Giving Mount Erebus a CAT Scan
Posted March 10, 2016
An international collaboration of scientists is using electromagnetic emissions from lightning strikes and solar wind to map the inner workings of Antarctica’s Mount Erebus, the world’s southernmost active volcano. The research, led by scientists from New Zealand and the United States, will yield the clearest picture yet of the volcano’s interior.
Photo Credit: Peter Rejcek
Mount Erebus is one of relatively few active rifting volcanoes in the world.
“In a sense we’re trying to do a big CAT scan of the volcano,” said Phil Wannamaker, a research professor at the University of Utah in Salt Lake City.
The team of six just wrapped up their second of three seasons deploying electric and magnetic field sensors across Erebus to understand what lies beneath the surface of the volcano. They’re using a system called magnetotellurics to map its underground network of magma conduits and chambers.
The collaboration is supported by the National Science Foundation, which manages the U.S. Antarctic Program, and the Marsden Fund, of the Royal Society of New Zealand, through Antarctica New Zealand.
The team is braving the Antarctic elements because Mount Erebus can offer up insight into the structure and properties of an uncommon but important type of volcano.
“Mount Erebus is perhaps the best example in the world of a carbon dioxide dominated volcanic system,” Wannamaker said.
He went on to explain that most volcanoes around the Pacific Rim, like Mount Saint Helens in Washington State and Mount Fuji in Japan, are formed when two tectonic plates collide, and one is forced under the other, a process known as subduction. The plate pulls down with it a tremendous amount of water, which causes the top plate to melt and fissures of magma to flow to the surface.
However there are no subducting plates near Mount Erebus. Instead the tectonic plate it rests on is pulling apart and magma and carbon dioxide are working their way up through the cracks creating Mount Erebus and the other volcanoes along the Royal Society Range. It’s the same rifting process that’s happening in eastern Africa and the western United States.
Photo Credit: Lewis Baya
Members of the international team pose for a photo next to a New Zealand helicopter on Mount Erebus.
The researchers hope that by studying Erebus in depth, it can yield insights into how these kinds of rifting systems behave as a whole.
In order to peer through the kilometers-thick layers of rock that make up the planet’s crust, they use a technique that employs the naturally occurring electromagnetic waves that are constantly passing through the Earth’s surface.
Natural phenomena like lightning strikes and gusts in the solar wind subtly perturb Earth’s magnetic field and produce cascades of electromagnetic waves, essentially incredibly low frequency radio waves. These waves ripple out from their source, eventually reaching across much of the planet.
Because they’re of such low frequency, these waves pass deep into the Earth’s surface. They have little effect on most of the ordinary rock they pass through. However, electrically conductive materials, like metals, act as an underground antenna for these waves. They induce a subtle electrical current that sensitive instruments can detect.
“Conductivity in turn can be used to infer physical properties. For example, magma, a hot liquid… is quite electrically conductive against crystalline rock, so that will be a nice physical property contrast that we are looking for,” Wannamaker said.
Just like in a metal antenna, the electromagnetic waves induce a slight current in the magma, which in turn reemits its own electromagnetic waves. The team picks up these secondary emissions from the magma below by placing magnetic and electric sensors across Mount Erebus.
“We’re basically measuring the electric current and the magnetic field to say something about the conducting material down below,” said Graham Hill, a senior fellow at the University of Canterbury in New Zealand.
Each instrument is most sensitive to the region nearest to it, so the team has been deploying instruments to about 130 regularly spaced sites across Ross Island.
Photo Credit: Phil Wannamaker
The team buries one of the five-foot-long solenoids used to measure magnetic fields coming from below the surface.
“We have a network of these recorders all over,” Wannamaker said. “Erebus is essentially being covered.”
The signals from the magma are so weak, and the sensors in turn so sensitive, that their data can be easily contaminated by environmental noise from unexpected sources.
“For example, when the wind blows around the measurement location and the snow is actually moving, it picks up static electricity,” Wannamaker said. “You get this moving static electricity going around your sensor and that’s a source of noise.”
Each instrument needs at least a full day of quiet weather to record a clear picture. The team leaves each recorder in place for as long as their batteries last, up to two weeks, to collect enough viable data during bad weather.
There’s an ample amount of equipment to deploy at every site. Each unit has a central data recorder and battery, both about the size of a milk crate. Sticking out in different directions are three solenoids, each a five-foot-long tube of coiled wire that measure the magnetic fields. The team also has to lay out two, 150 meter-long wires to measure the electric field.
The exhausting part for the research team is that all of the sensors need to be buried under the snow on the volcano.
“It’s a little like construction work. You dig a lot of holes,” Hill said. “The sensors get buried to get them out of the wind, so they’re not moving and to keep them at a constant temperature.”
Typically, setup takes about two hours, but can be significantly longer in icy or steep terrain. On any given day of clear weather, the team is deploying or recovering multiple instruments, making for a full day of work.
Helicopters transport the team members across Ross Island to set up their network. Because it’s a joint program, the flights were split between the U.S. and New Zealand programs. Near the base of the volcano, the detectors are spaced as much as five kilometers (3.1 miles) apart, but near the peak, where the magma nears the surface, stations are as close as 1.5 kilometers (.9 miles) from each other.
The thin atmosphere at the peak, over 3,600 meters (12,000 feet) above sea level, makes deploying the instruments particularly strenuous.
“You can get a little winded. You’re moving slower in the summit area for sure,” Wannamaker said. “We’re certainly made aware of the possibility of altitude sickness.”
“We carry an altitude drug kit and an oxygen cylinder with us, when operating above 10,000 feet,” Hill added.
Wannamaker added that despite some of the logistical difficulties, burying the sensors under Erebus’s frozen snow is a lot simpler than trying it on warmer slopes.
“I would hate to plant 100 of these recordings into the fresh rock that you might face on a Mount Kenya or someplace like that,” he said.
With winter approaching, Wannamaker and Hill have returned to their respective institutions. They’ll spend the intervening months starting to process the gigabytes of data they’ve collected to ultimately produce a three-dimensional map of the volcano’s interior. They plan on returning to Erebus next season to capture the data at the remaining two dozen or so sites in their plan.
NSF-funded research in this story: Philip Wannamaker, University of Utah, Award No. 1243559 .