and stopped in early 1924. It is likely, from what William Ellis was told in 1823, that eruptions had been continuous or at least very frequent in the caldera "for many king's reigns past," probably since the early 16th century.

A vent just east of Kīlauea Iki, named the `Aila`au shield by volcanologists, erupted for about six decades during the 1400s. Flows from this shield moved away from the summit area and covered most of Kīlauea north of the east rift zone, extending to the sea at Kaloli Point.

Numerous radiocarbon ages obtained during the past several years document a series of nearly continuous, if not truly continuous, eruptions from the summit shield of Kīlauea in the 1200s and 1300s. These flows spread in all directions from the summit shield, which finally collapsed in about 1500 to form the modern caldera.

All of this evidence shows clearly that Kīlauea has often had longer eruptions than the current one or any other rift eruption that we know of. The length of the current eruption is unusual for a rift eruption, but it pales alongside the durations of summit eruptions.

Why is this? The summit is directly above the conduit conducting magma into the volcano from the hot spot. If magma gets into the volcano, it passes through the summit reservoir system. The summit is only a short distance higher, and it is easy for magma to erupt there. In order to erupt along a rift zone, the magma has to travel a much longer, more complex pathway from the summit reservoir. It is therefore simpler for magma to erupt at the summit than along a rift zone. Consequently, long eruptions are likely to be more common in the summit area than along a rift zone, just as the evidence indicates.

Volcano Activity Update

Eruptive activity of Kīlauea Volcano continued unabated at the Pu`u `O`o vent on this nineteenth anniversary. Lava moves away from the vent toward the ocean in a network of tubes and descends Pulama pali in several separate tubes. At times during the week, a bifurcating river of incandescent lava was seen on the pali. Many surface flows, mainly from breakouts of the Kamoamoa tube, are observed in the coastal flats. Lava enters the ocean at Kamoamoa and the area east of Kupapa`u.

The public is reminded that the benches of the ocean entries are very hazardous, with possible collapses of the unstable new land. The steam clouds are extremely hot, highly acidic, and laced with glass particles. Swimming at the black sand beaches of the benches can be a blistering or even deadly venture.

There were no earthquakes reported felt during the week ending on January 3.