(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

This thermal image was taken at a nearly vertical angle from a helicopter, in order to see the bottom of the extremely deep and narrow vent cavity. Prior to the drop in lava level, the lava lake was near its high lava mark, shown by the hot ring on the upper vent cavity walls. The lava level dropped considerably over the past several days, retreating to a narrow opening deep within the vent cavity.

The rapid draining of lava from the pit in Halemaumau on March 5 occasioned worst-case concerns of explosive eruptions at Kilauea’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 Feb. 21, the long-lived lava lake in Halemaumau 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 the Kilauea’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 Halemaumau 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 Halemaumau.

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 website:

• http://hvo.wr.usgs.gov/volcanowatch/1999/99_04_01.html
• http://hvo.wr.usgs.gov/volcanowatch/1999/99_05_06.html
• 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 Halemaumau. An intrusion (and eruption) took place on the middle east rift zone between Napau and Puu Oo. 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 Halemaumau, 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 Halemaumau eruption.

We are living in an uncertain time at Kilauea, 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.