Volcano Watch — Kīlauea plume: now you see it, now you don't

Several factors affect the plume's appearance. Changes in physical conditions at the vent, meteorology, and even chemical reactions occurring as gas boils out of the melt can conspire or act alone to produce the variably visible plume.

The summit plume is principally composed of water vapor (H₂O), carbon dioxide (CO₂), and sulfur dioxide (SO₂), all of which are clear and colorless gases. The visible part of the plume includes a minor amount of ash, along with a suspended mixture of tiny droplets and particles, called aerosol. Volcanic ash consists of rock dust and glass particles less than 2 mm (0.08 inches) in diameter. The aerosol in the plume is even smaller, less than 0.0025 mm (0.0001 inch) in diameter--about one tenth the diameter of a strand of human hair. Aerosol is composed chiefly of dilute sulfuric acid (H₂SO4), formed when SO₂ reacts in the presence of oxygen and water vapor.

Physical changes occur intermittently at Halema`uma`u as the unstable rim crumbles and releases rocks and debris into the vent. These rockfalls disturb the top of the magma column and sometimes result in the production of brief ash-rich plumes, ranging from gray to brown, before returning to their prior appearance.

Meteorological variations, including relative humidity, temperature, cloud cover, and wind speed also affect plume appearance. Because the plume is visible, owing partly to the presence of water-laden droplets, air temperature and relative humidity work in concert to changing the plume's visibility. Higher humidity and lower temperature conditions cause moisture condensation, producing a more visible plume. The converse temperature/humidity story is true, as well. Increasing wind speed tends to make the plume more compact, and usually more opaque, except at very high wind speeds.

Cloud cover diffuses the sun's rays; diffuse sunlight is scattered in a way that appears different to our eyes than does direct sunlight. The result of this effect can be that a plume appearing blue and translucent in bright sun light, turns milkier and denser under heavy cloud cover.

Chemical transformations occurring with the plume are of great interest with respect to their effects on plume appearance. These same changes also provide clues about processes occurring at the top of the summit magma column, a place often not directly visible from the surface.

As noted earlier, the visible aerosol is comprised of H₂SO4, formed by chemical oxidation of SO₂. A primary factor accounting for variability in the amount of H₂SO4 produced is obviously the amount of SO₂ bubbling out of the magma to start with, but the temperature of the vent has a strong effect, as well. Higher temperatures within the vent tend to oxidize more SO₂.

A useful lesson showing the complex effects of temperature and chemistry on the visibility of SO₂- and H₂SO4-bearing plumes occurred at coal-fired power plants on the mainland in the 1990s. Power companies worked hard to clean up sulfur and nitrogen oxide emissions by installing chemical scrubbers. The scrubbers reached their goal of effectively decreasing the amount of clear, colorless and noxiously polluting SO₂ in their exhaust gases, but, partly because of gas temperature and humidity effects, had the unintended consequence of producing dense blue plumes containing light-scattering H₂SO4.

The experience of the power stations underscores the importance of temperature and humidity on plume visibility. It also demonstrates how decreases in SO₂ emissions do not always result in a less visible plume. Conversely, a more dense plume doesn't necessarily mean more SO₂, either. At Kīlauea, we confirmed this lesson by using ultraviolet spectrometers to measure SO₂ emission rates of dense white plumes and thin, wispy blue ones. We've found that plumes appearing thin and wispy often contain as much or more SO₂ then dense white ones.

A substantial and valuable part of the science of volcanology is based on simple but careful observations. Combining these direct observations with instrumental ones has helped volcanologists answer numerous questions about how volcanoes work. As our toolbox of high-tech methods bulges, we remind ourselves of the value and place of simple human-based observations.

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Volcano Activity Update

Surface flows have been active on Pulama pali within the Royal Gardens subdivision and on the coastal plain west of the subdivision. There has been little forward advancement toward the ocean, however, due to last week's disruption in lava supply caused by a deflation/inflation (DI) event at Kīlauea's summit. Rapid deflation at Kīlauea's summit started again on Thursday and will probably lead to a slow-down in surface activity. When the volcano reinflates, there will likely be a corresponding increase in surface activity on the pali as the system recovers.

At Kīlauea's summit, a spattering and roiling lava surface, deep within the collapse pit inset within the floor of Halema`uma`u Crater, was visible via Webcam. For much of the week, the lava surface was seen to cyclically rise and fall. This behavior may stop as the lava surface lowers in response to the ongoing DI deflation, and may start again when the volcano reinflates. Volcanic gas emissions remain elevated, resulting in high concentrations of sulfur dioxide downwind.

One earthquake beneath Hawai`i Island was reported felt during the past week. A magnitude-3.4 earthquake occurred at 10:41 a.m. on Tuesday, February 23, 2010, H.s.t., and was located 8 km (5 miles) southeast and offshore of Kalapana at a depth of 44 km (28 miles).