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? Kilauea and Kilauea, 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.
How can Kilauea 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 Kilauea erupts explosively or effusively. They can be excused, because the caldera today is only a shadow of its former self. At a depth of 400 feet, it is only 20 percent or less of its past depth of at least 2,000 feet.
At its deepest, the floor of Kilauea 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 Kilauea’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 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 Kilauea’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 Kilauea’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. Kilauea is in an effusive mood now, and people know it can be unpleasant.
Active volcanoes are inherently unstable, uncertain places to live. Whether in an explosive or effusive mood, Kilauea will always present hazards to the populace. We should be realistic, but not fatalistic, about the situation. We can’t command Kilauea, but we can control our actions in the face of its hazards.
Kilauea activity update
The lava flow from Puu Oo that began on June 27 remains active. Lava continues to advance toward the northeast along the western edge of the older flow field within the Wao Kele O Puna Forest Reserve. As of Wednesday, the leading edge of the flow was 3.3 miles upslope from the Pahoa Transfer Station on Apaa Street. There was no significant change in activity at Puu Oo.
The level of the summit lava lake within Halemaumau Crater fell during the week, in concert with gradual deflation at Kilauea’s summit. As of Wednesday morning, the lava lake was about 170 feet below the rim of the Overlook crater.
Two earthquakes were reported felt on Hawaii Island. On Nov. 22 at 7:58 a.m., a magnitude-3.3 earthquake occurred 19 miles northwest of Kailua-Kona at a depth of 4 miles. On Nov. 23 at 7:35 a.m., a magnitude-3.3 earthquake occurred 1 mile west of Volcano at a depth of 1 mile.
Visit the HVO website at hvo.wr.usgs.gov for activity updates and more; call 967-8862 for a Kilauea summary; email questions to askHVO@usgs.gov.