Volcano Watch — Volcanic gases provide clues to how volcanoes work

In a double handful of molten magma (weighing about a pound), there is less than a tenth of an ounce, by weight, of dissolved gas—roughly the same weight as a pinch of table salt. Yet this tiny amount of gas can drive spectacular lava fountains hundreds of feet into the air. The fountaining occurs because dissolved volcanic gases expand when pressure on the magma is released. Anyone who has shaken a bottle of soda and opened it quickly has gotten the full value of this basic principle of physics.

The Volcano Watch column for this week will be on volcanic gases. What gases come out of volcanoes like Kīlauea and Mauna Loa, where the gases come out from, and what happens to them afterwards.

Gases are trapped (dissolved) in magma at depth, where pressures within the Earth's crust are very great—many thousands of pounds per square inch. As the magma rises to the surface and is erupted, the pressure decreases, and some of the gas is released. The main gases dissolved in magma are water vapor, carbon dioxide, and sulfur gases, with lesser amounts of hydrogen, hydrochloric acid, and hydrofluoric acid. In our pinch-of-salt-to-a-double-handful-of-magma illustration earlier, most of the "pinch" is water vapor, followed by lesser amounts of carbon dioxide and sulfur gases with a few "grains" of hydrogen and the other acid gases. At HVO, we study gases emitted from Kīlauea and Mauna Loa in order to assess the state of eruptive activity, and to help improve our understanding of how volcanoes work.

At the summit of Kīlauea, most of the gas released—aside from water vapor—is carbon dioxide (CO₂), the same gas found dissolved in soda (and beer). Kīlauea emits about 1,300,000 tons of carbon dioxide each year, an amount that seems large, but is actually about 1/17,000 th of the man-made CO₂ generated each year, and contributes very little to the greenhouse effect.

Sulfur dioxide (SO₂) and lesser amounts of hydrogen sulfide (H₂S) are the most abundant sulfur gases emitted in volcanically active areas that are very hot, or where molten magma is close to the surface. These areas include the summit of Kīlauea, the eruptive sites on and near Pu'u 'O'o, and the lava-tube system going down to where lava enters the ocean. In geothermal areas, such as Pohoiki, however, hydrogen sulfide is essentially the only sulfur gas emitted. Hydrogen sulfide, sometimes called "sewer gas", is a toxic gas that irritates the eyes, nose, and throat and has a rotten egg odor. It is an insidious poison, because although our noses can initially detect H₂S at very low concentrations, under prolonged exposure to higher levels of the gas, our sense of smell may become fatigued. Because of this fatigue, we may no longer be able to detect H₂S but could still suffer from its effects. Furthermore, our ability to distinguish between moderate and very high concentrations of H₂S is limited. Sulfur dioxide, in contrast, is the biting, choking gas that you smell right after you've lit a kitchen match, or the sharp odor you sense at the Halema'uma'u overlook. Sulfur dioxide is also toxic but is typically so irritating to the nose that it provides its own warning when concentrations reach toxic levels.

An interesting chemical relationship exists between the sulfur dioxide and the hydrogen sulfide released by the volcano. These two gases react quickly (within minutes) with each other to produce sulfur particles and water vapor. Both of the products of this reaction are odorless and are less toxic than either H₂S or SO₂. Most of the hydrogen sulfide released in eruptive areas on Kīlauea is consumed and is converted to sulfur particles by this process, because there is much more sulfur dioxide than hydrogen sulfide coming out of the volcano. This is why you seldom smell hydrogen sulfide at the summit caldera or along the eruptive east rift. The volcano has its own hydrogen sulfide abatement system! Geothermal areas, by contrast, have no large quantities of SO₂available for reaction, so any H₂S released is removed by reaction with oxygen in the air to form sulfur dioxide, a process that takes a day or more.

Scientists at HVO measure the amount of sulfur dioxide released from Kīlauea using a correlation spectrometer (COSPEC). The COSPEC is mounted in a vehicle that is driven beneath the volcanic gas plume. The COSPEC "looks" up through the plume and measures the SO₂ as it is blown across the road and away from the volcanic vent. By recording how much sulfur dioxide the COSPEC sees and by knowing the speed of the wind blowing the gas plume, we compute the amount of SO₂ emitted from the volcano. Before the current east rift eruption began 10 years ago, Kīlauea released about 150 tons of SO₂ each day, nearly all of which was emitted from the summit caldera. Sulfur dioxide emissions were as high as 30,000 tons per day during the high fountaining episodes at Pu'u 'O'o. These episodes occurred every three to four weeks from 1983 to 1986 and typically lasted 24 hours or less. Starting from 1986 when the Kupaianaha lava pond formed, Kīlauea released as much as 1,800 tons of SO₂ each day, between 150 and 300 tons of which were from the summit and the balance from the east rift eruptive sites. This sulfur dioxide emission rate has continued at roughly the same or a slightly lower level ever since. During eruptive pauses emission rates are much lower. By comparison, Pinatubo Volcano in the Philippines released 19,000,000 tons of SO₂ during its catastrophic eruption on June 15, 1991.

Sulfur dioxide is released at the summit, at Pu'u 'O'o, at the 51-53 vents, and at the skylights along the tube system. The sulfur dioxide reacts chemically with oxygen, dust particles, sunlight, and water in the air to form a mixture of sulfate-aerosols (tiny particles and droplets), sulfuric acid, and other oxidized sulfur species known as "vog." What happens to the vog is determined primarily by wind direction and intensity. When moderate to strong northeasterly trade winds blow, the vog is typically carried to the southwest towards South Point, where the wind patterns wrap around the island, sending the vog up the Kona coast. During Kona winds, or in the absence of strong trade wind conditions, however, vog stays on the east side of the island, affecting people, animals and plants, sometimes from Ka Lae to Hilo.

Gas release of another form occurs where lava enters the ocean. Again, a chemical reaction occurs, this time between molten lava and sea water. The reaction is vigorous because of the intense heat, generating a large, white plume cloud containing a mixture of hydrochloric acid and sea water, commonly called laze. During along-shore or on-shore winds, this laze plume can drop rainwater on people along the coast. This rain is often more acidic than lemon juice, with a pH of 1.5 to 2.5, but is more corrosive than lemon juice to skin and clothing. Hydrochloric acid is toxic and causes irritation to the throat, lungs, eyes, and nose. Visitors should heed park warning signs and avoid standing directly in or under the coastal-entry plume.