Categorized | Sci-Tech

Volcano Watch: Now you’re monitoring with gas!


An HVO scientist collects a volcanic gas sample from a fumarole on the rim of Halema‘uma‘u Crater within Kilauea caldera. USGS photo

An HVO scientist collects a volcanic gas sample from a fumarole on the rim of Halema‘uma‘u Crater within Kilauea caldera. USGS photo

As part of Volcano Awareness Month (January 2011), the Hawaiian Volcano Observatory is devoting this month’s Volcano Watch column to a discussion of different types of volcano monitoring methods. In the past two weeks, we covered geophysical and geologic methods. Today, we will focus on gas geochemistry.

Everyone in Hawai`i has first-hand experience with volcanic gases. The vog (volcanic air pollution) that is a byproduct of gas emissions from Kilauea Volcano is a part of daily life, not just on Hawai`i Island, but across the state.

Similar to a bottle of carbonated soda with the lid screwed on, magma inside Kilauea contains dissolved gas—in fact, a mixture of gases composed primarily of carbon dioxide (CO2), water (H2O), and sulfur dioxide (SO2). As magma rises to the surface, the pressure on the magma decreases. This pressure drop allows the dissolved gases to exsolve, or bubble out, from the magma the same way that gas bubbles out of soda when the lid is loosened.

Volcanic eruptions are driven by exsolving gas bubbles. These bubbles rise, pushing magma upwards towards the surface. The different gases in magma exsolve at different pressures. CO2 exsolves at the greatest pressures and depths, perhaps 30 km (20 mi) beneath the surface. H2O and SO2, however, form bubbles at much shallower depths—less than about 1 km (0.6 mi) beneath the surface. Measuring the abundance of different gases helps volcanologists formulate a picture of the physical conditions of a volcano’s magma plumbing system.

But how do we study gas emissions? One method is to go to a volcanic gas vent, called a fumarole, and collect a gas sample in a sealed bottle, then return to the laboratory and analyze the composition of the sample. This method, which provides a chemical fingerprint of the gases, has been used for over 100 years, but field conditions can be hazardous to humans, and this technique requires a lot of time and sophisticated laboratory equipment to carry out thoroughly.

SO2 is one of the easiest gases to detect, partly because, unlike H2O and CO2, there is very little naturally occurring SO2 in the Earth’s atmosphere. Since the 1970s, SO2 has been measured using the COSPEC and, more recently, FLYSPEC instruments that detect the amount of sunlight absorbed by SO2 in a gas plume. By driving (or flying) back and forth beneath a gas plume and measuring the decrease in ultraviolet energy from the Sun, the amount of SO2 in the plume can be calculated. These measurements have been done at Kilauea since 1975—the longest record for any volcano in the world!

Newer spectrometers allow volcanologists to measure other important gases based on absorption of infrared light. In this case, in order to detect gas compositions, the instrument only needs a heat source, such as a heat lamp or even a lava flow. Since 2008, HVO gas geochemists have used this instrument to measure the gases that make up Kilauea’s summit plume. The resulting data have been useful in identifying potential health hazards, as well as the quantity of magma that is circulating within the summit vent.

Sulfur dioxide can also be detected via satellite. This method is particularly useful for measuring the amount of sulfur emitted from volcanoes, even in very remote locations on Earth, and also for tracking large volcanic plumes that travel long distances, like that of the 2010 Icelandic eruption.

Next week, in the fourth part of our series on volcanoes monitoring methods, we will center our attention on seismology.

Meanwhile, check out the Volcano Awareness Month activities that HVO has scheduled for this week: an update on Kilauea’s east rift zone eruption at the Hawai`i Volcanoes National Park Visitor Center on January 18; a talk about the current status and history of Mauna Loa at the Pu`uhonua o Honaunau National Historical Park amphitheater on January 19; and a talk about how HVO monitors Hawaiian volcanoes at the University of Hawai`i at Hilo on January 21. Details about these and other Volcano Awareness Month activities are available at or by calling 808-967-8844.

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