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March 4, 2004

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

Two tiny kipuka tell secrets about Kilauea?s past

About 2.5 km (1.5 miles) down the Hilina Pali Road in Hawai`i Volcanoes National Park are two tiny kipuka that help geologists decipher obscure parts of Kilauea?s past 1,500 years.

The lava flow in the kipuka, first mapped by former HVO staff member Tina Neal, is named the Hope`o (wasp) flow, after yellowjackets in the area. It is covered by volcanic ash from two different periods of Kilauea?s explosive activity. The younger period was from about A.D. 1500 to 1790. The older dates to about A.D. 800 and is the more important for this discussion.

Some ash fell just before the Hope`o flow came out. Radiocarbon ages for charcoal in the ash above and below the flow bracket its age as 1,200-1,300 years old, that is, A.D. 800-700.

Just as certain characteristics define the Stone Age, the Iron Age, and other historical eras, so distinctive magnetic characteristics place the Hope`o flow in a particular magnetic era recognizable elsewhere on Kilauea.

Magnetic minerals in a lava flow acquire and lock in the direction of the earth?s magnetic field. As the flow cools, the direction is preserved and does not change thereafter.

However, the earth?s magnetic field is constantly changing direction. For example, the little diagram at the bottom of a USGS quadrangle map shows the declination?the amount that the compass direction to magnetic north differs from that to true north. That declination is shown as 11 degrees east on the 1981 edition of the Volcano quadrangle but only 10 degrees on the 1997 edition. The magnetic direction slowly changed in the intervening 16 years. A lava flow erupted in 1997 would have a declination 1 degree less than that of a flow erupted in 1981.

Over geologic time, the directions can change by tens of degrees. This allows one to estimate the age of a lava flow with a particular magnetic direction, if another flow with a similar direction has been dated by radiocarbon techniques.

Unfortunately, the direction can loop back upon itself, so that the same direction may have appeared more than once in the past. We need independent evidence before the age of a magnetic direction can be confirmed rather than just suspected.

The Hope`o flow has an unusual magnetic direction. If lava flows with that direction occur elsewhere on Kilauea, they might be of the same age.

Former HVO staff member Robin Holcomb, and colleague Duane Champion, discovered long ago that a lava flow with the Hope`o magnetic direction occurs low in the wall of Kilauea?s caldera (in Uwekahuna Bluff, below HVO), in the northeast wall of Kilauea Iki, and in Pauahi Crater on the east rift zone.

Their findings suggest flows of about the same age as the Hope`o flow, but other evidence must confirm it.

That evidence has been recognized recently by Smithsonian and HVO geologists. Each lava flow with the Hope`o direction has an explosion deposit directly above it. This consistent pattern makes it likely that all of the flows with Hope`o magnetic direction erupted at about the same time (within perhaps 50 years) as the Hope`o flow itself.

If so, we can estimate the rate at which lava flows erupted after Hope`o time. At Uwekahuna, about 120 m (400 ft) of younger lava flows accumulated in about 450 years (A.D. 900-1350), 25 m (80 ft) per century, on average. These flows formed part of the Observatory shield, which grew before the caldera collapsed in about A.D. 1500.

The rates of accumulation were slower at Kilauea Iki and Pauahi Crater. About 70 m (230 ft) of lava flows younger than Hope`o erupted over about 550 years (A.D. 900-1450) at the east end of Kilauea Iki, an average accumulation rate of 13 m (40 ft) per century. The rate was a little slower at Pauahi Crater, where about 60 m (190 ft) of flows were erupted in the same time?an average rate of 11 m (35 ft) per century.

The Hope`o direction will acquire even more significance as it is discovered elsewhere on Kilauea and Mauna Loa. And it all started in those two tiny kipuka along the Hilina Pali Road!

Activity update

Eruptive activity at the Pu`u `O`o vent of Kilauea Volcano continued unabated during the past week. Lava flows erupted from the rootless shield complex along the Mother?s Day lava tube and from the south base of Pu`u `O`o. Lava also spread from Dave's pit vent inside the Pu`u `O`o crater, and many of the other crater vents remain incandescent. No active flows are on Pulama pali or the coastal flat below Paliuli. No lava is entering the ocean.

No earthquakes were felt on the island during the week ending early March 4.

Mauna Loa is not erupting. The summit region continues to inflate slowly. Seismic activity remains very low, with 2 earthquakes located in the summit area since early February 26.

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Updated: March 9, 2004 (pnf)