February 22, 1998

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.

Volcano: Will it Flow or Blow?

A gentle, effusive style of activity has characterized the ongoing eruption of Kilauea for well over a decade now. But remember the explosive episodes of 1983, `84, `85 and the first half of `86? Pu`u `O`o burst forth periodically with towering lava fountains that could be seen for miles around. Ever wonder why the change? Just what causes a volcano to flow rather than blow?

This question has intrigued and inspired scientists for decades. Along with our colleagues at the USGS in Menlo Park, California, we think we may have a new answer to this age-old question. And it all hinges on bubbles--tiny bubbles!

Scientists have long known that the driving force of an eruption comes from the build-up and release of magmatic gases. Magma that is deep within the earth, where the pressure is high, contains dissolved water-, carbon-, and sulfur-rich gases. As it rises to the surface, where the pressure is lower, these gases are released, and the magma starts to "fizz" or bubble, just as a can of soda does when you pop the top. This fizzing, technically known as "vesiculation," is the force that propels the magma out of the vent during an eruption.

What puzzles scientists is why vesiculation sometimes leads to an explosive ejection of magma, while at other times it produces a more passive outpouring, like the current activity at Pu`u `O`o.

At first thought, it may appear obvious. There must be a different amount of gas released under the two circumstances. More gas, bigger bang. Right? Not necessarily.

Let's return to the soda analogy and conduct an experiment. Start with two cans of soda pop. Gently shake the first can, then pop the top. Soda wells up out of the opening, flows over your hand, and spills onto the kitchen floor. Now take the second can and shake it hard. Stand back, and pop the top. Soda explodes out, spraying your ceiling. The same amount of gas in both cans--two very different styles of "eruption."

What is different in the two experiments is not how much gas is released, but how fast the gas is released. Of course, at a volcano, the rate of gas release is not controlled by shaking, but, instead, by differences in the depressurization rate of the rising magma.

The hypothesis that HVO scientists are working with is this: Magma that rises to the surface slowly experiences slow depressurization. Gases are released gradually, and the ensuing eruption is gentle. A fast-rising magma, in contrast, undergoes very rapid depressurization. The gases are given off in a violent rush of vesiculation, and magma explodes out of the vent.

This hypothesis is being tested in a rock-melting laboratory at the USGS center in California. Specialized furnaces and pressure vessels are used to subject molten rock to the conditions a magma encounters as it travels to the Earth's surface.

Ultimately, the results of these experiments can be coupled with advances in seismic and geodetic monitoring that allow scientists to track the rise rate of magma beneath an active volcano. It thus may be possible to predict how explosive an eruption is likely to be--before it occurs.

Eruption and Earthquake Update, 22 February

There were no felt earthquakes on the Big Island last week, but at 7:12 AM on February 18th there was a magnitude 3-3.5 quake approximately 50 miles SE of Oahu. The East Rift eruption of Kilauea continues without significant change.