DAYS NOW SLIGHTLY SHORTER
Chilean Quake Likely Shifted Earth’s Axis, NASA Scientist Says
BY Alex Morales / March 1, 2010
The earthquake that killed more than 700 people in Chile on Feb. 27 probably shifted the Earth’s axis and shortened the day, a National Aeronautics and Space Administration scientist said. Earthquakes can involve shifting hundreds of kilometers of rock by several meters, changing the distribution of mass on the planet. This affects the Earth’s rotation, said Richard Gross, a geophysicist at NASA’s Jet Propulsion Laboratory in Pasadena, California, who uses a computer model to calculate the effects. “The length of the day should have gotten shorter by 1.26 microseconds (millionths of a second),” Gross, said today in an e-mailed reply to questions. “The axis about which the Earth’s mass is balanced should have moved by 2.7 milliarcseconds (about 8 centimeters or 3 inches).” The changes can be modeled, though they’re difficult to physically detect given their small size, Gross said. Some changes may be more obvious, and islands may have shifted, according to Andreas Rietbrock, a professor of Earth Sciences at the U.K.’s Liverpool University who has studied the area impacted, though not since the latest temblor. Santa Maria Island off the coast near Concepcion, Chile’s second-largest city, may have been raised 2 meters (6 feet) as a result of the latest quake, Rietbrock said today in a telephone interview. He said the rocks there show evidence pointing to past earthquakes shifting the island upward in the past.
“It’s what we call the ice-skater effect,” David Kerridge, head of Earth hazards and systems at the British Geological Survey in Edinburgh, said today in a telephone interview. “As the ice skater puts when she’s going around in a circle, and she pulls her arms in, she gets faster and faster. It’s the same idea with the Earth going around if you change the distribution of mass, the rotation rate changes.” Rietbrock said he hasn’t been able to get in touch with seismologists in Concepcion to discuss the quake, which registered 8.8 on the Richter scale. “What definitely the earthquake has done is made the Earth ring like a bell,” Rietbrock said. The magnitude 9.1 Sumatran in 2004 that generated an Indian Ocean tsunami shortened the day by 6.8 microseconds and shifted the axis by about 2.3 milliarcseconds, Gross said. The changes happen on the day and then carry on “forever,” Benjamin Fong Chao, dean of Earth Sciences of the National Central University in Taiwan, said in an e-mail. “This small contribution is buried in larger changes due to other causes, such as atmospheric mass moving around on Earth,” Chao said.
SPINNING EVEN FASTER
Chilean quake shortened Earth’s day, but not by much
BY Carla Hall / March 03, 2010
When a magnitude 8.8 earthquake struck South America last weekend, the ground rumbled in Chile, the sea rose in the Pacific, and a day on Earth got shorter. Not by much. Earthlings ended up losing 1.26 millionth of a second of a day. You can’t sense it. Nor is your dog aware of it. But while other experts charted the shift of tectonic plates and the swell of ocean waters wrought by the quake, geophysicist Richard Gross mathematically calculated the temblor’s disruption of the length of the day.
The thrust-fault quake — in which plates under the Earth’s surface moved vertically — caused mass to be redistributed, said Gross, who works at the Jet Propulsion Laboratory in La Cañada Flintridge. “On average, the mass of the Earth got a bit closer to the rotation axis,” he said. As a result, Gross said, the planet rotates faster — “just like a spinning skater brings her arms in closer to her body to rotate faster.” When the planet rotates faster, the day shortens, he said. Gross studies the Earth’s rotation and how it is affected by cataclysmic forces of nature. “Anything that moves mass around on the Earth I take a look at,” he said. And it takes a mega-earthquake to attract Gross’ attention.
The magnitude 6.7 Northridge earthquake didn’t even register on the scale of throwing off the Earth’s rotation. “I didn’t look at that earthquake,” he said. “It takes something like the Chilean or Indonesian earthquake before I look at it.” This earthquake also shifted the axis around which the Earth rotates, Gross said. Although the Chilean quake shortened the day by 1.26 microseconds — the unit of time for millionths of a second — the 2004 Indian Ocean earthquake that triggered the catastrophic Asian tsunami shaved 7 microseconds off the day, according to Gross’ calculations.
