Hawai'i Volcano Blog

USGS: Possible Outcomes at Kīlauea Caldera

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With contributions by Big Island Now News Director/Editor Debra Lordan

This screenshot was captured by Seattle Architect & Urban Designer AJ Taaca, who has the livestream of Kīlauea running on his computer while he works.

In a recent report to Hawai‘i Volcanoes National Park, the U.S. Geological Survey assessed potential volcanic hazards at the summit of Kīlauea.

The USGS’ June 29, 2018, update is a guide for understanding current activity and hazard at and around the summit of Kīlauea Volcano. The report summarizes activity from late April through June 29,
details possible future outcomes and reviews hazards associated with these outcomes. View the full report here.

Few processes outlined below are known sufficiently well for the USGS to be able to assign quantitative probabilities to possible future events. Instead, the USGS ranks future possibilities in qualitative terms of likelihood based on its understanding of current data.

Comparison of the topography of Halema‘uma‘u Crater in 2009 (left) and on June 21 2018 (right). The 2018 data shows extensive cracking and faulting has occurred around the crater, and the crater has enlarged through collapses of wallrock. The depth of the crater floor has increased by more than 1,100 feet since early May. Image courtesy of the U.S. Geological Survey and Office of Aviation Services, Department of the Interior, with support from the Hawaiian Volcano Observatory and Hawai‘i Volcanoes National Park.

The USGS’ understanding of Kīlauea and the current eruption is continually evolving as scientists obtain new information; their analyses may change in response.

Kīlauea Collapse/Explosion Events


Early collapse/explosion (CE) events (before late May) ejected ash and gas to heights above 25,000 feet and large ballistic fragments tens of centimeters in size pelted the area immediately surrounding the vent.

Observations and preliminary models suggest that these explosions may have been caused not by interaction of magma with groundwater, as previously believed to have occurred at Kīlauea in 1924, but rather by exsolution, expansion and release of gases which were dissolved in the magma.

CE events have continued to occur semi-regularly, with repose periods (time between events) of roughly .5 to two days.

Early collapse/explosion events have continued to occur semi-regularly, with repose periods (time between events) of roughly .5 to two days.

Seismicity increases in the hours preceding explosions, leading to cycles of earthquakes that are felt in the summit area.

Eight-ton block ejected from Halema‘uma‘u during a 1924 explosion at Kīlauea Volcano, located 0.6 miles SE of Halema‘uma‘u’s center. PC: USGS


The mechanism producing CE events is not well understood. The USGS infers that withdrawal of magma towards the LERZ continually works to reduce pressure in the shallow reservoir. When the reduction in pressure becomes too great, the rock that forms the floor of Halema‘uma‘u and parts of surrounding Kīlauea Caldera slump down into the shallow magma reservoir in a CE event.

The rock that slumps down into the reservoir replaces magma that has migrated into the LERZ and abruptly increases reservoir pressure (as measured by ground deformation instruments). Similar processes have been observed during caldera formation at other volcanoes.


Oct 11, 2008, explosive event Halema‘uma‘u’s Overlook Crater. VC: USGS HVO.

In this way, magma evacuation is accompanied by relatively non-hazardous slumping and enlargement of Halema‘uma‘u, rather than sudden large-scale collapses and more powerful explosions.


Since the beginning of May, the crater’s volume has more than quadrupled and since May 29, a GPS station near the north rim of Halema‘uma‘u crater measured a drop of more than 330 feet during CE events.

Halema‘uma‘u Crater’s volume has more than quadrupled and its floor has dropped more than 330 feet during collapse/explosion events.

At the end of May, collapse of rock from surrounding crater walls blocked the Overlook vent (which formerly contained the lava lake) and changed the character of summit activity. Since then, background gas emissions at the summit have greatly decreased, CE events generally have produced only weak ash plumes that do not rise higher than 6,000 feet above the crater rim and no ballistic fragments are known to have been ejected.

Although the plumes have become less vigorous, these more recent events have been preceded and accompanied by larger amounts of seismic shaking, and reservoir pressurization (as measured by ground deformation instruments) during the events has increased.

The character of subsidence at the summit has also changed, with deformation becoming more localized around Halema‘uma‘u Crater, but occurring at a higher rate. At the current time, subsidence of the caldera continues at a high rate due to magma withdrawal from the Halema‘uma‘u magma reservoir.

Possible Outcomes

Ground subsidence will continue for as long as magma is withdrawn from the summit reservoir(s) at a rate exceeding the rate of magma supply, but the rate, style and geographical extent of the subsidence—along with associated hazards—may vary. The scenarios below are considered under the condition of continued net magma withdrawal.

