VOLCANO WATCH: “What’s in a name?”




Media release | USGS, Hawaiian Volcano Observatory

“That which we call a rose by any other name would smell as sweet.” The Kilauea version of this Shakespeare quote could be “That which we call an explosive eruption by any other name would still be just as hot!”

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 pahoehoe or `a`a 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 Kilauea. 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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