Volcano Watch — A sensory tour of Kīlauea's sulfur gases

Gravy and Hostess cupcakes—both delicious in their own way, but not necessarily in combination. Although they are extremely different from each other—each with a unique flavor, texture and aroma, each providing a unique culinary and sensory experience—they share a fundamental ingredient. This ingredient, flour, is usually white, light, and powdery. It is characteristically different than either of the final products to which it contributes, yet this substance is essential for the creation of these two varied comestibles.

In a similar way, certain substances released from Kīlauea, although containing a common essential ingredient, have very different structures, characteristics, and aromas. At the summit of Kīlauea, many sensory experiences greet the visitor. One that is hard to ignore is the aroma of volcanic gases. Two gases that tend to define a visitor's nasal experience, and that geochemists use to define conditions within the volcano, are sulfur dioxide and hydrogen sulfide.

Hydrogen sulfide (H₂S) is the gas that smells like rotten eggs. Sometimes if one eats too much gravy and cupcakes (especially in combination), a rebellious digestive system will create some of this gas, and the hapless individual will taste and smell hydrogen sulfide as it is released into the atmosphere. Visitors and geochemists alike can sample this gas at the Sulfur Banks area adjacent to the Visitor Center in Hawaii Volcanoes National Park.

Sulfur dioxide (SO₂), on the other hand, is the biting aroma you smell when you light a kitchen match, or set off lots of fireworks on a still New Year's eve. Currently, it might be known as Kīlauea's signature gas. According to a report by the EPA, which ranks sources of sulfur dioxide emissions, Kīlauea is the largest stationary source of this gas in the nation. When you stand at the rim of Halema`uma`u, it is sulfur dioxide that gives you the tickling, choking sensation in the back of your throat. In east Hawai`i it is also the component that lends the characteristic smell to vog.

Kīlauea's hydrogen sulfide and sulfur dioxide gases, though extremely different from each other, are formed from a common source. Their common ingredient, sulfur, is dissolved in the magma deep in the volcano—much like the gas bubbles in soda or beer. In the case of soda, the closed chamber of the can keep the liquid soda pressurized so that the gases inside can stay dissolved. When the can is opened, the pressure is released, and the dissolved gas forms bubbles and can escape.

Likewise, as the magma works its way to the surface of the volcano, pressure caused by the weight of the heavy column of material atop it decreases, and dissolved sulfur can form bubbles and escape from the magma. This begins to happen at depths where pressures are about 20 times greater than at the earth's surface.

The dominant sulfur product released at the ground's surface is controlled by a complex combination of environmental conditions both within and at the exit point of the volcano. These include pressure and temperature conditions, the type of rock surrounding the system, and the chemical reactions that occur among sulfur-containing substances. The main influence in determining which sulfur species we predominantly experience is the abundance of ground water in the volcano. Because sulfur dioxide dissolves easily in water, a dry pathway to the surface must exist before appreciable amounts of sulfur dioxide can make it to the light of day. Observatory staff witness this phenomenon at the summit of Kīlauea, when we see a decrease in sulfur dioxide gas emissions immediately after a ground-saturating rainstorm.

Because Kīlauea usually has relatively dry gas pathways, an abundance of sulfur dioxide is almost always released. In special areas like Sulphur Banks or the lower east rift zone geothermal area, generous amounts of ground water effectively scrub the sulfur dioxide, and hydrogen sulfide is the dominant sulfur gas. One thing for sure is that whichever gas pathway you choose, sulfur gases are part of the complete volcano experience.

Volcano Activity Update

Eruptive activity of Kīlauea Volcano continued unabated during the past week. Lava is erupting from Pu`u `O`o and flowing through a network of tubes toward the sea coast. One flow enters the ocean at Lae`apuki and is forming a large bench. The public is reminded that the ocean-entry area is extremely hazardous, with explosions accompanying unpredictable collapses of the new land. The active lava flow is hot and has places with very thin crust. The steam clouds are highly acidic and laced with glass particles.

Another flow is located near the eastern boundary of the National Park and has cut the Royal Gardens access road in two places. That flow has not advanced seaward since last week and is still about 800 meters (0.5 mile) from the coast. A higher section of the flow is burning trees on Pulama pali.

A resident of Leilani Estates subdivision in lower Puna felt an earthquake at 6:28 p.m. on Saturday, January 22. The magnitude-3.0 temblor was located 3.5 km (2.1 mi) southeast of Pu`ulena crater at a depth of 1.7 km (1.0 mi).