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Research at the Museum: Volcanic Volatiles

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Volatiles, gasses that drive volcanic eruptions, are shown here in a glass bubble. Photo courtesy of J. Webster

Jim Webster leans over a worktable coated with pliers, wires, and scraps of material, plucking a small, sealed capsule of white gold-palladium alloy out of the ordered chaos. Inside the capsule rests 50 milligrams of crushed stone and liquid, a combination that Webster, a curator in the Department of Earth and Planetary Science within the Division of Physical Sciences at the American Museum of Natural History, uses to understand why some volcanoes erupt explosively.

In his lab on the fourth floor of the Museum, Webster designs experiments to study the processes that caused such explosive volcanic eruptions as Mount St. Helens in 1980, Pinatubo in 1991, and much older volcanoes like Mt. Mazama. More commonly known as Oregon’s Crater Lake, Mt. Mazama is an ancient volcano that explosively erupted nearly 7,000 years ago, eventually spewing so much magma, gas, and ash that it collapsed on itself, leaving a crater where the mountain had stood. In his experiments, Webster uses samples from this ancient explosion that are compositionally equivalent to eruption stages at Mt. Augustine, Mt. St. Helens, and Mt. Pinatubo.

The destructive nature of explosive volcanic events such as the one at Mt. Mazama is partially due to dissolved gases within the magma. These gases, known as volatiles, or ‘fluid’ to a volcanologist, expand rapidly during a volcanic eruption. If there are enough volatiles present, they can cause the eruption to be sudden and violent, hurling bits of molten rock and mountain thousands of feet into the atmosphere and blanketing the surrounding landscape in deadly volcanic ash.

For all the mayhem they cause, the volatiles themselves (water vapor, carbon dioxide, sulfur, chlorine, and fluorine) take up only a very small proportion of the magma—roughly 5.6 percent of the total volume.  However, the ways in which they interact with each other and the magma can make a violent eruption more or less explosive. “It’s always the fluid that drives the eruption,” says Webster.

But while volcanologists know that explosive eruptions are caused by these volatiles, figuring out how the volatiles interacted with the rock after they have dispersed into the atmosphere can be difficult. A common mineral, apatite, which is found in most volcanic rocks, may provide the answer.

Because of its chemical composition and mineral structure, apatite interacts with volatiles around it, preserving small samples of the volatiles that scientists can find and analyze. But the information preserved is sparse and inconclusive without some standard of interpretation, much like an ancient text written in a language that no one can decipher.

In order to decode the information contained in erupted apatite, scientists like Webster are working to create a body of knowledge that will act as a Rosetta Stone of sorts, experimentally melting fixed proportions of volatiles with apatite to see what is preserved.

Webster’s capsule is made of white gold-palladium, which can withstand the intense temperatures and pressures that the experiment requires, and won’t react with any of the volatiles being examined. The capsule is carefully fitted into a small cylindrical furnace, easily held in one hand. The furnace is then inserted into the lab’s workhorse, an internally heated pressure vessel where rings of neoprene, copper and steel swell when pressurized to create a tight seal.

The vessel came with Webster from the University of Edinburgh, where Webster was doing his post-doctorate work. Over 200 years ago, scientists at Edinburgh had come up with the first, primitive version of the device that Webster uses today. “They sealed local basalt and water in a cannon, threw it in a nearby industrial furnace, and then cooled it quickly,” says Webster. No cannons are used for modern scientific endeavors, but the process remains similar: melt the rock, cool the rock, then look at it and see what happened.

Inside the vessel, the furnace heats the capsule to 1,120 degrees Celsius while Argon gas within the vessel applies a pressure of 2,000 bars, or 29,000 pounds per square inch. It stays at that temperature and pressure for a week or more, melting most of the rock within the capsule and mimicking conditions of magma usually found 4.4 miles into the earth’s crust. That’s a lot of pressure for any instrument to handle, and the entire vessel is ensconced behind a 600-lb steel box.

Webster then uses an array of machines, including a fourier transform infrared spectrometer (FTIR) and an electron microprobe, to measure the different elements within the sample. He attempts to determine exactly how the volatiles in the capsule interacted with the sample rock. “All the information we have about volatiles in magmas is based on experiments like this,” says Webster.

His work may pave the way for scientists to analyze igneous rocks from the field and figure out how they erupted, and which gases caused them to be expelled from their molten home deep within the earth.

This Research in Action article was initially provided to Live Science in partnership with the National Science Foundation.

Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation. See the Research in Action archive.


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