Volcano Watch — Hard rain at Halape

Two months ago, we discussed early findings suggesting that Kīlauea had an explosive eruption far larger than we dared think. The study has progressed since then, and the early findings have been confirmed and extended.

Our search for rocks thrown from Kīlauea's summit found them at all examined locations from Kipuka Keana Bihopa, just west of Hilina Pali Road, to the Kealakomo waena area below Holei Pali, along the Chain of Craters Road—an area 16 km (nearly 10 miles) wide. The rocks decrease in size systematically southward from the new Kulanaokuaiki campground, 7 km (4 miles) from Kīlauea's summit, to the coast, about 16 km (9.5 miles) from the summit.

At the campground, dense stones more than 10 cm (4 inches) in diameter are easily found--the largest yet is 16 cm (6.3 inches). At the coastal sites of Halape and Keauhou Landing, they are no larger than about 1.5 cm (0.6 inches) in diameter. Though small, they would have constituted a hard rain indeed on these idyllic coastal retreats.

By plotting the sizes of the ejected rocks on a map, it is immediately obvious that their distribution forms an elongate lobe aimed from Kīlauea's summit to the Keauhou Landing-`Apua Point area, some 18 km (11 miles) away. Sizes are largest along the axis of the lobe and decrease to either side as well as down the lobe toward the sea.

The explosive eruption took place between A.D. 600 and A.D. 1000; consequently the deposit is covered by younger lava flows in many places. Accordingly, we can find neither the ejected rocks closer to the summit than Kulanaokuaiki campground nor the absolute eastern and western sides of the lobe. Nonetheless, the pattern of distribution is remarkably complete and well defined.

The large rocks are too far from Kīlauea's summit to be explained by a single huge explosion. The observations seem to require a different kind of eruption-one with a sustained, up-rushing column of ash and rocks akin to those of very powerful eruptions from stratovolcanoes, such as Mount St. Helens and Pinatubo. These are called Plinian eruptions, after the famous Roman who observed the eruption of Vesuvius that destroyed Pompeii.

Modeling by academic colleagues shows that Plinian eruptions can boost fist-sized rocks to great heights. These rocks, together with ash and pumice, eventually spread out from the top of the column and move laterally, creating what are called mushroom, or umbrella, clouds. The rocks fall from the mushroom cloud, the larger ones nearer the vent.

Models of Plinian eruption columns allow one to calculate the height of the column and the wind velocity in the atmosphere during the eruption. If these models are applicable to Kīlauea, the new data suggest that the eruption column and its umbrella top reached a height of 25-30 km (15-18 miles) above the vent! This is higher than the 1980 umbrella cloud from Mount St. Helens. And, there was likely a wind of 10-20 m/sec (20-40 mph).

Plinian eruption columns are not driven by steam from heated ground water, as are the smaller explosions from Kīlauea. Instead, gases dissolved in magma expand rapidly to propel the liquid, and rocks scraped from the volcano's plumbing system, high into the air. Likely gases causing such energetic eruptions at Kīlauea are carbon dioxide and water dissolved in the magma. Of these, carbon dioxide is more abundant and less soluble than water and may be the most important propellant.

Plinian eruptions spread ash over wide areas, even upwind because of high-elevation air flow. They would present problems for the entire island, not just that part impacted by falling rocks. We don't know what the precursors would be or if they would differ from those leading to a typical Kīlauea eruption. Fortunately such events are rare. In at least the past 2,000 years, this is the only eruption of its type that we know about—so far.

Volcano Activity Update

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. Lava is entering the ocean at Kamokuna located 1.6 km (1 mi) west-southwest of Waha`ula. No surface flows were observed.

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 plumes are highly acidic and laced with glass particles.

Two earthquakes were reported felt during the week ending on December 7. Workers in Hawaii Volcanoes National Park reported feeling an earthquake at 7:51 a.m. on Tuesday, December 5. The magnitude-3.2 earthquake was located 1.5 km (.9 mi) beneath the summit of Kīlauea Volcano. A resident of Pa`auilo felt an earthquake at 4:47 p.m. on Thursday, December 7. The magnitude-3.5 earthquake was located 28 km (17 mi) south of Waimea at a depth of 39 km (23.4 mi).