A new technique for measuring the Earth's magnetic field back to the days of the dinosaurs and beyond has revealed that the magnetic field was as much as three times stronger in ancient Earth than previous techniques suggested. The new method could help scientists better understand ancient Earth, including how its molten core behaved in its early days. The results of the first field test of the new technique appear in the March 2 issue of Science.
Scientists use the record of the Earth's magnetic field locked in rocks to tease out secrets of the geodynamo-the currents of molten rock that seethe beneath the Earth's crust, causing everything from earthquakes and volcanoes to the drift of the continents themselves. The Earth's magnetic field also protects us from much of the sun's dangerous radiation, so understanding how it works can help scientists predict its fluctuations and look into what effect those fluctuations could have had on the development of life on Earth.
Researchers have known that the magnetic poles have flipped several times during our planet's lifetime-meaning a compass 100,000 years ago could have pointed south instead of north. The record of the field is captured in tiny pieces of magnetic particles in new lava. The particles orient themselves just like a compass, until the lava cools around them, locking them into place. Great bands of rock displaying north-south flips are laid across the ocean floors.
"We know a lot about the directions of the Earth's magnetic field," says John Tarduno, professor of geophysics and chair of the Department Earth and Environmental Sciences at the University of Rochester and first author of the Science paper. "It's how we unravel plate tectonics and learn something about the core. But to understand the way the field works, you also need to know the field's magnitude, and we don't know nearly enough about that."
The traditional approach to measuring the ancient Earth's magnetic field strength (called paleointensity) was developed more than four decades ago, and has changed little until Tarduno's technique. In the old method, a piece of igneous rock about an inch across is heated and cooled in a chamber that is shielded from any magnetic sources. The magnetism is essentially "drained" from the magnetic particles in the rock, like siphoning water out of a jug. The researchers then "refill the jug," measuring how much magnetism the particles can hold. Two significant drawbacks result from this method, however: a piece of rock hundreds of millions of years old often becomes contaminated over time, and the process often imparts a magnetism to the rock-like water leaking into the jug before you refill it. As a consequence, very ancient samples seem to hold little magnetization, further confounding results that were already in question because of contamination.
Tarduno decided to see if he could use the University's Superconducting Quantum Interference Device (nicknamed "SQUID"), a device normally used in computing chip design, which is extremely sensitive to the tiniest magnetic fields. "With the SQUID we realized that we could start measuring single crystals instead of whole rocks," says Tarduno. "That let us use samples we knew had no contamination."
Early tests showed that feldspar, the most common mineral on the Earth's surface, worked well since it created a microscopic shell around slivers of magnetite, protecting them from contamination. Tarduno's team took samples from a 1955 lava flow in Hawaii and tried to determine if the paleointensity reading would match the actual Earth's magnetic field strength in 1955. It did. Tarduno was essentially doing the same heating/cooling test that had been done for 40 years on large samples, yet doing it on samples the size of a grain of sand, without the possibility of contamination and with much more accurate results.
"We can now measure paleointensity in places we could never measure anything before," says Tarduno. "And the results are more reliable than ever before."
With the method tested, it was time for Tarduno to see what it revealed about the magnetic field back in the days of the dinos. His team took dozens of samples from lava flows in India that were nearly 100 million years old-an unusual time in Earth's history when the field was not reversing-and found that the intensity of the field was three times stronger than the old method suggested. Besides possibly giving T-Rex a better northern lights show, the field strength gives researchers a glimpse into what the Earth's hot, molten core was doing back then.
"Our findings suggest that there is a relationship between magnetic reversals and paleointensity," says Tarduno. "Such a relationship fits very well with supercomputer models. It's an exciting time. We're really starting to understand how the heart of our planet works."
Tarduno will use the new method to plot the paleointensity of different eras in ancient Earth's past. Some of his more challenging work is in the paleointensity of rocks 2.5 billion years old-more than halfway back to Earth's very beginning. The task is especially challenging because scientists believe that the core of the Earth that controls the magnetic field was still forming.
Post doctoral student Rory Cottrell and graduate student Alexei Smirnov, both from the University of Rochester, are also authors on the Science paper.