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“Kīlauea is a Moody Volcano”

November 28, 2014

Small pāhoehoe toes and surface flows on November 23. (USGS)

The following is this week’s edition of “Volcano Watch” from the USGS Hawaiian Volcano Observatory:

Trekkers to the summit were used to seeing explosions, but nothing like this. Within minutes, several hundred perished as a cloud of hot ash, fist-size rocks, and steam swept across them at hurricane velocity. Some were scorched to death and others suffocated, right along the trail they thought was safe. It was by far the worst tragedy known to have occurred at Volcano 1.

At Volcano 2, lava flows moved slowly through the forest and village, advancing here, stopping there, eventually destroying everything in their path. They presented little threat to human life, but disruption of society was high and long lasting. The on-off nature of the lava advance was inevitable for such a lava flow and nerve-racking for the villagers.

What volcanoes are these? Kīlauea and Kīlauea, one and the same volcano, acting in two very different moods. One mood is violent and explosive, the other relentless and effusive—almost a dual personality. The explosive mood caused the fatalities in 1790, and the effusive mood produced lava flows that disrupted society in the 15th, 19th, 20th, and now the 21st centuries.

An explosive eruption of Kilauea in 1924. Men in foreground are John Stokes of the Bishop Museum on left and Harold Palmer of the University of Hawai`i on right (USGS).

How can Kīlauea experience such different moods? Its behavior has to do with the presence of a deep depression—a caldera—at the summit. Visitors today peer into the caldera, not realizing its importance in determining whether Kīlauea erupts explosively or effusively. They can be excused, because the caldera today is only a shadow of its former self. At a depth of 120 m (400 ft), it is only 20 percent or less of its past depth of at least 600 m (2000 ft).

At its deepest, the floor of Kīlauea Caldera intersects the water table, and groundwater can interact with hot magma and rocks forming the walls of the magma conduit. Water can’t get in when the conduit is filled with magma. But when the conduit has emptied for some reason, water can flow into it, flash to steam, build pressure if the conduit becomes temporarily plugged by wall collapse, and finally explode. Explosions can also occur if magma erupts through a lake on the floor of the caldera.

In Kīlauea’s past 2,500 years, such a deep caldera has apparently existed about 60 percent of the time, leading to two long periods of sporadic violent explosions. One lasted 1,200 years, ending in about 1000 CE; the other lasted 300 years between 1500 and 1800 CE. Tragedies such as that in 1790 can occur during these periods.

Studies show that a deep caldera forms and persists when the magma supply rate to the volcano drops to only a few percent of its value during times of effusive activity. Frequent explosions are the ultimate result. Eventually magma supply picks up, the caldera fills, and effusive eruptions dominate, as they have for 40 percent of the past 2,500 years, most recently for the past 200 years.

Both of Kīlauea’s moods have good and bad sides. In the explosive mood, the explosions take place in the caldera and can threaten life within a radius of several kilometers. The hazard is severe but local to the summit. However, falling ash can be disagreeable many kilometers away, and airborne ash from future large explosive eruptions will threaten air traffic. Such explosive periods last decades to centuries and could create economic hardship around the summit and beyond. On the other hand, ash falls contribute to soil fertility. Former settlements on Kīlauea’s barren south coast were made possible by pockets of fertile ash.

In the effusive mood, the eruptions take place in the summit region and along the two rift zones. Potentially, lava flows can destroy or damage communities anywhere on the volcano. With few exceptions (for example, methane explosions and lava-delta collapses), the flows are not life-threatening but can be life-changing. Kīlauea is in an effusive mood now, and people know that it can be unpleasant.

Active volcanoes are inherently unstable, uncertain places to live. Whether in an explosive or effusive mood, Kīlauea will always present hazards to the populace. We should be realistic, but not fatalistic, about the situation. We can’t command Kīlauea, but we can control our actions in the face of its hazards.

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