Volcano Watch — Plumbing the depths of Pu‘u ‘Ō‘ō

At its highest in 1986, Pu‘u ‘Ō‘ō stood 255 m (835 ft) above the pre-eruption landscape and was about 1 km (0.6 mi) across at its base. Today, Pu‘u ‘Ō‘ō is about 171 m (560 ft) high and nearly all sides of the cone have been buried by lava flows. These flows form a broad shield, 2 to 3 km (1 to 2 mi) across and up to 150 m (500 ft) thick, that almost completely encircles Pu‘u ‘Ō‘ō. Of the original cone, only the northwest flank and a narrow sliver of the upper southeast flank remain exposed.

Pu‘u ‘Ō‘ō's summit is defined by a crater that is 450 m (490 yd) long and 300 m (330 yd) wide and is filled with solidified lava. Following the onset of the June 27th lava flow in 2014, a new crater about 240 m (260 yd) across and 30 m (100 ft) deep formed in the northeastern part of the older, filled crater. This new crater developed as magma beneath Pu‘u ‘Ō‘ō drained away to feed the June 27th flank eruption and the overlying crater floor fell into the resulting void.

Lava rises close to the surface in several spots along a fracture system that defines the perimeter of the new, smaller crater. Lava also erupts from a fracture lower on Pu‘u ‘Ō‘ō's northeast flank, feeding the active lava flow near Pāhoa. These surface vents are connected to a magma storage reservoir hundreds of feet below, probably by long, narrow conduits.

Mapping the shape of the magma storage reservoir and delivery system beneath Pu‘u ‘Ō‘ō is critical to understanding current monitoring data and forecasting how the eruption might behave in the future. But, because the entire plumbing system cannot be observed, we must rely on inferences from geological, geophysical, and geochemical data to "see" beneath the crater floor.

Studies conducted more than a decade ago suggested that the magma storage reservoir beneath Pu‘u ‘Ō‘ō had a vertical extent of about 300 m (1000 ft), with its top about 70 m (230 ft) below the pre-eruption ground surface. The size and shape of the crater formed above this reservoir is thought to approximate the reservoir’s horizontal dimensions.

Many changes have occurred at Pu‘u ‘Ō‘ō since those studies, including four major and several minor crater collapses and large fluctuations in magma supply, so the current shape of the Pu‘u ‘Ō‘ō magma reservoir is not known. However, the small size of the crater formed after the onset of the June 27th lava flow suggests that the reservoir may be relatively small.

The magma reservoir beneath Pu‘u ‘Ō‘ō is connected to the much deeper East Rift Zone system that transports magma from Kīlauea's summit to Pu‘u ‘Ō‘ō. Earthquake locations and modeling of other geophysical data suggest that magma travels through the East Rift Zone at a depth of about 3 km (2 mi), which is about 2 km (1.2 mi) below sea level.

The simplified model presented here is inferred from a wide range of data and will evolve as the tools and techniques used to study eruptions improve. Moreover, the plumbing beneath Pu‘u ‘Ō‘ō has changed dramatically through past decades, so aspects of the current view will have to be revised as the eruption changes over time. Regardless, this model provides some constraints for understanding how the plumbing system beneath Pu‘u ‘Ō‘ō might work.

Using these insights, USGS Hawaiian Volcano Observatory scientists hope to better understand the plumbing system that feeds the June 27th lava flow—to connect what we observe at Pu‘u ‘Ō‘ō to activity at the distal end of the lava flow near Pāhoa. This could eventually enable us to better forecast the behavior of threatening lava flows based on changes that occur at their sources—certainly an important goal given the ongoing crisis in Puna.

January is Volcano Awareness Month, during which HVO scientists will present a number of talks about Hawaiian volcanoes. Please visit http://hvo.wr.usgs.gov, email askHVO@usgs.gov, or call 808-967-8844 for details about this week's programs.

The staff of the USGS Hawaiian Volcano Observatory wishes everyone a Happy New Year.

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Volcano Activity Update

Kīlauea's East Rift Zone lava flow advanced only about 150 m (yd) over the past week. As of Wednesday, December 31, the front of the flow was stalled about 0.5 km (0.3 miles) upslope from the Pāhoa Marketplace, but small breakouts just behind the stalled front remained active. Additional small breakouts were scattered sparsely over a broad area extending several kilometers (miles) upslope, including weak breakouts near the abandoned True/Mid-Pacific geothermal well site and just downslope of Pu‘u ‘Ō‘ō. There was no significant change in activity at Pu‘u ‘Ō‘ō.

The level of the summit lava lake generally tracked changes in summit deformation, fluctuating between about 50 and 55 m (165–180 ft) below the rim of the Overlook crater during the week.

There was one earthquake reported felt in the past week on the Island of Hawai‘i. On Thursday, January 1, 2015, at 6:15 a.m. HST, a magnitude-2.8 earthquake occurred and was located 4.4 km (2.7 mi) northwest of Kawaihae at a depth of 25 km (16 mi).