Volcano Watch: Infrared (IR) measurements help HVO scientists unravel Kīlauea gas chemistry

Volcano Watch is a weekly article and activity update written by scientists and affiliates of the US Geological Survey Hawaiian Volcano Observatory.

Most people in Hawaii are familiar with sulfur dioxide (SO2), the main component of vog. But, have you ever found yourself wanting to know the SO2 / HCl (sulfur dioxide / hydrogen chloride) ratio in volcanic gas? Or the amount of CO2 (carbon dioxide) dissolved in volcanic glass?

If you’re not a volcanologist or a geologist, you probably aren’t. But at the USGS Hawaiian Volcano Observatory (HVO), we need to know these things. Magma contains dissolved, or volatile, gases that cause eruptions. And although gases in air and volatiles dissolved in molten magma (preserved as glass and glass inclusions in minerals) do not seem to be measured in the same way, we can use the same principles. and techniques to measure both and help us understand eruptive activity.

To perform each of these types of volatile measurements, we use an instrument called a Fourier Transform Infrared Spectrometer, or FTIR. FTIR instruments detect incoming infrared (IR) radiation, the type of radiation associated with hot / hot objects, having wavelengths slightly longer than visible light we can see with our eyes.

An HVO scientist aims the “FTIR field” to splash lava from the current eruption in Halema’uma’u crater, atop KÄ«lauea, October 4, 2021. USGS photo by P. Nadeau.

But how do you know which gases are present?


It turns out that gases absorb radiation and each gas, like CO2, HCl, SO2, H2O (water vapor) and others, has its own unique signature as to how much it is. absorbs at different wavelengths.


A slightly different, but common, example of an absorbent gas that many people are familiar with is ozone (O3). Ozone in the atmosphere protects us from some of the sun’s harmful ultraviolet (UV) rays (wavelengths shorter than visible light) by absorbing UV wavelengths very strongly. SO2 also absorbs strongly in the UV range; HVO uses UV to measure SO2 emission rates.

However, many large volcanic volatiles do not absorb UV very well, but they absorb strongly in the IR range. So we turn to the FTIR to measure them. Or, more precisely, one of our two FTIRs! Yes, HVO has two different types of FTIR, which we use for different applications.

To measure the gases in the air, we take our “field FTIR” and head to where the volcanic plume is. We need an IR power source and luckily lava is a great source of IR because it is very hot! So when we have an eruption, we can aim the FTIR on hot, glowing lava. If there is no lava around, we can still measure the gas by directing the FTIR at a special lamp that generates IR.


Once we have the IR source, we need to position the FTIR so that the volcanic gas is between the IR source and the FTIR. The FTIR measures the relative amounts of IR radiation at different wavelengths, some of which is absorbed by volcanic gases. We then process the data and calculate important gas ratios, like CO2 / SO2 and SO2 / HCl, which can give us information about how magma and volatiles are transported through the volcanic plumbing system.

But what about other FTIRs?

This would be our “laboratory FTIR”, which does not venture near the volcanic plume like the “field FTIR” does. Instead, the “FTIR lab” is used to measure small amounts of H2O and CO2 dissolved in minerals and volcanic glass.

The principles are the same: the FTIR laboratory has an IR source and a radiation detector that measures IR intensity at many different wavelengths. But instead of a volcanic plume passing between them, we insert a tiny chip of carefully polished mineral or glass into the path between the IR source and the detector.

Minerals and glasses, especially those that erupt from gas-rich volcanoes, often contain still dissolved CO2 and H2O, which will absorb infrared at these characteristic wavelengths, just like volcanic gases in the air. As long as we know how thick the tiny glass or mineral chip is, then the FTIR can tell us how many of those gases are dissolved in those little solid pieces. Once we know this, we can determine things like how deep the erupting material is coming from and how quickly it erupted.

Even if Santa isn’t bringing you an FTIR this year, don’t worry, the HVO has two, and we’ll continue to use them to learn more about Hawaii’s volcanoes, their eruptions, and the gases that drive them. .

Updates of volcanic activity

The KÄ«lauea volcano is erupting. Its USGS volcanic alert level is at WATCH (https://www.usgs.gov/natural-hazards/volcano-hazards/about-alert-levels). KÄ«lauea updates are released daily.

Lava continues to emerge from a single vent in the western wall of Halema’uma’u Crater after a 2-day break in activity. All of the lava is confined to Halema’uma’u Crater in Hawai’i Volcanoes National Park. Sulfur dioxide emission rates remain high and were measured at around 130 tonnes per day on December 21, 2021. Seismicity is high but stable, with few earthquakes and an ongoing volcanic tremor. Summit inclinometer data recorded several deflation and inflation trends over the past week. For more information on the current eruption of KÄ«lauea, see Recent eruption (usgs.gov).

Mauna Loa is not erupting and remains at the ADVISORY volcanic alert level. This alert level does not mean that a rash is imminent or that progression to a rash from the current level of unrest is certain. Mauna Loa updates are released weekly.

Last week, around 113 low-magnitude earthquakes were recorded below the summit and upper elevation flanks of Mauna Loa, the majority of which occurred at depths below 10 kilometers (6 miles). Global Positioning System (GPS) measurements show no major distortions over the past week. The gas concentrations and the temperatures of the fumaroles at the summit and at Sulfur Cone in the Southwest Rift Zone remain stable. The webcams do not show any change in the landscape. For more information on the current monitoring of Mauna Loa, see: https://www.usgs.gov/volcanoes/mauna-loa/monitoring.

There have been 4 events with 3 or more felt reports in the Hawaiian Islands in the past week: an M3.3 earthquake 7 km (4 mi) east of Pāhala 32 km (20 mi) ) deep on December 17 at 2:25 p.m. HST, an M3.1 earthquake at 3 km (1 mi) SSE of Pāhala at 34 km (21 mi) deep on December 17 at 8:49 a.m. HST, an earthquake M3.5 earthquake at 6 km (4 mi) ENE of Pāhala 32 km (20 mi) deep on December 17 at 8:27 a.m. HST and an M3.5 earthquake at 7 km (4 mi) at l 21 mi (34 km) deep east of Pāhala on December 17 at 4:17 a.m. HST.

The HVO continues to closely monitor the ongoing eruption of KÄ«lauea and Mauna Loa for any signs of increased activity.

Visit the HVO website for old Volcano Watch articles, Kīlauea and Mauna Loa updates, photos of the volcano, maps, information on recent earthquakes, and more. Email your questions to [email protected].

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