Volcano Watch: Colorful plumes — can we see volcanic gases?

  • CHRISTOPH KERN/U.S. Geological Survey

    A volcanic gas cloud over the lower East Rift Zone on May 19. In this picture, the sun is illuminating the volcanic gas plume from behind. The plume appears orange in color, as the blue component of the sunlight has been preferentially removed by scattering on tiny sulfate aerosols. This looks a bit like a sunset, but note that the meteorological clouds below the volcanic plume do not show this discoloration.

  • ALLAN LERNER/U.S. Geological Survey volunteer

    Fissure 8 on June 5. The sun is behind the observer taking this photo, making the plume appear bluish because of sunlight being scattered by tiny sulfate aerosols. The gas plume is against a very dark cloud background so no backlighting is affecting the plume view.

*NOTE: This week’s Volcano Watch article was submitted Sunday, June 17. Volcano Watch normally appears every Sunday in the Tribune-Herald.

When volcanic gases are released into the atmosphere, resulting plumes sometimes appear to have a faint color.

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Is this color indicative of a certain gas present? Answering this question requires describing what makes a plume visible in the first place.

High temperature volcanic gases consist mostly of water vapor. In fact, more than 90 percent of the emitted gas is just steam. So, gas plumes often are whitish because of sunlight being scattered on water droplets condensing within a plume as it cools.

This process is similar to condensation in meteorological clouds.

Thus, volcanic plumes often are hard to distinguish from simple water clouds. It also explains why the temperature and humidity of the background atmosphere has significant control over the visibility of a volcanic plume.

Volcanic gases emitted into cooler, wetter air will condense more efficiently and degassing might appear more vigorous than during warmer, drier conditions even if the gas emission rate remains constant. This makes visual assessment of degassing intensity difficult.

On occasion, volcanic plumes appear slightly off-white and at other times they have a faint blue tinge. Other plumes look yellow or brown when compared with nearby water clouds.

So, are these the different volcanic gases we are seeing?

The short answer is no.

All gases emitted by volcanoes are completely invisible to humans in the concentrations at which they are present. Water is only visible once it condenses, not in the form of water vapor.

So what causes these colorful plumes?

The key to this mystery again lies in the scattering of light within the plumes. Steam plumes appear white because water droplets are highly reflective. These droplets also are much larger than the wavelengths of visible light and therefore scatter all visible light with approximately the same efficiency, thus leading to a white color.

Besides water vapor, some volcanic plumes also contain significant amounts of sulfur dioxide. Exposure to these plumes is uncomfortable and can be hazardous, as sulfur dioxide is an irritant and reacts to sulfuric acid when in contact with water. This reaction can occur in your eyes or respiratory tract and lead to a burning sensation. Exposure to sulfur dioxide concentrations of more than 100 parts per million would be an immediate threat to your health and even your life.

But sulfur dioxide has another characteristic. Once emitted, it reacts with atmospheric gases to form sulfate aerosols. Aerosols are tiny solid particles that are quickly coated and then dissolved in water, since there is plenty of that available in the plume.

Compared with meteorological clouds, however, the reaction of sulfur dioxide to sulfate creates many more, much smaller particles in volcanic plumes. In fact, these sulfate aerosol particles are so small that they are comparable in size to the wavelengths of visible light (about 400-700 nanometers).

It turns out that the scattering process is different for such small particles. These tiny particles will scatter the short wavelengths of blue light (about 400 nm) more efficiently than the longer wavelengths of red light (about 700 nm). If white light enters such a plume, e.g. by pointing a flash light at it or by standing with the sun at your back, then the returning light might have a slight bluish color to it because of this enhanced back-scattering of blue wavelengths.

Some volcanic plumes appear to have an orange color to them, rather than a blue tint. Interestingly, this is caused by the very same effect, just in different illumination geometry.

Imagine the sun is behind a volcanic plume containing significant sulfate aerosols. Some of the light entering the plume will be scattered, with blue light preferentially being redirected. Therefore, the blue component will be missing if we observe the light that makes it all the way through the plume, and the plume will appear orange, brown or even slightly red. This phenomenon is observed whenever the plume is back-lit; for example, when viewed against the sun or some bright white meteorological clouds in the background.

The bottom line is that we can’t see the volcanic gases themselves, but we can see the effect of light scattering on aerosols formed by reaction of these gases with air.

So whenever you see a blue or orange plume, beware that there are tiny particles in it, and think about from where these particles might be coming. If it’s a volcanic plume, then chances are there’s a significant amount of invisible sulfur dioxide there as well, and you should keep clear!

Volcano activity updates

As of this writing during the weekend, on Kilauea Volcano’s lower East Rift Zone, lava continued to erupt from fissure 8. As of June 17, lava fountains were feeding a well-established channel flowing east toward the ocean entry in the Kapoho Bay area. Residents in the lower Puna District of Hawaii Island should remain informed and heed Hawaii County Civil Defense closures, warnings and messages (www.hawaiicounty.gov/active-alerts). At Kilauea’s summit, inward slumping of the rim and walls of Halema‘uma‘u continues in response to ongoing subsidence at the summit.

Mauna Loa is not erupting. Rates of deformation and seismicity have not changed significantly during the past week. The number of monthly and weekly earthquakes recorded beneath the volcano has decreased to near background levels.

One hundred, eighty-four earthquakes were reported felt in Hawaii during the past week.

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Visit the HVO website (http://hvo.wr.usgs.gov) for past Volcano Watch articles, Kilauea daily eruption updates and other volcano status reports, current volcano photos, recent earthquakes and more; call (808) 967-8862 for a Kilauea summary update; email questions to askHVO@usgs.gov.

Volcano Watch (http://hvo.wr.usgs.gov/volcanowatch/) is a weekly article and activity update written by U.S. Geological Survey`s Hawaiian Volcano Observatory scientists and colleagues.