December 6, 2001
A weekly feature provided by scientists at the Hawaiian Volcano Observatory.
A lua can be a fine place for a volcanologist
The roadway into Hawai`i Volcanoes National Park is an obstacle course now, as construction is underway for the new entrance station. The new station will have a toilet--something the old one didn't. A lua requires a drain-field pit, and a pit anywhere--particularly in the national park--is an unusual opportunity for volcanologists to examine past explosive eruptions of Kilauea.
Thanks to timely phone calls from Nick Heinrich and Bobby Camara of the park, and to the cooperation of Isemoto Contracting, the walls of the pit were examined before the pit was backfilled.
The pit was located 400 m (400 yards) from the inner rim of Kilauea caldera and was about 1.8 m (6 feet) deep and 5.5 m (18 feet) square. About 55 square meters (More than 1,900 square feet) of the vertical, freshly dug walls were accessible for examination and sampling. This amount of clean outcrop so near the upwind side of the caldera was unprecedented.
The bottom of the pit is in the glassy top of a pahoehoe flow-one of the last erupted before the caldera formed. This "flow of Volcano Village" is about 600 years old.
Resting on top of the flow is a pink-to-gray volcanic ash made of tiny bits of pulverized rock. This innocuous-looking ash, up to 8 cm (3 inches) thick, records the first explosion from the caldera, probably in the period 1460-1490.
A deposit of reticulite, up to 33 cm (13 inches) thick, overlies the ash. The reticulite, an extremely porous kind of pumice, is common throughout Volcano Village, encountered during almost any excavation down to bedrock. It was erupted in the period 1470 to 1500, according to numerous radiocarbon ages obtained for charcoal above, within, and below the deposit. High, vigorous lava fountains produced the reticulite.
A pink deposit of heavy rocks and sand rests on top of the reticulite. This deposit records a powerful, hot explosion that hurled rocks across the area now containing the park's housing area. The Volcano House and the entrance station are near the northern margin of the deposit. Its thickness is 40 cm (16 inches) in the pit but almost 1 m (3.3 feet) thick beneath the housing area. Single rocks more than 10 cm (4 inches) across plummeted to the ground and punched into the fluffy reticulite, some penetrating all the way down to the top of the pahoehoe flow.
The upper part of the "Housing Area deposit" is pink ash that likely records the fine fallout of the explosion. Small pieces of charcoal in, and on top of, the ash were probably burned during the explosion. The charcoal provides an opportunity to date the explosion, which probably occurred in the 16th century.
The rest of the deposits up to the ground surface total about 1.3 m (4.3 feet) thick and indicate a repetitive series of eruptions, mostly explosive but including thin beds of spatter and pumice from lava fountains. Most or all of the debris in the deposits apparently fell almost vertically, rather than sweeping across the landscape in surges. The continuous exposures along the walls of the pit failed to reveal any telltale structures related to surges.
Of great significance is the evidence for two periods of incipient soil formation and erosion. One occurs about 70 cm (28 inches) from the top of the pit, the other about 47 cm (18 inches). Each erosional surface cuts down into slightly red or orange, weathered ash. Charcoal occurs along each surface, indicating that vegetation grew in the soils. Two similar soils have been observed in the deposits south of Kilauea Iki. The soils indicate periods of time without explosions long enough for soil to begin forming-at least one to several decades.
The uppermost 35 cm (14 inches) are rocky deposits, partly or entirely exploded in 1790, when between 80 and 800 of Keoua's troupe were killed.
So, thanks to the need for a toilet at the entrance station, we have the most complete record of Kilauea's latest period of major explosions on the windward side of the caldera.
Eruptive activity of Kilauea Volcano continued unabated at the Pu`u `O`o vent during the past week. Lava moves away from the vent toward the ocean in a network of tubes and descends Pulama pali in several separate tubes. Surface flows are observed above the pali and at various places along the trace of the Kamoamoa and Kupapa`u tube systems. Field crews report a flow within 230 m (750 ft) of the coast at Kupapa`u. Lava continues to enter the ocean in two locations: Kamoamoa and the area east of Kupapa`u. A film crew from the A&E television series, "Behind Closed Doors," shot spectacular footage at the east Kupapa`u entry on Monday evening. Earlier in the day, show hostess Joan Lunden accompanied USGS personnel and collected fresh lava samples from a surface flow.
The public is reminded that the benches of the ocean entries are very hazardous, with possible collapses of the unstable new land. The steam clouds are extremely hot, highly acidic, and laced with glass particles. Swimming at the black sand beaches of the benches can be a blistering or even deadly venture.
One earthquake was reported felt during the week ending on December 6. Residents of Kapulena, Waimea, and Pa`auilo felt an earthquake at 11:28 a.m. on November 30. The magnitude-3.0 earthquake was located 13 km (7.8 mi) southwest of Honoka`a at a depth of 7 km (4.2 mi).
Updated: December 11, 2001 (pnf)