Volcano Watch - Low sulfur emissions mean a new focus on a different volcanic gas

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Currently, SO₂ emissions on Kīlauea are drastically lower than they have been for decades. However, as mentioned in our November 21, 2018, Volcano Watch article (https://volcanoes.usgs.gov/observatories/hvo/hvo_volcano_watch.html?vwi…), some SO₂ is still present, in addition to rotten egg-scented hydrogen sulfide (H₂S).

So, sulfur smells are still sometimes detected around the island, but it's another gas emitted by Kīlauea that has become more important lately—carbon dioxide (CO₂).

CO₂ is a significant volcanic emission, along with the familiar sulfur gases, water vapor, and trace amounts of other gases, such as hydrogen chloride, hydrogen fluoride, and helium. What's important about CO₂ specifically is that it can give clues about the depth of magma.

To explain, we'll use something familiar to everyone—a bottle of soda, which has only one gas (CO₂) dissolved in it. This CO₂ stays dissolved as long as the bottle is sealed, because the bottle creates enough pressure to keep the CO₂ in the liquid. As soon as the bottle is opened, pressure on the liquid decreases and the CO₂ creates bubbles that escape to the atmosphere.

Unlike soda, magma has many different gases dissolved in it, and they don't all behave the same way. With soda, one pressure decrease is enough to release all the gas from it, but with magma, different degrees of pressure decrease result in the release different gases.

Since pressure on magma is created by the weight of the earth above it, the deeper magma is, the higher the pressure it feels. As magma rises to shallower depths, it feels lesser amounts of pressure and different gases are able to escape along the way.

When magma is shallow or actually reaches the surface, the pressure on it is quite low, so SO₂ can easily escape, leading to the vog that impacted island residents for so long. But when magma is deeper, as is the case now, there is enough pressure to keep the SO₂ dissolved.

However, CO₂ is less soluble than SO₂ in magma and can escape even when magma is deep and the pressure is high. This is why, even with no lava erupting at the surface, Kīlauea is currently producing significant amounts of CO₂. (It's important to note that these amounts are very small compared to anthropogenic, or human-caused, CO₂ emissions.) It's this CO₂, in conjunction with the small amounts of sulfur gases still being emitted, that can provide clues to how deep Kīlauea's magma is.

Because CO₂ can escape even when magma is deep but SO₂ mostly stays dissolved until the magma is shallow, deeper magma produces a high ratio of CO₂ to SO₂. Geochemists typically use this CO₂/SO₂ ratio as an indication of magma depth.

Deep magma may begin with a high CO₂/SO₂ ratio, but that ratio will drop as magma moves to shallower depths and more SO₂ begins to escape. Therefore, if we can measure Kīlauea's CO₂/SO₂ ratio over time, any changes in it could tell us whether magma is once again rising through the system.

The catch is that we must be able to measure the CO₂/SO₂ ratio. However, accurately measuring the CO₂/SO₂ ratio in volcanic gas is tricky because of the large and variable amount of CO₂ that already exists in the atmosphere. At Kīlauea, the situation is further complicated by collapse events that rearranged the summit caldera and damaged roads and other means of access to degassing areas.

As this new phase of inactivity at Kīlauea continues, gas geochemistry scientists at the USGS Hawaiian Volcano Observatory will explore new avenues for measuring the helpful CO₂/SO₂ ratio, including the installation of multi-gas sensors at Kīlauea's summit and the use of gas sensors mounted on unmanned aerial systems (UAS). Our goal in collecting such gas data is to document changes that could eventually indicate an increased likelihood of renewed activity at Kīlauea.

Volcano Activity Update

Kīlauea is not erupting. Rates of seismicity, deformation, and gas release have not changed significantly over the past week.

Three earthquakes with three or more felt reports occurred in Hawai‘i during the past week: a magnitude-1.6 quake 4 km (2 mi) southwest of Volcano at 3:02 a.m. HST on Feb. 17; a magnitude-3.0 quake 10 km (6 mi) south of Kapa‘au at 12 km (7 mi) depth on Feb. 17 at 12:04 a.m. HST; and a magnitude-3.3 quake 13 km (8 mi) east of Honoka‘a at 6 km (4 mi) depth on Feb.13 at 4:42 p.m. HST.

Deformation signals are consistent with refilling of Kīlauea Volcano's deep East Rift Zone (ERZ). Sulfur dioxide emission rates in the lower ERZ have been below detection limits since early September 2018. Sulfur dioxide emissions at Kīlauea's summit and middle ERZ remain low, with rates steady over the past several weeks.

Hazardous conditions still exist at both the lower ERZ and summit of Kīlauea. Residents and visitors in the lower Puna District and Kīlauea summit areas on the Island of Hawaiʻi should stay informed and heed Hawai‘i County Civil Defense closures, warnings, and messages (http://www.hawaiicounty.gov/active-alerts). HVO continues to closely monitor Kīlauea for any sign of increased activity.

The USGS Volcano Alert level for Mauna Loa remains at NORMAL.