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Time-lapse July 5–12, 2012 HD movie of Pu‘u ‘O‘o crater

(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

The use of fingerprints, first considered as a means of identification in the 1880s, is now ubiquitous in murder mysteries, TV crime shows, and standard police investigations.

A more recent development is the concept of “fingerprinting” magma within a volcano to characterize its depth and composition. These characteristics tell us whether magma is rising to shallow levels within the volcano and provide clues on how volcanoes work.

The magmatic fingerprints are provided by the volcanic gas “signature.” As magma rises from deep within the volcano, the pressure on the magma decreases. Dissolved volcanic gases bubble out of magma, much like dissolved gases bubble out of a carbonated beverage as you open the container. Because different gases have varying abilities to stay dissolved in magma, they are released at different pressures and depths below the surface of the volcano. Therefore, identifying how much of which gases are present in a volcanic plume can help identify whether magma is still deep within the volcano or close to the surface and more likely to erupt.

One of the gases that volcanologists use to determine whether magma is close to the surface is sulfur dioxide (SO2). It begins to bubble out of the melt in significant amounts at a depth of about 1 km (0.6 mi), so it is a good indicator that magma is close to the surface. Since the amount of sulfur dioxide gas in clean background air is very low, distinguishing SO2 from a volcanic source can be fairly simple. Sulfur dioxide gas strongly absorbs ultraviolet light, such as sunlight, so the total amount of SO2 in a plume can be calculated by measuring the amount of sunlight that hasn’t passed through the gas plume. An ultraviolet spectrometer, an instrument that measures ultraviolet light intensity, is the right tool for this job.

Typically, at Kilauea, we mount an ultraviolet spectrometer system on a vehicle and drive beneath the entire width of the gas plume to calculate the amount of SO2 released from the volcanic vent.

This method of measuring gas emission rates has been in use at Kilauea for several decades and has been very useful in tracking changes in the volcano’s behavior. For example, during the months preceding the opening of the vent in Halema`uma`u Crater in 2008, background summit SO2 emissions increased by an order of magnitude, from around 200 to 2,000 metric tonnes per day. This suggested that magma had worked its way close to the surface and that there was an increased likelihood of an eruption.

While the spectrometer traverse technique is quite straightforward, the frequency of data collection is limited, since it requires a dedicated operator and specific environmental conditions. The shortest time period in which changes in gas emission rates can be measured using this technique is on the order of tens of minutes. Many questions remain that can only be addressed by more frequent measurements.

In order to increase the frequency and efficiency of SO2 data retrieval, a novel approach is being developed and tested at Kilauea. Rather than drive a single spectrometer beneath the gas plume, we have positioned an array of multiple stationary spectrometers along the width and downwind of the plume to measure SO2 as it passes overhead. This 10-spectrometer array operates during daylight hours, calculating an SO2 emission rate every 10 seconds, producing as many as 3,000 measurements per day when the winds are favorable. Compared to the vehicle-based method which typically yields 8 to 20 measurements collected over a period of 1‒2 hours a few times each week, this represents a dramatic increase in data frequency.

The high data-collection rate of the stationary spectrometer array will allow gas data to be examined at a frequency similar to that of other geophysical and geologic observations, such as seismicity, ground deformation, and lava-pond level. Thus far, the data from the stationary spectrometer array have confirmed that changes in SO2 emissions occurring over a period of minutes to hours are correlated with changes in seismic energy release, as well as the rise and fall of the lava pond in the vent at Kilauea’s summit.

By developing new tools that provide higher frequency gas-emission measurements and a more detailed look at degassing behavior, we hope to reveal additional mysteries of magmatic movement, short-term dynamics, and volcanic processes, in general.