Volcano Watch — When is Kīlauea erupting?

I answered the phone last week to a perplexed voice asking, "The paper says Pu`u `O`o is erupting but there aren't any lava flows. How can it be erupting, then?" I explained that lava is ponded deep in the crater of Pu`u `O`o but is not overflowing the rim. "But why do you say it is erupting if all the lava stays in the crater?" I tried to explain, but the caller hung up, more confused then ever.

The fact is that the word "eruption" means different things to different people. What do we at the U.S. Geological Survey's Hawaiian Volcano Observatory mean when we use the word?

We actually use "eruption" in three different ways. The first we can all agree on. When lava rises to the ground surface and forms lava flows, that is an eruption. No problem there, except that some highway signs directing the visitor to the "eruption site" really mean to the "lava flow site" or the "viewing site for lava." The eruption takes place at the vent, or "eruption site"; what moves away from the vent is a lava flow.

The second meaning of eruption is less obvious. What about when lava stays in a crater? Nothing leaves, except for gas and perhaps small bits of spatter. In 1967-68, thousands of visitors flocked to Halema`uma`u to see the eruption, which was entirely within the crater. No one questioned that this was an eruption. We don't, either. The rationale is simple but not obvious. Lava had reached the earth's surface, which is the floor of the crater! So, whether or not a lava flow goes out of the crater is immaterial.

The third meaning of "eruption" is the most confusing of all. We use "eruption"for certain times when we see no lava anywhere on the surface--in other words, when there is no eruption at all by the previous two usages! An example is the Pu`u `O`o-Kupaianaha eruption. There have been times during this long eruption, most recently between January 31 and February 23, when no lava was visible anywhere. Yet we reported that the eruption was continuing, because we expected lava to return to the ground surface soon on the basis of geophysical information, such as continued volcanic tremor, ground deformation, or high gas output.

This usage is time honored. It was applied to the famous Kīlauea Iki eruption in 1959, to the Mauna Ulu eruptions of 1969-71 and 1972-74, and to the Mount St. Helens eruption of 1980-1986. Each of these eruptions had "dead" times when no lava was visible. Yet monitoring of each eruption showed notable geophysical links between quiet and active times. Perhaps we need another word for such an "interrupted" or "punctuated" eruption. But we know of no better terminology, despite the richness of the English language. As always, it is best to understand the word in the context of its use.

Sometimes we subdivide a long-lasting eruption into episodes, like books into chapters. Episodes denote vigorous new eruptive activity either from a different vent or after a pause or slowdown. For example, Episode 54 of the Pu`u `O`o-Kupaianaha eruption began on January 30 at a new location (Napau Crater), and Episode 55 began on February 24 after a 24-day pause. Some episodes are well defined; others are rather arbitrary. Overall they provide convenient markers along the course of an eruption.

"Eruption" and "episode" are not important words in themselves except as shortcuts in communication. One should always read beyond them to understand what is really happening.

Volcano Activity Update

Eruptive activity within Pu`u `O`o crater continues with the lava pond about 60 meters (195 ft.) below the lowest section of the rim. On March 28, lava was reported seen outside of the crater for the first time since January 30. Lava was observed in a collapse pit at the southwestern base of Pu`u `O`o. By Thursday morning, April 3, lava had reoccupied about 1.5 km (1.0 mi.) of the old tube system, then had broken out of the tube at the 700-m (2300-ft) and 690-m (2270-ft) elevations. The two surface flows from these breakouts were nearly stagnant by late Friday afternoon. The distal end of the lower flow was about 4 km (2.5 mi.) from Pu`u `O`o and nearly 6 km (3.5 mi.) from the ocean. If lava output continues at the current rate, flows are expected to cascade over Pulama Pali within a week. Visitors traveling to the end of the Chain of Craters road in Hawaii Volcanoes National Park should have a great view of the lava.

A magnitude 1.9 earthquake located 7 km (4.2 mi.) west of Captain Cook was felt by a resident of that town. The 12.7 km (7.6 mi.) deep temblor occurred at 3:59 a.m..on Wednesday, April 2.