Kīlauea was Jumping and Rocks were Flying
The following is this week’s edition of “Volcano Watch” from the Hawaiian Volcano Observatory (USGS):
Ninety years ago this month, Kīlauea was exploding. For some 20 days starting May 11, 1924, more than 50 explosive bursts, each lasting a few minutes or less, spread ash across the eastern third of the island, from Hilo to Makuʻu to beyond Pāhala. Blocks weighing up to 12 tons fell around Halemaʻumaʻu. Truman Taylor, a young accountant from Pāhala, was killed by a falling rock near today’s Halemaʻumaʻu parking lot, and others, including the national park superintendent, suffered minor injuries. It was indeed a month very different from any since!
The prelude to the eruption involved draining of a lava lake in Halemaʻumaʻu, we think down to below the water table, about 615 m (2,020 feet) below Jaggar Museum. That allowed water to pour into the emptied conduit that fed lava to the lake. The hot walls of the conduit heated the water to steam, which billowed up the conduit and out of Halemaʻumaʻu. Every few hours, on average, part of the conduit wall collapsed, temporarily damming the steam, and observers often noted a clearing of the air above Halemaʻumaʻu. For several minutes, steam pressure built up below the rock-fall dam, eventually bursting through it, propelling rocks and ash with it in an explosion. Once the dam was breached, steam pressure dropped and relative quiet ensued, only to be broken again by another rock fall.
Impressive though they were, the 1924 explosive events left little trace that remains today. Ash spread widely, disrupting life and even temporarily closing the Kapoho-Hilo railroad in Makuʻu, but it was thin and quickly removed by water and wind. On the rim of Kīlauea Caldera, one has to search very hard to find even tiny remnants of the ash. Large blocks and scattered ash still dot the caldera floor near Halemaʻumaʻu, but future lava flows will eventually cover them, erasing all evidence of the 1924 eruption.
In striking contrast, earlier explosive events, between 1500 and 1800, left thick deposits—as much as 11 m (35 ft)—all around the caldera. That means that the 1924 explosions were really very small, mere runts compared to their older siblings.
But size doesn’t necessarily matter. It is true that a large eruption has a wider “footprint” than a small eruption and the potential to affect society more. There is a very important distinction, however, between the size of an explosive eruption and its impact on people: no people, no impact. This reflects the difference between hazard—what the volcano can do, and risk—its impact on people. A very small eruption could present more risk than a large one, depending on where people are and the complexity of societal infrastructure.
In 1924, most people, except those few in the caldera, were affected in only small ways, because the population was low (relatively low risk). If a 1924-scale eruption were to occur today, the risk would be much higher, because of increased population. Such an eruption could occur with only a few days or week’s warning, if the lava lake in Halemaʻumaʻu were to drain down to the water table.
Given today’s situation, an eruption larger than in 1924 would present a still more serious problem. However, geologic studies show that past large explosive events were clustered into periods lasting several centuries. Since we are not now in such a period, the near-future chance of a large explosive eruption is probably small. But the future will eventually become the present, the volcano will enter a new explosive period, and the risk posed by a large explosive eruption will be high.