Volcano Watch — What to expect form explosions at Kīlauea

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The most recent small explosions took place at Pu`u `O`o in the early 1990s. The most recent lethal explosion was in 1924 from Halemaumau. The most fatalities from an explosion occurred in 1790, when between about 80 and 800 were killed; this tragedy culminated 300 years of recurring explosions from Kīlauea's caldera. The largest known explosion took place about 1,200 years ago, when rocks 2 cm (nearly 1 inch) across were thrown to Halape, 16 km (about 10 miles) from the caldera.

Except for the very largest explosions, steam pressure from heated groundwater apparently drives the explosions. To trigger such steam explosions, it is probably necessary to drop the floor of the caldera, or of one of the pit craters in the caldera, to a depth below that of the water table, about 490 m (1,600 feet) below the floor of the caldera.

We will certainly recognize such a collapse! We will see it, feel the earthquakes, and eat the dust from rockfalls.

There is absolutely no evidence that the volcano is building up to such a collapse. But as soon as it starts, we'll know it.

It is wise to consider the consequences of large explosions, because they will happen eventually. If you are prepared, the explosions need be only a nasty inconvenience; if unprepared, they could be worse.

What can nearby residents expect during a period of major explosions, such as that between 1500 and 1790?

Rocks may be flying through the air in residential areas. Volcanic ash, probably wet and heavy, will fall. In the worst case, there could be surges of hot, wet ash and rocks moving at hurricane velocities. Such surges would do the most damage and are the most likely cause of death.

During explosions, we'll feel many small earthquakes. Thunder and lightning, and very heavy rain mixed with ash, are common during steam explosions.

Just as with tsunami, don't expect only one event. Most likely, there will be repeated explosions over a period lasting days to weeks. The 1924 series of explosions lasted more than two weeks.

In nearby residential areas, breathing may be hard because of ash in the air. Roads will be slippery because of the wet ash. Hot stones in surges could set fires. Water catchment systems will be fouled. Power lines may collapse or short out in heavy ash fall. Roofs will collapse under the ash load. (In 1924, ash washed off the roof of Hirano Store in Glenwood into gutters that collapsed under the weight. This was upwind of the caldera.)

Tree limbs will be downed by heavy, wet ash and may block evacuation routes. If the caldera were collapsing, it would be prudent to trim limbs hanging over roadways. This was done in Rabaul, Papua Guinea, in 1984 in preparation for anticipated explosions that took place several years later.

Ash isn't snow. It doesn't melt. Extensive clean-up operations are needed once the explosions end.

Highway 11 will likely be closed by ash, downed limbs, and litter. Even roadways far from the summit, and upwind, could be affected. In 1924, the Hilo-Kapoho train had to stop in Maku`u because of slippery mud on the tracks.

Finally, a column of ash may rise into air-traffic lanes during very strong explosions. Ash and jet engines don't mix. Engines flame out. There have been several close calls with jumbo jets losing power in ash clouds, eventually restarting only after plunging thousands of feet to escape the ash.

The most powerful explosions may be triggered, not by steam, but by pressurized volcanic gas deep within the volcano. Such explosions may or may not have precursors. We simply don't know. Fortunately, they are rare on human scales, though one took place about 1,200 years ago. The consequences of such an explosion will resemble those given above, but on a magnified scale.

Kīlauea is a wonderful volcano to enjoy. Take care to know what to expect during the few bad times.

Volcano Activity Update

Eruptive activity of Kīlauea Volcano continued unabated and effusively at the Pu`u `O`o vent during the past week with lava being occasionally seen in the Pu`u `O`o crater. Bright glow persists over the "rootless" shield area, where short flows emanate from overflows of the perched ponds and from leaks at the base of the shields. Surface flows from breakouts of the tube system on Pulama pali and also on the fan at the base of the pali are frequently observed. Some of the flows extend onto the coastal flats. There are no ocean entries.

One earthquake was reported felt during the week ending on April 4. A resident of Hawaiian Ocean View Estates felt an earthquake at 52 minutes after midnight on April 1. The magnitude-3.0 earthquake was located 19 km (11.4 mi) northwest of Na`alehu at a depth of 9.1 km (5.5 mi).