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On Jan. 25, 1998, scientists monitoring undersea volcanoes were alerted to a sudden swarm of earthquakes, which occurred as magma began moving toward the surface and into the south rift zone on the Pacific Ocean floor. The lava flow then inflated and drained out over the next two hours. In the following days, the entire volcano summit gradually subsided as magma oozed out of the summit reservoir and spread over the seafloor. This eruption of the undersea Axial Volcano on the Juan de Fuca mid-ocean ridge off the coast of Washington state spewed lava that engulfed a seafloor instrument placed on the volcano to measure pressure and vertical deformation of the seafloor.
The Volcanic System Monitor (VSM), also known as a rumbleometer, was resting on the caldera’s south rift zone. It survived the eruption and is here to tell the tale, for the first time providing a glimpse into the heart of one of Earth’s most powerful and least understood phenomena.
“The instrument was simply in the right place at the right time, with the right sensors, and happened to have the right physical design to survive the eruption,” write Christopher Fox, William Chadwick and Robert Embley, all with NOAA’s Pacific Marine Environmental Laboratory (PMEL) Vents Program, in the Aug. 16 Nature. PMEL scientists rescued the lost rumbleometer 18 months after the volcano erupted. They found it in three feet of solidified lava on the ocean floor, at a depth of 1,525 meters.
As is common during basaltic volcano eruptions, the pattern observed by the VSM was a brief burst at a high effusion rate, followed by a longer period of waning rates, and then subsidence during lava withdrawal. Even though the entire inflation and drainout cycle lasted only two hours, the basaltic lava spread over the ocean floor to create a lava plain more than 350 meters wide.
When the frigid ocean water met the thinly spread sheet of lava, a solid upper crust formed, serendipitously entombing the rumbleometer in solid crust while the molten interior continued flowing. The temperature near the rumbleometer peaked at 9.5 degrees Celsius, only 7.5 degrees Celsius above the normal water temperature of 2 degrees Celsius, even though the temperature of the erupting lava was 1200 degrees Celsius.
“Our principal focus is to see how undersea eruptions affect the ocean environment,” Fox says. “We are also interested in the basic research questions of how deep-sea volcanoes work, their time scales, dynamics and so on.”
Before the rumbleometer’s fortuitous entrapment, scientists had only been able to survey and observe undersea eruptions after they happened. The VSM recorded the duration, character and effusion rate of this eruption while it happened — and made the first in situ ground deformation measurements of a submarine eruption.
The data also helped researchers begin to unravel a few deep-sea mysteries. They can now calculate the amount of magma withdrawn during the eruption and the depth of the magma reservoir beneath the caldera. And the pressure measurements, which document the change in the depth of the instrument during the eruption, provide, for the first time, the means to directly estimate the effusion rate for a submarine eruption.
Submarine volcanoes are the primary sites for the exchange of heat and chemicals from Earth’s interior to its surface. Scientists have just begun to understand undersea volcanoes, as they have only been able to detect submarine eruptions for the last decade. Their primary detection tool has been SOSUS, a classified U.S. Navy Sound Surveillance System, currently “the only means we have to monitor volcanic activity in the deep sea in real time,” Fox says.
“Our primary goal now is to directly observe a deep-sea eruption in progress,” Fox says. “This will require pre-positioning instruments on Axial Volcano that can be commanded from shore to investigate active eruptions.
“The ocean is an extremely active part of the earth that still harbors many unknowns. We have a lot of new technology to work with in the deep sea and are attempting to unravel some of these mysteries.”
Meg Rudolph, contributing writer
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