Will our biological sleep clocks notice? Circadian rhythms can be affected by even a shift in minutes, said Michael Terman, director of the Center for Light Treatment and Biological Rhythms at Columbia University Medical Center. “Circadian biology . . . is indeed sensitive to the Earth’s rotation, but a change of 1.26 microseconds won’t have significant impact — I hope!” Terman said. Of course, losing just 1.26 microseconds a day takes a couple of millenniums to add up to one single second of lost time. (2,174 years to be more precise.) Gross suggests it’s not worth tallying that way. “It takes a lot of these big earthquakes to add up to even a second,” he said. Far from evoking that textbook illustration of a smooth round ball of continents and blue oceans, Gross describes Earth as a planet of unevenly distributed mass wobbling as it rotates, imperfectly balanced, around its axis, its physique woefully pear-shaped. “It’s a bit fatter south of the Equator,” Gross said. “The Earth is not completely elastic. It’s kind of like putty,” he said. “If you have a sudden shock to it, it will continue to deform later in response to that shock.”
Chilean Quake May Have Shortened Earth Days / March 01, 2010
“The Feb. 27 magnitude 8.8 earthquake in Chile may have shortened the length of each Earth day. JPL research scientist Richard Gross computed how Earth’s rotation should have changed as a result of the Feb. 27 quake. Using a complex model, he and fellow scientists came up with a preliminary calculation that the quake should have shortened the length of an Earth day by about 1.26 microseconds (a microsecond is one millionth of a second). Perhaps more impressive is how much the quake shifted Earth’s axis. Gross calculates the quake should have moved Earth’s figure axis (the axis about which Earth’s mass is balanced) by 2.7 milliarcseconds (about 8 centimeters, or 3 inches). Earth’s figure axis is not the same as its north-south axis; they are offset by about 10 meters (about 33 feet).
By comparison, Gross said the same model estimated the 2004 magnitude 9.1 Sumatran earthquake should have shortened the length of day by 6.8 microseconds and shifted Earth’s axis by 2.32 milliarcseconds (about 7 centimeters, or 2.76 inches). Gross said that even though the Chilean earthquake is much smaller than the Sumatran quake, it is predicted to have changed the position of the figure axis by a bit more for two reasons. First, unlike the 2004 Sumatran earthquake, which was located near the equator, the 2010 Chilean earthquake was located in Earth’s mid-latitudes, which makes it more effective in shifting Earth’s figure axis. Second, the fault responsible for the 2010 Chiliean earthquake dips into Earth at a slightly steeper angle than does the fault responsible for the 2004 Sumatran earthquake. This makes the Chile fault more effective in moving Earth’s mass vertically and hence more effective in shifting Earth’s figure axis. Gross said the Chile predictions will likely change as data on the quake are further refined.”
email : richard.gross [at] jpl.nasa [dot] gov
(THAT SAID : ROTATION IS MUCH MORE AFFECTED BY WIND)
Richard Gross, geophysicist at NASA’s Jet Propulsion Laboratory, explains / January 17, 2005
Q: Did the undersea earthquake affect the earth’s rotation?
A: Models predict that the earthquake should have affected rotation of the earth by shortening the length of a day by about three microseconds, or three millionths of a second. This happens because during the earthquake one of the tectonic plates [the India plate] subducted down beneath another plate [the Burma plate]. The downward mass movement of the plate changed the earth’s rotation just like a spinning ice skater bringing her arms closer to her body increases her rotation. When the earth spins faster, the days are shorter.
Q: Has this shift been measured?
A: This rotation change is a prediction from a model, and the data [collected by ground- and space-based position sensors] is being analyzed to see if the predicted change actually occurred. The data comes in every day, but it will take a few weeks for the most accurate data to be received and analyzed.
Q: Is this change permanent, or will it shift again?
A: The length of the day changes all the time in response to many different processes such as changes in the atmospheric winds or ocean currents. Changes in winds have by far the greatest effect on the length of the day: their effect is actually about 300 times larger than that predicted to be changed by this earthquake.
Q: Did the tilt of the earth’s axis change as well?
A: The earth wobbles as it rotates because its mass is not balanced about its rotation axis, just like a tire on a car will wobble as it rotates if the tire is not perfectly balanced. The size of the planet’s wobble is usually about 33 feet. As the India plate subducted beneath the Burma plate, the mass of Earth was rearranged, not only causing the speed of rotation to change, which causes the length of the day to change, but also causing the wobbling motion of the planet to change by about an inch. The wobble is also affected by other influences, such as changes in atmospheric pressure.
