Why didn't draining of lava from Halema‘uma‘u cause explosions?

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The rapid draining of lava from the pit in Halema‘uma‘u on March 5 occasioned worst-case concerns of explosive eruptions at Kīlauea's summit. Thank goodness these concerns didn't materialize. Still, it is worth describing how these seemingly disparate processes could be related and why the explosive scenario didn't become a reality.

The concerns were based on the events of February–May 1924. On February 21, the long-lived lava lake in Halema‘uma‘u disappeared. Earthquakes began along the east rift zone, finally settling in the Kapoho area, where an intrusion, fed by magma that moved through the east rift zone from Kīlauea's summit reservoir, cracked the ground in April.

In late April, the intrusion ended, but the caldera immediately began shaking. Surrounding the drained lake, the floor of Halema‘uma‘u started to collapse until it dropped out of sight. From the collapsed crater, an explosive eruption on May 10–11 started 17 days of explosive activity that killed one person and doubled the width of Halema‘uma‘u.

Geologists think that the explosive eruptions resulted as ground water entered the hot conduit emptied by the draining lava. The water was heated to steam but couldn't escape passively, because rock-fall debris formed a pressure seal. Steam pressure had to build before it could explode through the seal. Unknown then was how deep the ground water was.

For more information about these dramatic events, see three articles on HVO's Web site: http://hvo.wr.usgs.gov/volcanowatch/1999/99_04_01.html, http://hvo.wr.usgs.gov/volcanowatch/1999/99_05_06.html, and http://hvo.wr.usgs.gov/archive/2001_05_18.html

Now to March 5, 2011. Lava drained rapidly from the pit along the southeastern edge of Halema‘uma‘u. An intrusion (and eruption) took place on the middle east rift zone between Nāpau and Pu‘u ‘Ō‘ō. Rock falls occurred from the walls of the emptying pit. In these ways—though faster and on a smaller scale—the events of 2011 resembled those of 1924.

Research since 1924 shows that the water table, below which rocks are saturated with water, is about 500 m (1,600 feet) under the present caldera floor. If lava were to drain below that depth, models suggest that ground water would enter the conduit and potentially trigger steam-driven explosions.

The depth from the caldera floor to the point where lava disappeared on March 9–10 was about 305 m (1,000 feet), 60 percent of the way to the water table. We don't think the lava dropped much farther, though, because sounds of gas escaping the lava could be heard from the rim of Halema‘uma‘u, even when lava couldn't be seen. On March 14, lava reappeared in the pit, accompanying summit inflation and ending the draining.

Aside from the lack of explosive eruptions this time, how did the 1924 and 2011 draining episodes differ? Perhaps the most important difference was that the crater's floor collapsed in 1924, whereas only liquid drained in 2011, with relatively small rock falls from the walls of the pit and no floor collapse. In fact, a nearly flat platform, once drowned by lava, reappeared in the pit during the March draining and did not drop down in the next several days. Aside from the expected rock falls from the vertical and overhanging walls, the rest of the pit maintained its integrity pretty well.

These differences probably relate to scale. In 1924, a much larger volume of magma left the summit reservoir, and wholesale collapse was the outcome. Summit deflation in 2011 and the volume of lava that drained from the pit were small, by comparison, and no collapse took place.

Ironically, the narrow feeding conduit exposed by the 2011 draining probably increased the chance of another kind of explosion—one driven by gas escaping from the lava but trapped beneath rock-fall debris. Such debris could have choked the conduit, forcing gas to pressurize before breaking through. It didn't happen this time, but something similar took place on March 19, 2008, when the first explosion since 1924 ushered in the ongoing Halema‘uma‘u eruption.

We are living in an uncertain time at Kīlauea, but not a very explosive one. To hear about what one of those is like, come to the After Dark in the Park lecture on April 5 concerning the explosive eruption of 1790, the largest in the past 1,000 years.


Volcano Activity Update

The Kamoamoa fissure eruption, originating from vents just west of Pu‘u ‘Ō‘ō cone, ended on the evening of Wednesday, March 9, and no lava has been erupted on the east rift zone since that time. Pu‘u ‘Ō‘ō, which collapsed during the fissure eruption, remains filled with rubble, and lava flows have ceased on the Thanksgiving Eve Breakout (TEB) flow field.

The lava lake in Halema‘uma‘u crater drained during the Kamoamoa eruption, leaving a small amount of lava at a depth of 220 m (722 ft) by Monday, March 7. By Thursday, March 10, the lava surface at the bottom of the vent cavity was buried in rubble, and the nightly vent glow diminished. By Monday, March 14, however, lava reappeared deep in the vent cavity, and as of Thursday, March 17, a mostly crusted lava surface remains very deep. Along with the current low levels of activity, the volcanic gas emissions have decreased over the past week but remain elevated, resulting in high concentrations of sulfur dioxide downwind.

Ten earthquakes beneath Hawai‘i Island were reported felt this past week. The earthquakes, which all occurred between 8:18 pm on Thursday, March 10, and 2:04 pm on March 11, 2011 HST, were located in the vicinity of Kalapana with magnitudes between 1.9 and 4.6, and occurred at depths of 8–10 km (5–6 mi).