Almost nothing is enough: reticulite helps measure depth of Kīlauea's caldera 500 years ago

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Pumice is glassy volcanic ejecta so full of bubbles that it floats in water. Reticulite is an extreme form of pumice in which all bubbles have burst and become interconnected, tenuously held together by glassy threads. The resulting high permeability causes reticulite to sink quickly in water despite its light weight, like chicken wire—there's almost nothing there, but it sinks anyway.

Almost nothing is enough: reticulite helps measure depth of Kīlauea...

Reticulite sample. USGS Photo by J.D. Griggs.

(Public domain.)

Pumice and less frothy ejecta form in typical lava fountains at Kīlauea, but reticulite develops only in very high fountains, say 300 m (1,000 feet) or higher, that shoot rapidly from the vent. Reticulite rises higher than pumice and blows farther downwind before falling to the ground.

Some of the high fountains in 1959 at Kīlauea Iki, 1969 at Mauna Ulu, and 1983-86 at Pu`u `O`o produced reticulite, but it was mostly deposited far downwind from the vent. In contrast, the heavier pumice and other material fell near the vent to form a cone and surrounding apron of fallout debris.

What does this have to do with estimating the depth of Kīlauea's caldera just after it formed some 500 years ago?

Recent field work confirms the presence of a bed of reticulite that completely encircles the caldera, visible along cracks and faults near the caldera rim and in dug pits at KMC. The reticulite is the second oldest pyroclastic deposit on the rim of the caldera, overlying only a thin, gritty volcanic ash that probably fell during formation of the caldera.

Two characteristics of the reticulite bed are especially notable. First, it contains little material other than reticulite itself; Pele's tears and pumice are present but not abundant. Second, it commonly exhibits internal variations in grain size, color, or concentrations of Pele's tears that don't continue far—in some places less than 200 m (650 feet)—before disappearing or being replaced by other variations.

The purity of the deposit suggests a distant vent, beyond the reach of most pumice and heavier debris. This is unlikely, however, because the reticulite encircles the caldera, and its vent must be inside that circle. More likely, the vent was so deep within the caldera that little pumice could reach the rim. This depth must have been at least 300 m—the minimum height of a fountain that produces much reticulite—and was probably much more, given the thickness of reticulite, locally as much as 65 cm (2 feet).

Did a single lava fountain produce all the reticulite? There's no way to be sure, but the fact that the reticulite occurs completely around the caldera and has many internal variations argues against a single vent. It seems likely that the reticulite came from several fountains erupting more or less simultaneously.

Multiple high fountains are certainly not the norm for Kīlauea. Perhaps the highest recorded were twins that played several times during the Mauna Ulu eruption in 1969, rising from vents 150 m (500 feet) apart and sometimes reaching more than 200 m (650 feet) high.

In the chaotic time during and just after caldera formation, however, several faults bounding sections of the caldera floor may have penetrated into the emptying magma reservoir, allowing eruption from multiple vents. Indeed, eruptions from "ring fractures" are inferred for many old calderas on Earth.

Whatever the number or size of the vents, the caldera was already a few hundred meters deep when the reticulite erupted. The eruption took place about 500 years ago, just after, or during, collapse of Kīlauea's summit to form the caldera. The formation of the caldera apparently happened fast, likely in one event lasting hours to days rather than piecemeal over years or decades.

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Volcano Activity Update

Kīlauea summit and Pu`u `O`o continue to deflate. Seismic tremor levels continue to be low. Earthquakes were located beneath Halema`uma`u crater, the south flank area, and the lower southwest rift zone.

The July 21 eruption remains active. The erupting lava flows through an open lava channel and into a lava pond about 1/2 mile from the vent. Due to frequent overflows from the channel and pond, both have become perched well above the surrounding lava surface. In the past week, a few larger overflows from the pond have produced short-lived `a`a flows that reached into the forest to the northeast. A slow moving `a`a flow, fed by lava seeping from a rupture at the base of the perched pond wall, continues to move eastward along the south side of flows emplaced in early August.

Because `a`a lava flows cool relatively quickly, even the longest flows have been able to reach only 3 to 4 miles from the fissure before stagnating. 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 repeated several times in the past month. None of the more recent flows have traveled as far as the first couple of flows from this eruption. Instead, the flows are mostly accumulating in the area just north of Kupaianaha shield, as well as resurfacing the earlier flows from August.

For the last several days, no incandescence has been seen on the Webcam at night in Pu`u `O`o crater. The heavy fume coming from Pu`u `O`o completely obscures any view into the crater. As has been seen in years past, Pu`u `O`o could be acting as temporary storage for lava that passes beneath the cone on its way to the erupting fissure. There have also been a number of collapses in Pu`u `O`o crater in the past few weeks, and cracks on the north rim and south flank of the cone seem to be widening.

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).

Two earthquakes beneath Hawai`i Island were reported felt within the past week. A magnitude-2.0 earthquake occurred at 5:39 a.m. H.s.t. on Sunday, September 2, and was located 9 km (5 miles) east of Waiki`i, at a depth of 22 km (13 miles). A magnitude-3.3 earthquake occurred at 1:29 p.m. on Monday, September 3 and was located 36 km (22 miles) offshore west of Kailua at a depth of 36 km (22 miles).

Mauna Loa is not erupting. No earthquakes were located beneath the summit. Extension between locations spanning the summit, indicating inflation, continues at steady, slow rates, which have slowed further since May 2007.