MEANWHILE : MAGNETIC POLE MOVING FAST
North Magnetic Pole Is Shifting Rapidly Toward Russia
BY Brian Vastag / December 15, 2005
New research shows the pole moving at rapid clip—25 miles (40 kilometers) a year. Over the past century the pole has moved 685 miles (1,100 kilometers) from Arctic Canada toward Siberia, says Joe Stoner, a paleomagnetist at Oregon State University. At its current rate the pole could move to Siberia within the next half-century, Stoner said. “It’s moving really fast,” he said. “We’re seeing something that hasn’t happened for at least 500 years.” Stoner presented his team’s research at the American Geophysical Union’s meeting last week in San Francisco. Lorne McKee, a geomagnetic scientist at Natural Resources Canada, says that Stoner’s data fits his own readings. “The movement of the pole definitely appears to be accelerating,” he said.
Not a Reversal
The shift is likely a normal oscillation of the Earth’s magnetic field, Stoner said, and not the beginning of a flip-flop of the north and south magnetic poles, a phenomenon that last occurred 780,000 years ago. Such reversals have taken place 400 times in the last 330 million years, according to magnetic clues sealed in rocks around the world. Each reversal takes a thousand years or more to complete. “People like to think something special is happening in their lifetimes, but despite the dramatic changes, I don’t see any evidence of it,” Stoner said. “It’s probably just a normal wandering of the pole.” The north magnetic pole shifts constantly, in loops up to 80 kilometers (50 miles) wide each day. The recorded location of the pole is really an average of its daily treks, which are driven by fluctuations in solar radiation. The pole is currently at about 80º north latitude and 104º west longitude, in the Canadian territory of Nunavut.
Importance of the Pole
Pinpointing the precise location of the north magnetic pole is important for navigation: As you move closer to the pole, the direction north indicated by your compass becomes less accurate. The pole also plays a role in the Northern Lights, which form when solar radiation bounces across the magnetic field in the upper atmosphere. As the north magnetic pole drifts, it will take the Northern Lights with it. But for scientists, studying the field provides a tantalizing glimpse into the fiery center of the Earth. The planet’s outer core of molten iron spins constantly, acting as a giant dynamo, or electromagnet. This energy interacts with the rocky mantle of the Earth, which is also shifting, resulting in a complex, ever-changing magnetic field. “We’re close to having a much better understanding on how the field fluxes,” Stoner said.
The first readings of the north magnetic pole date to 1831, when Sir John Ross and his ship searching for the Northwest Passage became ice-bound. To pass the time he sent out a team with a compass to take readings, and the team soon found a dipole—an area with compass readings pointing both north and south—in what is now Nunavut. It was the north magnetic pole. While historical readings date back almost two centuries, Stoner’s team wanted to take a deeper look into the past. They went to the Arctic and pulled 4.5-meter-long (15-foot-long) cores of mud and clay from the bottom of frigid lakes. Each year, snowmelt deposits a layer of silt at the bottom of the lakes, which is then covered with a layer of clay. “There are these distinct couplets every year,” Stoner said. “It’s a lot like counting rings in a tree.”
Back at his laboratory at Oregon State University, Stoner and his team sliced the cores into thin sections. They then ran each section through an instrument that reads tiny magnetic particles in the silt to reveal both the direction and intensity of the magnetic field. Each section comprises five to ten layers, or five to ten year’s worth of magnetic readings. “We can’t get down to the yearly scale yet,” Stoner said, “but that’s getting to be a pretty tight resolution.” In contrast, similar techniques used to measure magnetism in rock have yielded much coarser resolutions of thousands to tens of thousands of years. Besides recording the movement of the pole, the silt cores also show a recent drop in the strength of the magnetic field, Stoner said, a phenomenon that often accompanies north-south reversals. But research by French scientists published in 2003 suggests that such “jerks” in the magnetic field—abrupt shifts in intensity and direction—occur often, not just during reversals.
