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May 25, 2000

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.


Dissolved gases improve our understanding of eruptive processes

"Why should I believe something I can't see, especially since I already don't believe so many things that I can see?" This verbal barb is sometimes used by volcanologists during good-natured, "tongue-fu" exchanges, as when arguing about which discipline - say, geology, seismology, geophysics -- is "best" for determining what the volcano has done or might do next.

Those of us who study the composition of gases as they bubble out of magma, a field called gas geochemistry, have certainly received our share of the above remark. After all, the gases we study are mostly invisible and, in the case of Kilauea, account for less than one half of one percent of the weight of the lava that comes out. What can be important about something so seemingly insignificant? Anyone who has shaken and then rapidly opened a bottle of champagne knows the answer to the question. The answer is: "Big things sometimes come in small packages."

Recently, we completed a detailed study of Kilauea's sulfur dioxide (SO2) emissions, and we're excited about the results. But before we get to the results, a little background is in order. Gases are dissolved in magma deep within the volcano, where pressure is great. This pressure decreases as magma approaches the surface, allowing some of the gas to escape.

Of the several gases emitted by volcanoes, sulfur dioxide is the one with the biting, choking odor, like what you smell when you've just lit a kitchen match. It's also the principle gas responsible for the formation of volcanic smog (vog). Since 1986, when the volcano went into a state of nearly continuous eruption, Kilauea has released more than twice as much SO2 as the most notorious polluter on the EPA's "top 100" list. Kilauea contributes about 5 percent of the SO2 emitted by the 600 or so active volcanoes worldwide, but globally, humans generate nearly 10 times the amount of SO2 released by volcanoes.

Kilauea's SO2 emissions have been monitored longer and more regularly than at any other volcano on Earth. Among the fruits of this long-term monitoring, which began in 1979, is our improved appreciation for how closely SO2 release is tied to magma movement and especially to eruption rate. Kilauea was in a fitful eruptive state from 1979 through the end of 1982, a period distinguished by three very short eruptions, along with numerous intrusions of magma close to, but not breaking, the surface. Sulfur dioxide release during this time was variable as well, but it jumped tenfold when the current eruption began in 1983.

The reason for the distinct increase in SO2 emissions is simple, yet has useful implications. When one kilogram (2.2 pounds) of lava erupts, 0.002 kilograms (0.06 ounces) of dissolved SO2 gas bubbles out. If more lava erupts, proportionately more gas is released. Thus, the inference is that the more gas is released at the eruption site, the more lava is being erupted.

One of our most pressing questions related to monitoring the eruption is "How much lava is being erupted?" We have historically answered this question of eruption rate using a couple of techniques. One way employs field mapping of the coverage and thickness of these flows as they are emplaced. The other involves using geoelectrical measurements to determine the amount of lava flowing through lava tubes near the surface.

The relationship between dissolved gas and lava is now providing a third technique for measuring eruption rate. Results using the dissolved gas technique are consistent with both the geoelectrical and field mapping methods. Unfortunately, each of these several methods has limitations. Field mapping is difficult (and wet!) if you're trying to map lava which has gone directly into the sea. Geoelectrical measurements can be compromised if lava tubes or channels are poorly developed. Gas emission measurements can be spoiled, literally, if the wind blows gases the wrong way. By melding results from the several techniques together the eruption is better understood. Remember that, for gases, though, "Believing is not necessarily seeing!"

Eruption Update

Eruptive activity of Kilauea Volcano continued unabated during the past week. Lava is erupting from Pu`u `O`o and flowing through a network of tubes toward the coast near the eastern boundary of Hawai`i Volcanoes National Park. A large skylight of the main tube is visible on Pulama pali. The tube system is now well-established and fully developed. Surface flows from breakouts of the tube system are rarely seen in the coastal flats these days. Lava is entering the ocean primarily at Waha`ula and in lesser amounts at two other sites. The public is reminded that the ocean-entry areas are extremely hazardous, with explosions accompanying sudden collapses of the new land. The active lava flows are hot and have places with very thin crust. The steam clouds are highly acidic and laced with glass particles.

No earthquakes were reported felt during the week ending on May 25.

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Updated: 30 May 2000