Volcano Watch — "What's in a name?"

An important part of HVO's mission is to document the activities of Hawaiian volcanoes. In order to do that well, we sometimes need to name processes or structures in order to make it easier to describe other aspects of their activity. For example, we can save a lot of text by calling flows either pāhoehoe or ‘a‘ā instead of describing them each time.

The explosive expulsion of rocks and spatter from the Halema‘uma‘u vent since mid-March 2008, seemed to be an obvious explosive eruption at first. The intermittent glow and the discovery of small bits of spatter on the rim of Halema‘uma‘u Crater suggested the presence of lava deep down in the vent, but it wasn't actually seen until September of that year.

Between March and the end of 2008, there were several more explosive events, but it was also clear that a lava lake existed within the Halema‘uma‘u vent, making this a continuous eruption punctuated by explosive expulsion of spatter and chunks of the vent wall.

The products of these explosive events looked like those of other explosive eruptions—a mixture of rock dust and chunks and bits of fresh spatter in the form of Pele's tears and hair and a variety of other geometric forms. But what exactly was making the lava lake explode?

Establishment of several Webcams tracking developments in Halema‘uma‘u Crater allowed scientists to safely witness the events that preceded several explosive events. They found that many were triggered by vent wall collapses, that is, portions of the wall would collapse and rain down on the lava lake. This would initiate vigorous degassing—enough to eject gas, bits of lava from the lake, and fragments of the wall rocks that appeared to explode out of the narrow vent.

From the first explosive event, our colleagues at the Infrasound Laboratory of the University of Hawai‘i (ISLA) pointed out that atmospheric pressure variations caused by the Halema‘uma‘u "explosive" events did not match those from other explosive eruptions.

Truly explosive eruptions produce an excess atmospheric pressure that sometimes hurt observers' ears. For example, the explosive eruptions of Halema‘uma‘u in 1924 were immediately preceded by concussions painful to the ears. The 2008-2010 Halema‘uma‘u events produced an initial drop in atmospheric pressure, followed by a rapid increase—the increase being associated with the forceful ejection of solid rocks and spatter out of the vent. The drop in atmospheric pressure could be associated with the rocks hitting the lava lake, followed by the pressure increase as the gases explosively expanded. They differentiated these from those initiated by excess pressure by calling them "degassing bursts."

The recent Halema‘uma‘u explosive events are triggered in a different way than other explosive eruptions. Normally, volcanoes explode when the strength of the volcano is no longer capable of containing the internal magmatic pressure. When the volcano fails, gases dissolved in the magma are able to expand explosively ejecting debris to potentially great heights and great distances. Another way that an explosive eruption can be triggered is the introduction of water into a hot volcano's interior. Upon contact with molten magma, the water can explosively flash to steam as is thought to have driven the May 1924 explosive eruptions of Kīlauea. Neither of these two traditional processes are triggering the recent explosive events at Halema‘uma‘u.

HVO discoveries have highlighted a third way that explosive eruptions are triggered in lava lakes. Rock falls from vent walls must cause the release of a small amount of built-up gas pressure beneath the lava lake crust, resulting in small explosive releases. These explosive events will probably never be as large or as hazardous as those triggered in other ways.

So what's in a name? To paraphrase Shakespeare geologically-speaking, "That which we call an explosive eruption or a degassing burst would still produce the same rock deposits."

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Volcano Activity Update

At the end of last week, two new breakouts high on the TEB tube robbed some of the lava supply feeding breakouts farther downslope. As a result, the ocean entries and coastal plain breakouts west of Kalapana stalled. The highest of the new breakouts was from the lava tube just a short distance downslope from the TEB vent. These flows flowed south several hundred meters but were inactive by Wednesday, February 9. The other breakout was from the lava tube near the top of the pali, and, as of Thursday, February 10, lava is accumulating over the breakout point and making a new rootless shield. Breakouts from the tube also remain active on the pali upslope and to the west of Kalapana.

At Kīlauea's summit, the circulating lava lake in the collapse pit deep within the floor of Halema‘uma‘u Crater has been visible via Webcam throughout the past week. Sporadic, short-lived lava high stands have brought the lava surface to within about 95 m of the floor of Halema‘uma‘u. Volcanic gas emissions remain elevated, resulting in high concentrations of sulfur dioxide downwind.

Four earthquakes beneath the Hawaiian Islands were reported felt this past week. A magnitude-2.4 earthquake occurred at 10:03 p.m. HST on Tuesday, February 8, 2011, and was located 4 km (3 mi) southeast of Kīlauea summit at a depth of 3 km (2 mi). A magnitude-3.2 earthquake occurred at 1:08 p.m. on Wednesday, February 9, and was located 3 km (2 mi) south of Kīlauea summit at a depth of 3 km (2 mi). A magnitude-3.8 earthquake occurred at 8:30 a.m. and a magnitude-2.2 earthquake occurred at 8:37 a.m. on Thursday, February 10. Both were located 5 km (3 mi) southeast of Kīlauea summit at a depth of 4 km (2 mi).