Dust cloud caused by the collapse of the Halema‘uma‘u Crater floor at Kīlauea Volcano, Hawai‘i, May 9, 1924. PC: USGS


The most likely activity for the immediate future is continued subsidence of Kīlauea Caldera,
episodic slumping into a widening Halema‘uma‘u Crater, felt earthquakes (some large enough
to be damaging) and small to intermediate ash plumes that remain below 10,000 feet above
sea level.

Most likely for the immediate future—continued subsidence of Kīlauea Caldera,
episodic slumping into a widening Halema‘uma‘u Crater, felt (sometimes damaging) earthquakes and small to intermediate ash plumes less than 10,000 feet high.

As the reservoir deflates, cracking and slumping is gradually engulfing a broader extent of Kīlauea Caldera (as observed in high rates of ground deformation and propagating cracks around Halema‘uma‘u); this process will likely continue to enlarge Halema‘uma‘u and may involve larger slump blocks than previously. This activity is impressive in scale—and may ultimately involve much or even all of the current Kīlauea Caldera—but it need not necessarily involve new or more hazardous explosive activity.

Hazardous explosive activity cannot be ruled out, however. It is possible that a large section of the Halema‘uma‘u wall could abruptly collapse into the crater. Because a broad region E and NE of Halema‘uma‘u is currently deforming, it is difficult to predict how large such a collapse might be or its impact on the explosion hazard. Most likely, such an event would generate only strong seismic shaking and a robust ash plume.

Should activity continue as described, primary hazards of concern are:

  • Damaging earthquakes (potentially exceeding equivalent M-5).
  • Ash plumes, ashfall (associated with CE events and large rockfalls)
  • Large and sudden collapses into the expanding Halema‘uma‘u Crater
  • Ground cracking and continued rockfall activity along steep caldera walls
  • Vog (although sulphur dioxide output is approaching low pre-2008 levels)

March 3, 2011, wall collapse in Halema‘uma‘u’s Overlook Crater. VC: USGS HVO


Several mechanisms could change the nature of activity and associated hazards. These are considered less likely but cannot be ruled out. The likelihood of some of these processes may increase if the net rate of magma outflux from the summit increases.

Below, the USGS considers the possibility of 1) more hazardous explosions occurring during ongoing subsidence and enlargement of Halema‘uma‘u, and 2) sudden collapse of the larger caldera system.

This list may not include all possible future outcomes and hazards.

1. Larger, more hazardous explosions during ongoing subsidence and enlargement of Halema‘uma‘u

Activity could become more hazardous over short-time scales. This could be triggered in one of several ways, including a) rapid pressure change or other perturbation of the reservoir, b) opening of new pathways between the reservoir and the surface, or c) interaction of magma with groundwater. Rapid pressure change could be caused by a large, sudden landslide from the crater’s steep, faulted rim; alternatively, sudden larger-scale collapse of rock into the reservoir could perturb reservoir pressure above levels seen during previous CE events.

Activity could become more hazardous over short-time scales.

New pathways could be formed by explosive ejection of rubble in the vent or downward propagation of cracks. Groundwater could enter the magmatic system at sufficient rates to produce steam-driven explosive eruptions. Some of these mechanisms could be preceded by detectable changes in monitoring data, but others could happen with no warning. If larger explosions do occur, their style and magnitude cannot be predicted; it is possible that they could produce more ballistics and ash, and
possibly also pyroclastic surges.

Spectators flee explosion from Halema‘uma‘u at Kīlauea Volcano, 11:14 a.m., May 18, 1924. PC: USGS

2. Sudden collapse of broader caldera system and catastrophic failure of high caldera

Even less likely but more hazardous scenarios exist. Large explosive eruptions have occurred in Kīlauea’s past after caldera formation or during the last stage of its formation. It is possible that these eruptions were triggered by rapid collapse of broad regions of the caldera along caldera-bounding faults due to withdrawal of large quantities of magma from the summit storage system.

A more hazardous scenario: rapid collapse of broad regions of Kīlauea Caldera, and associated lava fountaining, dangerous explosions and pyroclastic surges.

Based on USGS’ understanding of the magmatic system, this activity should be preceded by significant changes in earthquake activity and ground deformation. At this time, satellite radar data show that high rates of deformation are concentrated in a well-defined area bordered by caldera-boundary faults on the west and south and on the east and northeast along a line roughly 600 to 900 yards from the caldera walls. These data do not suggest that extensive deformation is occurring outside of the caldera. Additionally, the USGS currently sees no evidence that major caldera-bounding faults are moving, although some cracks have been detected that probably result from ground shaking.

Additional hazards associated with rapid, broad-scale caldera collapse could include high lava fountains and larger and more dangerous explosions producing pyroclastic surges. However, the USGS emphasizes that current data do not suggest that a larger, sudden collapse scenario is likely at present.

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