Volcano Watch — Speculations about when Kīlauea's caldera formed

Scientific concepts are often thought to result from thinking about hard facts. Speculation is sometimes considered out of bounds for scientists. The reality is, however, that speculation often encourages the search for facts and so can be an important part of the research process. Today's article deals with speculation about an important part of Kīlauea's history. When did Kīlauea's caldera form?

1790 is the date that most geologists accept. Before that, Kīlauea's summit was thought to have been indented by a pit or crater of unknown dimensions. Then the summit subsided, leaving a great depression perhaps 300-500 m (1,000-1600 ft) deep. Near or just after the end of the collapse, powerful explosions erupted debris that killed part of Keoua's army, which had camped the night before at the summit. The explosive debris totals 12 m (40 ft) deep in places.

This hypothesis has been pieced together over a number of years by geologists from HVO and several universities. But certain cultural questions remain to be answered before the hypothesis can be accepted.

Why are there no Hawaiian legends or oral histories of such a tremendous collapse in 1790? Surely that would have been the event of a generation (at least!), especially since it involved the home ground of Pele.

Why did the Hawaiians fail to note a vast change in landscape outside the caldera resulting from a thick blanket of volcanic debris erupted in 1790? That debris would have covered trails and buried other cultural sites. Instead, history tells of the misfortune of Keoua's army but little about a cover of debris.

What if the caldera already existed in 1790? What if it simply deepened then, as the final act in a series of comparatively small collapses spaced over decades or even a century or two? The Hawaiians would have been accustomed to such a deep hole, and explosion debris from previous episodes of collapse would already have mantled the surrounding landscape. The explosions of the 1790 eruption might have added only a few new layers to the deposits already on the ground around the caldera. Thus, changes in 1790 would simply have been incremental and, to the Hawaiians, relatively minor.

These speculations challenge conventional thinking about the caldera. So, let's look at some of the "facts." The modern caldera—bounded by `Uwekahuna, Wahinekapu (Steaming Bluff), and `Uwealoha (Byron Ledge)—is bordered by faults that cut lava flows more than 450 years old. We can't say precisely when these faults actually formed, except that it must have been some time between about 450 and 200 years ago. Therefore there might have been at least 250 years (say, from about AD 1550 to 1790) during which the caldera could have sunk intermittently. If each episode of sinking were followed by explosions, then the thickness of debris outside the caldera would have gradually increased with no great disruption to cultural activities.

What eventful times they must have been! In the religious framework existing then, Pele and Kamapua`a must have battled again and again, just as the legends tell us about their tempestuous relationship.

So, armed with the awareness that the caldera could theoretically have formed over a protracted period of time, we now must evaluate all available evidence to see whether we can distinguish a single catastrophic event from a series of repeated smaller events. Geologists from the Smithsonian Institution and HVO are working to evaluate these competing hypotheses.

We need your help. We do not have an adequate grasp of Hawaiian legends, chants, or other cultural evidence that might help to distinguish between the two ideas. If you have, please contact HVO. Our speculation could be nipped in the bud if there are reliable legends that tell of one huge caldera collapse in 1790. Or, the speculation could be strengthened if the legends and chants describe repeated collapses or other evidence for a deep crater before 1790. We will combine the cultural and the geologic evidence in an attempt to resolve this issue, which has strong public safety as well as academic ramifications.

Volcano Activity Update

The east rift zone eruption of Kīlauea Volcano continued unabated during the past week. There was constant effusion of lava from the Pu`u `O`o vent, and the molten rock was confined to the tube system from the vent to the sea coast without any surface breakouts. Lava from the tube system entered the ocean at two locations - Waha`ula and Kamokuna. The public is reminded that the ocean entry areas are extremely hazardous, with explosions accompanying frequent collapses of the lava delta. The steam cloud is highly acidic and laced with glass particles.

Three earthquakes were reported felt during the past week. On January 30 at 4:25 p.m., an earthquakelocated 5 km (3 mi) west of Honoka`a was felt by residents of Ahualoa, Pa`auilo, and Waimea. The earthquake had a magnitude of 3.0 and originated from a depth of 13.3 km (8 mi). Residents of Waimea, Hawi, and Pa`auilo felt an earthquake at 8:57 p.m. on Sunday, February 1. The magnitude 3.7 earthquake was located 10 km (6 mi) northwest of Waimea at a depth of 13.6 km ( 8.2 mi). A magnitude 2.9 earthquake located 3 km (1.8 mi) west of the summit of Kīlauea was felt in Hilo. The 5 km (3 mi) deep earthquake occurred at 7:24 p.m. on Wednesday, February 4.