Volcano Watch — Kīlauea's caldera: more than just a hole in the ground

Many visitors come to the summit of Kīlauea, take a quick look into the caldera, say, "Hmmm, that's nice," shrug their shoulders, and move on. All they see is a hole in the ground, big to be sure, but not something that jumps out and grabs them. Well, there's more to it than that. What follows is the short history of the caldera-the modern caldera, that is, the one we see today.

The history begins some 1,000-1,200 years ago. A series of explosive eruptions was ending. These explosions, interspersed with high lava fountains, deposited beds of volcanic ash and blocks around the summit region of Kīlauea. One such deposit occurs near the base of Uwekahuna Bluff, below HVO. This deposit has recently been interpreted to have formed by explosions in about A.D. 800-1000 from an old caldera, the predecessor to the modern one.

As the explosions ended, lava began to fill the old caldera. Slowly in human terms, but quickly by geologic standards, the caldera was filled. As eruptions continued, lava flows built a shield-shaped edifice over the buried caldera. Construction of the entire shield was completed by about A.D. 1400. As the shield grew, it sent flows downslope in all directions. Some of these flows now underlie Volcano Village; others covered a mature koa forest, forming the Tree Molds site in Hawai`i Volcanoes National Park. Still other flows flooded the area south of the summit. Driving down the Hilina Pali Road, you cross some of these flows as far as Kipuka Nene and beyond.

People driving toward Volcano from Pahala, or down the Mauna Loa Strip Road, will note the shield shape of the summit area. That was formed by this series of eruptions. Trudging up the trail from Namakani Paio to HVO, you get a definite feeling for the presence of a significant slope to this shield.

The roof literally fell in on the shield in about A.D. 1475-1500. That is when the modern caldera formed. We know this because explosive and lava-fountain deposits dated at about A.D. 1500 are plastered against the vertical walls of the caldera; clearly the walls were there when the explosions occurred. How deep the caldera was then is unknown, but it was, at least part of the time in the next three centuries, 500 m (1500 feet) or more deeper than at present. There may have been periods when parts of the floor collapsed still farther, and such periods may be recorded by some events described in Pele-Hi`iaka chants, such as Hi`iaka digging for the body of Lohi`au. Explosions took place at times when the floor was below the water table, now about 600 m (2000 feet).

The first written description of the caldera was made by Ellis on August 1-2, 1823. The outline of the caldera then was the same as it is today, but the floor was far deeper, 540 m (1770 feet) then vs. 120 m (400 feet) now. During the 19th century, the caldera gradually filled because of nearly continuous eruptions on the floor from several lava lakes. Halema`uma`u, which was, when first seen, simply one among equals, eventually became the main lake, and it acquired its name in the late 1830s. Eruptions from it, ending in 1924, created a small shield whose shape you can easily see from Crater Rim Drive west of Keanakako`i.

So, the history of the modern caldera has been dynamic. There is much more there than just a hole. There is a past rich in lava eruptions and explosions, and there is a future with similar events in store. We happen to live in a time when the caldera seems quiet and passive. Had we been here in the 15th century, we would have seen a mountain, and in the 16th century, a hole. Nothing is permanent except change.

Volcano Activity Update

Kīlauea's eruptive activity continued unabated during the past week. A lava lake occupies the crater of Pu`u `O`o most of the time. Lava leaves Pu`u `O`o and flows through a tube southeastward to the 620-m (2050-ft) elevation, near the top of Pulama pali. There lava wells out to form a low shield with a perched pond on top. Since October 22, breakouts from the south side of the shield have been feeding a flow that has descended to the base of the pali and spread part way across the coastal flat. By October 28, the distal end of the flow is about 2.7 km (1.7 mi) from the sea coast.

There were two earthquakes reported felt during the week ending on October 28. Residents of Hawaii Volcanoes National Park felt an earthquake at 10:08 p.m. on Sunday, October 24. The magnitude-3.3 temblor was located 2.5 km (1.5 mi) beneath the south rim of Kīlauea Caldera. Early Tuesday morning at 1:23 a.m., residents of Leilani Estates felt the earth shake from a magnitude-3.2 earthquake located 3.3 km (2 mi) south of Pu`ulena Crater.