North Magnetic Pole Moving East Due to Core Flux
BY Richard A. Lovett / December 24, 2009
Earth’s north magnetic pole is racing toward Russia at almost 40 miles (64 kilometers) a year due to magnetic changes in the planet’s core, new research says. The core is too deep for scientists to directly detect its magnetic field. But researchers can infer the field’s movements by tracking how Earth’s magnetic field has been changing at the surface and in space. Now, newly analyzed data suggest that there’s a region of rapidly changing magnetism on the core’s surface, possibly being created by a mysterious “plume” of magnetism arising from deeper in the core. And it’s this region that could be pulling the magnetic pole away from its long-time location in northern Canada, said Arnaud Chulliat, a geophysicist at the Institut de Physique du Globe de Paris in France.
Magnetic north, which is the place where compass needles actually point, is near but not exactly in the same place as the geographic North Pole. Right now, magnetic north is close to Canada’s Ellesmere Island. Navigators have used magnetic north for centuries to orient themselves when they’re far from recognizable landmarks. Although global positioning systems have largely replaced such traditional techniques, many people still find compasses useful for getting around underwater and underground where GPS satellites can’t communicate. The magnetic north pole had moved little from the time scientists first located it in 1831. Then in 1904, the pole began shifting northeastward at a steady pace of about 9 miles (15 kilometers) a year. In 1989 it sped up again, and in 2007 scientists confirmed that the pole is now galloping toward Siberia at 34 to 37 miles (55 to 60 kilometers) a year. A rapidly shifting magnetic pole means that magnetic-field maps need to be updated more often to allow compass users to make the crucial adjustment from magnetic north to true North.
Geologists think Earth has a magnetic field because the core is made up of a solid iron center surrounded by rapidly spinning liquid metal. This creates a “dynamo” that drives our magnetic field. Scientists had long suspected that, since the molten core is constantly moving, changes in its magnetism might be affecting the surface location of magnetic north. Although the new research seems to back up this idea, Chulliat is not ready to say whether magnetic north will eventually cross into Russia. “It’s too difficult to forecast,” Chulliat said. Also, nobody knows when another change in the core might pop up elsewhere, sending magnetic north wandering in a new direction.
email : chulliat [at] ipgp.jussieu [dot] fr
Supercomputer models of Earth’s magnetic field. On the left is a normal dipolar magnetic field, typical of the long years between polarity reversals. On the right is the sort of complicated magnetic field Earth has during the upheaval of a reversal.
Considering that ships, planes and Boy Scouts steer by it, Earth’s magnetic field is less reliable than you’d think. Rocks in an ancient lava flow in Oregon suggest that for a brief erratic span about 16 million years ago magnetic north shifted as much as 6 degrees per day. After little more than a week, a compass needle would have pointed toward Mexico City. The lava catches Earth’s magnetic field in the act of reversing itself. Magnetic north heads south, and — over about 1,000 years — the field does a complete flip-flop. While the Oregon data is controversial, Earth scientists agree that the geological evidence as a whole — the “paleomagnetic” record — proves such reversals happened many times over the past billion years. “Some reversals occurred within a few 10,000 years of each other,” says Los Alamos scientist Gary Glatzmaier, “and there are other periods where no reversals occurred for tens of millions of years.” How do these flip-flops happen, and why at such irregular intervals? The geological data, invaluable to show what happened, registers only a mute shrug when it comes to the deeper questions.
For that matter, why is it that instead of quietly fading away, as magnetic fields do when left to their own devices, Earth’s magnetic field is still going strong after billions of years? Einstein is said to have considered it one of the most important unsolved problems in physics. With a year of computing on Pittsburgh’s CRAY C90, 2,000 hours of processing, Glatzmaier and collaborator Paul Roberts of UCLA took a big step toward some answers. Their numerical model of the electromagnetic, fluid dynamical processes of Earth’s interior reproduced key features of the magnetic field over more than 40,000 years of simulated time. To top it off, the computer-generated field reversed itself. “We weren’t expecting it,” says Roberts, “and were delighted. This gives us confidence we’ve built a credible bridge between theory and the paleomagnetic data.” Their surprising results, reported as a cover story in Nature (Sept. 21, 1995), provide an inner-Earth view of geomagnetic phenomena that have not been observed or anticipated by theory. Furthermore, the Glatzmaier-Roberts model offers, for the first time, a coherent explanation of magnetic field reversal.
