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Volcanowatch

November 17, 2005

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


New Age Found for Kaho`olawe's Most Recent Eruptions

Kahoolawe, a single shield volcano
Kahoolawe, a single shield volcano, consists of a nearly filled caldera and a rift zone that trends to the southwest. Kahoolawe is about 12 miles (20 km) across and the smallest of the major Hawaiian Islands. This photo is looking to the east. Photo by Jack Lockwood, U.S. Geological Survey, March 31, 1984.

Two questions commonly asked of any volcano are the age of its youngest eruption and the frequency of eruptive events. For the volcano that forms the island of Kaho`olawe, the first question was answered recently when ages of about 1 million years were obtained from lava flows on the east side of the island. This finding may surprise some, because publications have described an age less than 10,000 years for Kaho`olawe's youngest volcanic rocks.

With colleagues from the University of Kyoto, we visited Kaho`olawe in 2004 to collect samples from the youngest lava flows. At that time we estimated that the youngest volcanic rocks were at least 200,000 years old because of their depth and style of weathering.

The ages were obtained recently by Hiroki Sano, a graduate student at Kyoto. He used the potassium-argon method of dating, which relies on the natural decay of potassium atoms into argon. A gaseous element, argon escapes from magma prior to eruption, resetting the clock to zero. By measuring the amount of argon present today, Sano could calculate the time that has passed since the lava flows first crystallized and argon accumulated anew. Thus the age corresponds to the age of eruption.

The two more precise ages are 0.97 and 1.19 million years, give or take about 0.22 million years. This give-or-take is known as the analytical error. It indicates that Sano has 95 percent confidence that the actual age will be within the range of the error. We would look askance if someone at the grocery store counted our change with some analytical error, but it's a fact of life when dealing with difficult measurements like dating rocks. A third age, 1.41 million years, has a larger error, about 0.45 million years. The ages may appear to differ greatly, but they overlap completely when the analytical error is considered.

Fortunately, other facts help establish the age more precisely. For example, the lava flows recorded the Earth's magnetic direction at the time they formed, a consequence of the iron- and titanium-bearing minerals within them. Just as a compass responds to a magnet, these crystals lock in the direction of the magnetic field when the lava cooled and solidified. The most likely era to have had a similar magnetic direction was the time from about 900,000 to 980,000 years ago, given the choices allowed by the potassium-argon ages and their associated analytical error.

Another helpful fact is the age of underlying rocks. Dated in the late 1980s by a different group of scientists, the older lava flows of Kaho`olawe range in age from about 1.3 to 1.1 million years. Taken together, these different facts indicate that the youngest lava flows are closely related in time to the older lava flows, perhaps all of them within 300,000 years of each other. Kaho`olawe has been extinct ever since, for nearly one million years.

So where's the beef? How could the age of the topmost layers have been thought to be so much younger prior to Sano's dating? The answer lies in the geologic setting. The older lava flows were truncated by the head of a landslide that formed Kanapou Bay. Bouldery sediment was then deposited on the resulting slope. Sometime afterward, an eruption spewed the youngest lava flows. In other words, the youngest and the next youngest lava flows were separated by the formation of Kanapou Bay.

Geologists once believed that much time was required to erode an escarpment, accumulate draping gravel, and then shed a lava flow or two. Only recently have we come to understand that Hawaiian volcanoes can undergo large-scale landslides in brief episodes. Lava flows are dated with increasing precision, by a variety of methods, leading to discoveries on O`ahu, East Maui, and the Big Island that confirm the rapid change of some geologic landscapes. Perhaps the real surprise is the length of time it took to uncover this secret about Kaho`olawe's geologic history.

Activity update

During the past week, the count of earthquakes located beneath Kilauea remains at low levels. Inflation continues, but has slowed over the past few weeks.

Eruptive activity at Pu`u `O`o continues. On clear nights, glow is visible from several vents within the crater and on the southwest side of the cone. Lava continues to flow through the PKK lava tube from its source on the flank of Pu`u `O`o to the ocean, with a few surface flows breaking out of the tube. In the past week, flows were active on the steep slope of Pulama pali and visible at night (weather permitting) from the end of Chain of Craters Road.

As of November 17, lava is entering the ocean at East Lae`apuki, in Hawai`i Volcanoes National Park. Small bench collapses continue to occur at the ocean entry. Large cracks cross both the old and new parts of the bench. Access to the ocean entry and the surrounding area remains closed, due to significant hazards. If you visit the eruption site, check with the rangers for current updates, and remember to carry lots of water when venturing out onto the flow field.

There were again no felt earthquakes reported on Hawai`i Island within the past week.

Mauna Loa is not erupting. During the past week, the count of earthquakes located beneath the volcano remains at low levels. Inflation has resumed after having slowed over much of the previous month.

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Updated: November 28, 2005 (pnf)