KILAUEA IS A MOODY VOLCANO, report Hawaiian Volcano Observatory scientists is the current issue of Volcano Watch. The issue describes the effusive and explosive nature of Kilauea. “One mood is violent and explosive, the other relentless and effusive — almost a dual personality,” the article states. “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 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 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 that 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.” The full article and daily updates on Kilauea’s activity are available at hvo.wr.usgs.gov.
Posted on: Sun, 07 Dec 2014 00:15:28 +0000