The key to understanding Earth’s prehistoric magnetic field lies hidden in the rocks.
Specifically, igneous rocks, basalts that cooled from liquid magma spewed out of volcanos.
Geologist Lisa Tauxe of the University of California, San Diego, and her team, traveled to Antarctica to collect rock samples that can tell them more about the planet’s magnetic field over the past few million years. The research was funded by the National Science Foundation, which manages the U.S. Antarctic Program.
“The Earth has a magnetic field. It’s generated in the core and we can think about this field as being generated by a big bar magnet,” Tauxe said. “It behaves very much like that.”
Earth’s magnetic field makes compasses point north or south, but crucially it also protects the planet’s surface from cosmic radiation. The field extends far out into space, deflecting charged particles from the sun that could damage sensitive infrastructure. This makes understanding its future behavior a question of more than academic interest.
“If the field is less than a certain amount, then we have cosmic rays coming in, and cosmic ray bombardment will affect us,” Tauxe said. “For example we know that it affects our electrical grid every time we have a big magnetic storm we have power outages. Well that’s going to get worse as the field goes down.”
The researchers hope that their work will offer some insight into whether the field will keep going down, how low it can get and how fast it can get that low.
The Earth’s magnetic field is generated in a complicated, dynamic interaction deep in the planet’s core, a process that scientists are still working to fully understand.
Because of these complex processes, the field behaves much more unpredictably than that of a simple bar magnet. It fluctuates in strength over time and even completely reverses itself every few hundred thousand years. These changes are difficult to predict, in part because scientists still aren’t certain about some of the fundamental characteristics of the field. Tauxe is hoping to fill in a few of these gaps, in particular whether today’s measured field strength is typical.
“What we’re looking for is what is the average, what does the ordinary field do,” Tauxe said. “If we don’t know the basic answer of what is the average magnetic field strength over time, and has that changed, than we can’t answer that fundamental question, ‘What’s it going to do in the future?’ because we don’t know enough about what it’s done in the past to answer that.”
Clues to these previous changes are encoded in tiny magnetic particles embedded in the volcanic rocks throughout the region. McMurdo Station is located on Ross Island, which is itself composed of extinct and active volcanos. The nearby Transantarctic Mountains are likewise home to numerous extinct volcanos.
Antarctica is the perfect environment to pristinely preserve these rocks. In effect, they’ve been stored in a protective freezer since they formed.
“One advantage of the lava flows around McMurdo is there’s no vegetation and there’s no rainfall, and there’s no chemical weathering,” Tauxe said. “If you have rainfall and moist warm conditions with a lot of vegetation, rocks break down. They turn to dust, they turn to mud, [and] they turn to other minerals.”
Tauxe and her team were looking for a specific kind of igneous basalt; samples that rapidly cooled from liquid lava to solid rock.
“We’re looking for a certain type of material, really glassy material, material that quenched really quickly,” said Hanna Asefaw, a graduate student at UCSD. “Studies have shown that this material will give us our best results.”
The key to measuring ancient magnetic fields lies within tiny bits of magnetic minerals embedded within these rocks. These magnetized grains can act as a record of the planet’s ancient magnetic field, recording clues to its alignment and intensity as the rocks formed out of liquid lava.
However, not all grains are created equal. The researchers were looking for the rocks with the smallest domains because they are the most magnetically stable, they hold onto their original magnetic properties indefinitely.
“What controls grain size in lavas is how fast they cool,” Tauxe said. “If they cool very slowly, the crystals can grow larger and then they have this nasty habit of being magnetically unstable. We’re going for the things that cool very quickly and they look glassy, they’re actually glassy. Those have the tiniest magnetic particles and we hope they are magnetically stable.”
By analyzing the magnetic properties of the rocks’ grains, the researchers can infer what the planet’s magnetic field was like when the rock formed.
The team focused on finding rocks from the past 5 million years, a relatively short time in the geologic scale of the planet, but a good length of time to put together a detailed picture of its recent history.
In fact, the field strength can change significantly over a brief period of time.
“It’s decreased by about 8 percent over the last 100 years,” Tauxe said. “And it’s decreasing more now than it was at the beginning of the century, so it’s really cratering.”
The geologic record has also shown that the field flips every 200,000 to 300,000 years or so. That’s led some scientists to think that the Earth’s magnetic field is heading towards such a dramatic reversal, where the magnetic north and south poles swap.
Tauxe is unconvinced. There is evidence that the Earth’s magnetic field is has been particularly strong as of late, and the decrease they’ve been seeing is it returning to equilibrium. But the data to support that is incomplete, and she’s hoping that the samples that she and her team brought back from the Antarctic can help shed some light on the matter.
“We still don’t really know why it reverses,” Asefaw said. “It’s still an active area of research.”
The team is following up on previous projects to learn more about the intensity of the planet’s magnetic field. In 2004 and 2006 Tauxe and co-investigator Hubert Staudigel traveled to a number of sites around the continent collecting more than 100 samples from multiple lava flows. However those expeditions were looking for information about the direction of the planet’s magnetic field, not its strength. The samples they collected then were ones that took longer to cool and solidified deep inside the rock face, different from the quickly-cooled glassy outer layers that their current intensity experiments need.
“We took a gasoline powered drill and we drilled out samples and took orientations and all that,” Tauxe said. “We have an excellent idea of what the directions are, and now we just need that strength component. Only 12 of them worked for the intensity experiment, so we’re trying to bring that number up.”
The team traveled to numerous locations around the region, including spots throughout Ross Island, nearby Black Island and multiple sites along the Royal Society Range in the Transantarctic Mountains. They returned with 108 samples and are currently processing them using a sensitive magnetometer.
“[The season] went incredibly well,” Tauxe said. She added that they’re working to improve the estimated formation dates for their samples and are hoping to publish their final results sometime next year.