Volcano Watch — What will happen when the eruption ends?

Kīlauea has been erupting for nearly 18 years, and there is no sign of it stopping anytime soon. But all previous Kīlauea eruptions ended, and there's no reason to think this one is any different. What can we expect when the curtain finally falls?

We can look at what we know about the past and make educated guesses. The guesses range widely and depend on whether the supply of magma will continue into the volcano, whether the summit or a rift zone will be the next point of weakness, and whether the current eruption will last for a few years or a few decades longer.

Obviously the eruption will end if magma stops rising into the volcano, and nothing will happen at the surface until supply resumes. This is unlikely but possible. A more likely scenario is that magma entering the volcano will be stored somewhere in it, perhaps deep in the east rift zone or below the summit. This is what may have happened between 1924 and 1952, when Kīlauea was, for all intents and purposes, quiet. During the first half of 1924, the lava lake in Halema`uma`u drained, a large area near Kapoho opened and dropped, and explosions burst from Halema`uma`u. Apparently these events made room in the volcano to store incoming magma, and it wasn't until 1952 that the volcano was full and Kīlauea had to erupt.

Without such drastic events, we can expect more eruptions, either at the summit or along one of the two rift zones. In fact, this could even happen before the Pu`u `O`o eruption ends. Many people believe that extrusion at Pu`u `O`o removes pressure from the rest of the volcano, so that no eruption can occur elsewhere. During the Mauna Ulu eruptions (1969-71 and 1972-74), however, four other outbreaks did in fact take place, two in 1971 and two in 1973. Just after the Mauna Ulu eruption ended, two summit eruptions, and one in the upper southwest rift zone, occurred between July and December 1974.

The Mauna Ulu events show that, if the volcano remains pressurized, eruptions can take place during and soon after a long-lived eruption. And, the eruptions can be at the summit, anywhere in the rift zones, or both. Currently a barrier blocks the east rift zone near Pu`u `O`o and keeps magma from moving into lower Puna. How long this barrier will last, and how it will respond to the end of the Pu`u `O`o eruption, are anybody's guesses.

Perhaps the most striking scenario of all is suggested by the 15th-century eruption of Kīlauea and its aftermath. This summit or upper east-rift-zone eruption, named the `Aila`au eruption by geologists, lasted from about 1410 to about 1470, according to work by Dave Clague and associates. It is the longest rift eruption known at Kīlauea. By 1500, only 30 years later, a new caldera had formed at Kīlauea's summit. This close association in time suggests that the long-lasting `Aila`au eruption somehow led to the collapse of the caldera, perhaps by milking magma from the summit reservoir faster than it could be replenished. The collapse of the caldera ushered in nearly three centuries of episodic explosive activity.

If the current eruption continues several decades longer, could it result in renewed caldera collapse? Of interest in this regard is that the caldera has been sinking at a rate of nearly 10 cm (4 inches) per year since the eruption began in 1983. Is this sagging a prelim before the main event?

The bottom line is that the end of the current eruption could result in prolonged quiet, frequent new outbreaks, or caldera collapse and explosions. This complete spectrum of outcomes is not surprising. It reflects the exceedingly complex and dynamic nature of one of Earth's most active volcanoes.

Volcano Activity Update

The 81 cm (32 inches) of rain recorded at Pu`u `O`o on Thursday was not enough to douse the eruption. Eruptive activity of Kīlauea Volcano continued unabated during the past week. Lava is erupting from Pu`u `O`o and flowing southeast through a tube system down to the flats below Pulama pali and beyond to the ocean. Breakouts from the tube system are feeding small flows at the top of Pulama pali. Lava in the tube system is entering the ocean at Kamokuna located 1.6 km (1 mi) west-southwest of Waha`ula. The public is reminded that the ocean-entry areas are extremely hazardous, with explosions accompanying sudden collapses of the new land. The active lava flows are hot and have places with very thin crust. The steam clouds are highly acidic and laced with glass particles.

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