Volume 1, Issue 2
An Explosive Theory About Volcanoes
The hulking steel volcano simulator in UC Berkeley professor Michael Manga's laboratory is a far cry from the baking soda-and-vinegar science fair projects of our youth. Of course, that's to be expected. What's unusual is that Manga, a professor of earth and planetary science, is trying to answer the same question posed by the quintessential science class experiment: Why do volcanoes erupt?
Michael Manga is also part of a NASA-funded project to analyze Mars's natural history and the possibility that the planet could have supported life. Five of the 10 team members are professors in UC Berkeley's Department of Earth and Planetary Sciences.
More specifically, Manga's research explains why volcanoes sometimes erupt by oozing lava and other times violently burst ash into the air. Understanding what makes magma erupt in these two very different ways, sometimes from the same volcano, could help scientists determine how hazardous a particular volcano may be.
"Many people live on volcanoes," Manga says. "Knowing when a volcano may be dangerous could help us warn people appropriately. It's way too expensive to tell people to evacuate if nothing ends up happening."
The two types of volcanic eruptions are known as explosive and effusive. Mount Saint Helens' 1980 explosive eruption caused the death of 58 people and more than $1.2 billion in property damage. The slow creep of lava from Hawaii's Kilauea volcano is effusive--visitors flock to the area to watch the honey-like flow.
For many years, scientists believed that explosive eruptions are caused when rising magma breaks as it moves. To demonstrate, Manga grabs a hunk of Silly Putty.
"Silly Putty and magma are similar," he says, tugging on the rubbery substance. "If you deform it slowly enough, it flows. But if you pull it hard, it breaks."
Inside a volcano, the breakage of magma, known as fragmentation, releases gas bubbles trapped in the liquid. The pressure of the escaping gas was thought to propel the fragmented magma, in the form of ash, out of the volcano much like soda squirts from a soft drink bottle if you shake it before popping the top.
But in November, Manga and graduate student Helge Gonnermann published a hypothesis in the journal Nature proposing that fragmentation is also evident in effusive eruptions. According to their report, fragmentation is not the sole cause of explosive eruptions. In fact, they wrote, if the magma repeatedly fragments as it rises to the surface, the steady escape of the gas pressure prevents an explosive eruption. Volcanoes, they went on to say, only explode when the magma rises so quickly that the pressure builds faster than it's released.
"Whether a volcano erupts explosively or not depends on the amount of bubbles inside the magma and how much, or how quickly, the pressure changes," Manga says.
The Little Glass Mountain obsidian flow, the result of late Holocene eruptive activity on the eastern flanks of northern California's Medicine Lake shield volcano, the largest volcano in the Cascade Range. Mt. Shasta, an active volcano, is in the background. (courtesy Helge Gonnermann)
The researchers first realized that fragmentation may be universal across eruptions while studying obsidian created by an effusive eruption at California's Big Glass Mountain volcano. The multi-colored bands visible in the rock hinted that the magma had fragmented, reannealed, and deformed as it ascended.
A 2-cm wide slice through a piece of banded obsidian from Big Glass Mountain in northern California. The bands, which contain different concentrations of microcrystals of the mineral pyroxene, are created by shear forces as the magma rises under the volcano and as it flows on the surface. (courtesy Helge Gonnermann)
Currently, Manga and Atsuko Namiki, a post-doctoral researcher in the laboratory, are using the volcano simulator to test their theories and models about how and why magma fragments. A corn syrup-like liquid is held in the machine's sample chamber, separated from the vacuum chamber by diaphragms. Decreasing the pressure in the vacuum chamber causes the diaphragms to break. Once exposed to the pressure change, the sample liquid expands and blows out the top. A high-speed video camera captures 2,000 frames per second, enabling the researchers to analyze the liquid fragmentation.
"These kinds of experiments help us predict whether something like a big landslide at Mount Saint Helens, for example, would change the pressure enough to cause an explosive eruption," Manga says.
Along with volcano simulators, Manga and his colleagues build unusual models of various planets' entire geological systems. Massive plexiglass vats of corn syrup are heated and cooled from the top and bottom to provide insight into how rock melts and moves through a planet's mantle. In just one day, the researchers can simulate billions of years of Earth's geophysical history. Gaining a better understanding of geophysical fundamentals such as how fluids carry heat through a planet's mantle, Manga says, is important to all of their research efforts.
Post-doctoral research Atsuko Namiki with a volcano simulator. She mixes the various syrupy fluids used in the experiment herself so that they exhibit desired properties.
"Kids have been doing science fair projects about these things forever," he says. "But there are so many simple questions that we just can't answer yet."