Scientists find 'listening' helps to better study volcanoes

Scientists find 'listening' helps to better study volcanoes
February 9, 2017

A few dozen volcanoes begin erupting every year according to the Smithsonian Institute’s Global Volcansim Program (you can see an animation of all the eruptions worldwide since 1960 by clicking here). Geologists follow the seismic rumblings of each eruption, making note of where they are and how long the eruptions last.

Undersea eruptions blast lava, heat and nutrients into the ocean, so marine geologists study not only how these blasts change the landscape of the ocean floor, but also how they can affect the ecosystems that thrive on volcanic emissions: both important processes in the evolution of the ocean ecosystem.

Map of Axial Seamount

One ability that would make studying those processes easier is knowing when and where volcanoes are going to erupt. Geologists often use seismic data—rumblings of earthquakes and movements of rock that can disturb magma reservoirs—to predict eruptions. But researchers from the University of Washington and NC State University are trying a new tool to monitor and possibly predict eruptions: sound.

In a new study published in the journal Science, geophysicists used hydrophones—underwater microphones—in addition to the normal seismographic equipment to follow the eruption of Axial Seamount, an underwater volcano 300 miles off the coast of Oregon. This is the first study to combine acoustic and seismic data to track the progress of a volcano in real time.

NC State geophysicist Del Bohnenstiel and colleagues from the University of Washington and four other institutions tracked an eruption of Axial Seamount in April 2015. They chose Axial because it erupts on a somewhat frequent basis—three times in the last 20 years—and because its main geological feature, called a caldera, is common to many volcanoes, but scientists still do not understand exactly how lava flows in calderas work.

Calderas are craters left over from volcanic eruptions. They form either when volcanoes explode, flinging lava out and leaving an empty bowl, or when rock layers collapse into an empty magma chamber after the volcano has pushed the magma out. Kilauea in Hawaii is an explosive caldera that would blow its top every time magma would superheat the local ground water into highly pressurized steam. Crater Lake is also a caldera that used to be a mountain. As the magma underneath the mountain slowly poured out, the top of the mountain cracked and fell into space left behind.

Rocks at Axial SeamountCalderas often still erupt, however, and when they do, geologists want to know how the rock itself moves and how the lava gets flung from the volcano. Seismographs, the normal method of monitoring volcanoes and earthquakes are great at discerning what’s happening underground by picking up the vibrations created when rocks break and move through each other. The addition of the hydrophones, Bohnenstiel says, allows scientists to follow the lava as it breaks through the ocean floor. 

Bohnenstiel says that as soon as magma breaks through the seafloor, it creates a shockwave through the water that sounds like an explosion. At the most basic level, a “boom” can signify that magma has indeed erupted through the ocean floor. With enough hydrophones, the timing and intensity of the booms could map where and how much lava erupts from the caldera.

At Axial Seamount, the combination of the seismic and acoustic data showed that the eruption process begins as magma pools under the caldera. The heat softens the rock above, allowing it to slowly inflate as more magma pools in. When that inflation reaches its limit, magma, ash and steam blast through the caldera in an eruption. Faults around Axial allowed the caldera to deflate as the eruption process continued before finally dying back down.

Volcanoes represent one of the most awesome natural forces on Earth, and more than 80 percent of Earth’s volcanoes are located in the ocean. This research is important because it provides a concrete methodology to study this majority of volcanoes.

Further, the researchers found that hydrophones are a relatively cheap and easily accessible tool to detect volcanic eruptions, and the data they gathered about Axial specifically could help predict when it is likely to erupt again. All we need to do is listen.

—Daniel Lane

Daniel Lane covers science, medicine, engineering and the environment in North Carolina.

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