Life as we know it requires an atmosphere. It is the air we breathe, our shield from harmful ultraviolet rays, and our defense against extreme temperature swings, like those on Mars. But Earth’s atmosphere owes its persistence to the geomagnetic field, which thwarts the Sun’s rays from dispelling this gaseous veneer. And this protective geomagnetic field owes its existence to Earth’s core.
As the liquid part of the core (the outer core) swirls, the combination of molten iron alloys and Earth’s rotation results in a self-sustaining magnetic field called the geodynamo. As they archive evidence of the geodynamo’s billions-of-years-long existence, rocks can transcribe where north was (direction) and how strong the field was (intensity) at the time of formation. That transcription is possible as long as the rocks remain relatively untouched by high temperatures, fluids, or other traumas of tectonics.
Because the strength of Earth’s magnetic field relies on the vigor with which the liquid core churns, understanding how paleomagnetic intensity has changed at the surface can help scientists address when the core transitioned from a single ball of sloshing melt to a solid inner core wrapped in a liquid outer core. In other words, paleomagnetic intensity might tell scientists when the inner core began to form, with some suggesting that the answer is a little more than half a billion years ago (i.e., only in the last ~10% of Earth’s history).
In a new study published in the Proceedings of the National Academy of Sciences of the United States of America, paleomagnetist and University of California, Berkeley doctoral student Yiming Zhang and his coauthors collected and studied rocks from the (failed) North American Midcontinent Rift—a region where 1.1 billion years ago there was voluminous volcanism. The rocks that Zhang targeted are unique aggregations of crystals known as anorthosite xenoliths that formed deep in Earth’s crust but were brought close to the surface with magma that fed lava eruptions into the rift. The team found surprisingly high paleointensity values that signal a turbulent core—more spirited than might be expected for a liquid core lacking a solid center and stronger than Earth’s magnetic field today.
The Age of Earth’s Heart
The energy that causes the liquid core to move, or convect, comes from two different mechanisms. Thermally convecting liquid is driven by heat that’s wanting to rise and escape, said Courtney Sprain, a paleomagnetist at the University of Florida who was not involved in this study. In the other mechanism, compositional convection stirs the cauldron because of light elements. As the mostly iron inner core solidifies, it excludes lighter, more buoyant elements that proceed to rise through the liquid outer core. “We believe [that] today, that’s one of our main sources of energy driving the geodynamo,” Sprain explained.
Because Earth was much warmer billions of years ago and the inner core was not initially present, thermal convection may have been the primary driver generating the early magnetic field, said Richard Bono, a paleomagnetist at Florida State University who also was not involved in Zhang’s work. As Earth cooled, thermal convection—and the intensity of the magnetic field—should have tapered. But continued cooling eventually led to the beginnings of Earth’s solid metal heart, which should have boosted the waning magnetic field as compositional convection overtook its thermal counterpart.
This transition might have begun less than 700 million years ago (much younger than canonical estimates), according to experiments designed to determine how fast iron conducts heat at extremely high pressures and temperatures, said Zhang. However, such experiments have led to different results through different approaches, enough so that various scenarios of the age of the inner core are possible.
A 2019 paper led by Bono, in which scientists collated high-quality paleointensity data, supported this young inner core formation timeline, with the paleomagnetic field interpreted to decrease in intensity until a 565-million-year-old low, followed by a rise toward much higher values, signifying more mixing. This timeline led to the intriguing hypothesis that the inner core began to form sometime after 565 million years ago—remarkably young.
However, because older (1.14-billion-year-old) rocks have low paleointensity values, Zhang’s curiously high paleointensity data in 1.09-billion-year-old rocks could be interpreted as inner core nucleation similar to some previous estimates. “You need some really strong forces in the interior of the Earth to generate such strong [paleointensity] values,” said Zhang. If true, new explanations for later ebbs and flows of paleointensity are needed for around 565 million years ago, as well as at younger times of low to high field strength transitions. Nevertheless, these data don’t negate inner core nucleation 565 million years ago either, he said.
“If anything, this is telling us we need to start trying to understand some of the other added complexities” like plate tectonics, said Sprain. Subducting plates move through the mantle, sometimes settling into cold piles at the core-mantle boundary. Elsewhere along this boundary, buoyant plumes of hot material rise upward. These sunken slabs and upwelling plumes affect how heat escapes from the core, a process that itself affects how quickly the outer core can convect. The core’s pattern of exhaling heat changes as this geometry shifts, which could affect how the magnetic field is generated, she explained.
Snapshot or Long-Term Average?
“Our magnetic field is really crazy,” said Sprain. “It can change on timescales of seconds to millions of years.”
“When we’re trying to understand what the strength of the field is, we have to ask—how much time are we looking at, [and] how much time do we average?” said Bono. A rock that cools quickly, on the order of hundreds or thousands of years, will record a snapshot of the magnetic field. A rock that takes many tens or hundreds of thousands of years to cool smooths out the magnetic field’s short-term variation. “You really need to be looking at the time-averaged field strength” to understand what was happening in the core, he said.
In the new study, said Sprain, Zhang has data from seven sites but only one date for these rocks. Because these are very old rocks, each date’s margin of error would be on the order of 100,000 years to greater than 1 million; collecting more dates wouldn’t necessarily help resolve the relative timing between sites. “Even if there was more than 10,000 years between [multiple samples’] cooling times, we wouldn’t be able to resolve it [because] the ages would overlap,” Sprain said.
Nevertheless, the data are of high quality, and even averaging all the information together results in a higher-than-expected magnetic field for 1.1 billion years ago, confirming the findings of prior work, said Bono. In this prior work, Sprain found that slightly older volcanic rocks from the Midcontinent Rift also record a strong magnetic field similar to that on Earth today.
“What we need,” said Sprain, “is more high-quality data.” This is especially true for the Precambrian, whose rocks have had more time to endure upheavals that can erase their experiences.
—Alka Tripathy-Lang (@DrAlkaTrip), Science Writer