2008 will be known as the year that Kilauea Volcano had a mid-life crisis. After 25 years of almost continuous, effusive eruption from its east rift zone, an incandescent gas vent mysteriously appeared in Halema`uma`u Crater sometime on March 11 or 12, and exploded early on the morning of March 19, 2008.

Now that it happened, we are looking back to see whether we missed any precursors to the explosion. We're also trying to piece together exactly what happened. Here's what we know so far:

The event started at 2:55 a.m. with a few seismic events that were either small earthquakes, very small explosions, rockfalls, or a combination of all three. At 2:56 a.m., the steady incandescence of rocky rubble covering the top of the gas vent started to founder. At 2:58 a.m., a second series of even larger explosive earthquakes began.

At 2:59 a.m., the newly opened vent started to vibrate strongly at a frequency too low to hear but recorded by instruments operated by our colleagues at the Infrasound Laboratory of the University of Hawai`i at Manoa. Their sensors detect infrasound, or acoustic energy, that travels through the air. Seismic energy traveling through the ground was also recorded by our seismometer network as increased tremor.

At 3:02 a.m., HVO's automatic earthquake-location software concluded that the combination of energy from the earthquakes and explosions was a magnitude-3.7 earthquake in the upper east rift zone and posted that to our Web site. Several hours later, when an analyst had a chance to look at the signals, he realized that the vibrations were explosions, not earthquakes, and our Web site was updated.

At first light on March 19, scientists rushed to the rim of Halema`uma`u and found the nearby area scattered with angular rock debris that was coarser and thicker closer to the vent. Not surprisingly, the rock debris lay directly beneath the gas plume from the new vent. And that's not all! Some folks in Pahala and Wood Valley found their cars covered with a light coating of fine white grit 30 km (19 miles) downwind probably courtesy of this early-morning Halema`uma`u explosion.

Debris blanketing the area was composed of rock fragments blasted from Halema`uma`u Crater, probably from the rubble pile covering the gas vent. Many of the rock fragments were too hot to touch more than 5 hours after the explosion. But there was no lava. This was an explosive eruption that blasted out old rocks.

The largest rock observed on the rim of Halema`uma`u above the new gas vent was about 90 cm (2.6 ft) in average dimension. Assuming typical rock densities, that rock weighed about 2,000 kg (2.7 tons), and the explosion was powerful enough to propel it more than 72 m (230 feet) into the air!

We are still gathering details and trying to deduce the exact nature of the explosion. One explanation that HVO scientists are discussing is a "throat clearing" hypothesis. In this scenario, hot gas coming up along the east wall of Halema`uma`u Crater progressively undermined the overlying rubble and created an open conduit underneath. Early Wednesday morning, the surface rubble layer collapsed into the underlying open conduit and temporarily blocked gas from escaping. Pressure built up and, two minutes later, reached a level that blasted the rubble from the gas conduit.

Science is a process of asking questions, and the above hypothesis has raised many. The biggest question is whether pressure can build to explosive levels in only two minutes. Another is whether an explosion could happen at all without involving water that explosively flashes into steam as it did during the last series of explosive Kilauea eruptions in 1924.

Once we feel confident that we understand what happened on March 19, we will be able to assess whether future explosive eruptions are likely. Right now, we cannot rule them out.

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