April 4, 1997

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

When is Kilauea erupting?

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 Kilauea 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.

Eruption Update

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.