August 23, 2007
A weekly feature provided by scientists at the Hawaiian Volcano Observatory.
Oldest radiometric ages from Kilauea about 275,000 years
On an active volcano like Kilauea, it?s easy to point to the youngest volcanic features. Although the specific lava flow or vent may change daily, its age is ?now.? But looking back is more difficult for geologists. How far into Kilauea?s history can we see?
The answer deserves a three-part deliberation, because observation and sampling fall nicely into categories: surface outcrops, drill core, and submarine samples. The strictly outcrop-based geologist wants to put his or her hand on the rock, where it lies exposed for all to see. Those willing to delve deeper (so to speak) can gain insight from rock corings taken from drill holes on Kilauea?s east rift zone. But the deepest insight (and oldest ages) come from the submarine slopes of the volcano?s south flank.
Let?s cut to the chase: submarine samples are the oldest dated rocks from Kilauea. The samples were collected from the southern slope by remotely operated submersibles towed by ships. Their ages range from 275,000 to 225,000 years. They are from a type of lava thought to be characteristic of Kilauea?s early growth during its preshield eruptive episodes. Scientists consider these ages to be the oldest verifiable record of Kilauea?s early history.
The oldest ages for rocks above sea level are much younger?between 50,000 and 70,000 years ago. This knowledge is blended from a known age and an interpretation. The age, about 43,000 years, is that of charcoal beneath an ash layer exposed midway up Hilina Pali. The cliffs provide a view beneath younger lava flows that elsewhere have covered the surface. Hilina Pali is the one place above sea level where rocks older than 3,000 years have been dated, though the other fault scarps surely contain undated flows of old ages.
The lowest exposed lava flow in Hilina Pali lacks a radiometric age, and that?s where interpretation starts. Ages from the upper part of the pali indicate that lava flows and ash must accumulate quickly enough that, about every 1,000 years on average, another 6 m will have built up (about 20 vertical ft every 1,000 yrs). This rate, in conjunction with the radiometric age and the thickness of rocks exposed, yields the estimate that the oldest exposed Kilauea lava flow may be as old as 70,000 years.
Several ages determined from rocks in deep drillholes along the east rift zone are too old to agree with other information from the drill core. An age as old as about 450,000 years came from a sample at 1,700 m (5,580 ft) depth. But information on rock magnetism from the drill core, at least down to 800 m (2,620 ft) depth, suggests the strata at that depth are only about 50,000 years old and the holes didn?t penetrate into rocks older than about 125,000 years. Apparently the ages, obtained from rocks probably unsuitable for dating, are in error.
The paleomagnetic (rock magnetism of old rocks) method seeks to match properties in the drill core with changes in the Earth?s magnetic polarity through time. By comparing the drillhole results with the worldwide paleomagnetic database, a confident estimate of age versus depth is derived. In our Kilauea case, however, the method is limited to the upper 800 m (2,620 ft) of coring. Below that depth, the lava flows were erupted in a submarine setting, where they formed fragmental debris unsuitable for tracking the remanent magnetism.
Obtaining ages on rocks at Kilauea is a less than straightforward task. Every method has pitfalls, many of which we have experienced. It is a combination of different kinds of data, rather than one kind alone, that generally gives the most acceptable answer.
Kilauea summit and the July 21 fissures continue to deflate. The summit and Pu`u `O`o shared two DI (deflation-inflation) tilt events, once again demonstrating that they are hydraulically well connected. Seismic tremor levels are moderate in the vicinity of Pu`u `O`o and the July 21 fissure and low elsewhere. Earthquakes continue to be located beneath Halema`uma`u Crater and the south flank.
The July 21 fissure eruption remains active. Of the four original fissure segments, only fissure D---the fissure farthest east---is erupting lava. The lava enters an open lava channel that transitions into `a`a flows moving toward the northeast. Because `a`a lava flows cool relatively quickly, the flows have been able to reach only 3 to 4 miles from the fissure before they stagnate. Lava then piles up behind the stalled flow and is forced to jump out of the channel to make a new `a`a flow. This process has now been repeated three times---for a total of four `a`a flows---and each subsequent flow has followed along the north edge of the preceding flow. Because of the `a`a flow length limitation imposed by cooling, this behavior will likely continue as long as the eruption is producing `a`a flows.
The most recent `a`a flow to form (as of August 23) broke out from the lava channel Tuesday, August 21. The breakout point was closer to the fissure than the previous two channel overflows, so the new `a`a flow will have to travel even farther to catch up to the stagnant fronts of the previously emplaced flows. This will be further hindered by what may be a slight decrease in the volume of lava being erupted. The level of the lava in the channel has dropped, and the active vents along the fissure have partly crusted over.
While the July 21 fissure eruption continues to light up the night sky, Pu`u `O`o is still quietly fuming in the background. Steam and fume obscure the crater most of the time, but occasional glimpses of incandescence are still seen on the crater floor and in the West Gap. As has been seen in years past, Pu`u `O`o could be acting as temporary storage as lava passes beneath the cone on its way to the erupting fissure.
Vent areas are hazardous. Access to the eruption site, in the Pu`u Kahauale`a Natural Area Reserve, is closed (http://www.state.hi.us/dlnr/chair/pio/HtmlNR/07-N076.htm).
One earthquake beneath Hawai`i Island was reported felt within the past week. A magnitude-3.1 earthquake occurred at 4:15 p.m. H.s.t. on Thursday, August 16, and was located 2 km (1 mile) southeast of Kalapana at a depth of 10 km (6 miles).
Mauna Loa is not erupting. One earthquake was located beneath the summit. Extension between locations spanning the summit, indicating inflation, continues at steady, slow rates.
Updated: September 10, 2007 (pnf)