Journey to the Center of the Earth
Roughly speaking, Earth is like a chocolate-covered cherry — layered, with liquid beneath the surface and a solid inner core. Beneath the planet’s relatively thin crust is a thick, solid layer called the mantle. Between the mantle and the inner core is a fluid layer, the outer core. According to generally accepted theory — the dynamo theory — interactions between the churning, twisting flow of molten material in the outer core and the magnetic field generate electrical current that, in turn, creates new magnetic energy that sustains the field. “The typical lifetime of a magnetic field like Earth’s,” says Glatzmaier, “is several tens of thousands of years. The fact that it’s existed for billions of years means something must be regenerating it all the time.” How do we know if the dynamo theory is right? To the consternation of our desire to understand what’s happening inside the planet we live on, Jules Verne’s Journey to the Center of the Earth is still fiction. There’s no way to penetrate 4,000 miles to Earth’s center, nor to monitor fluid motions or magnetism in the outer core.
The Glatzmaier-Roberts computational model may be the next best thing to a guided tour of inner Earth. While other models have given good clues that theory is on track, they have been limited by a two-dimensional approach that required simplifying assumptions. Roberts and Glatzmaier set out to implement a fully three-dimensional model, based on a computer program Glatzmaier developed over many years, that would allow the complex feedbacks between fluid motion and the magnetic field to evolve on their own — in other words, to be solved “self consistently.”
Their objectives, in retrospect, were modest. “Mainly,” says Roberts, “we wanted to get a geomagnetic field that would maintain itself longer than the decay time. No one’s ever done that in a self-consistent manner.” After nearly a year running almost daily, as allocated computing time was about to expire, the model produced its Eureka moment. By itself, the reversal is strong confirmation of the model, and other details — magnitude and structure of the field — also agree well with surface features of Earth’s field. The simulation also offers precious insight into the dynamics that sustain the magnetic field and generate reversals. Contrary to what anyone guessed till now, the model shows that in the inner core the magnetic field has an opposite polarity from the outer core, and this stabilizes the field against a tendency to reverse more frequently. “No one even dreamed about this,” says Glatzmaier. “That’s the nice thing about a supercomputer. You can just let it do its thing, solve these equations over and over — a large set of variables affecting each other with nonlinear feedback, very hard to figure out. It’s a beautiful problem for a supercomputer, and it’s really exciting to see this structure and dynamics that no one imagined.”
email : glatz [at] es.ucsc [dot] edu
email : roberts [at] math.ucla [dot] edu
Earth’s Inconstant Magnetic Field / December 29, 2003
Every few years, scientist Larry Newitt of the Geological Survey of Canada goes hunting. He grabs his gloves, parka, a fancy compass, hops on a plane and flies out over the Canadian arctic. Not much stirs among the scattered islands and sea ice, but Newitt’s prey is there–always moving, shifting, elusive. His quarry is Earth’s north magnetic pole. At the moment it’s located in northern Canada, about 600 km from the nearest town: Resolute Bay, population 300, where a popular T-shirt reads “Resolute Bay isn’t the end of the world, but you can see it from here.” Newitt stops there for snacks and supplies–and refuge when the weather gets bad. “Which is often,” he says.
Scientists have long known that the magnetic pole moves. James Ross located the pole for the first time in 1831 after an exhausting arctic journey during which his ship got stuck in the ice for four years. No one returned until the next century. In 1904, Roald Amundsen found the pole again and discovered that it had moved–at least 50 km since the days of Ross. The pole kept going during the 20th century, north at an average speed of 10 km per year, lately accelerating “to 40 km per year,” says Newitt. At this rate it will exit North America and reach Siberia in a few decades.
Keeping track of the north magnetic pole is Newitt’s job. “We usually go out and check its location once every few years,” he says. “We’ll have to make more trips now that it is moving so quickly.” Earth’s magnetic field is changing in other ways, too: Compass needles in Africa, for instance, are drifting about 1 degree per decade. And globally the magnetic field has weakened 10% since the 19th century. When this was mentioned by researchers at a recent meeting of the American Geophysical Union, many newspapers carried the story. A typical headline: “Is Earth’s magnetic field collapsing?”
Probably not. As remarkable as these changes sound, “they’re mild compared to what Earth’s magnetic field has done in the past,” says University of California professor Gary Glatzmaier. Sometimes the field completely flips. The north and the south poles swap places. Such reversals, recorded in the magnetism of ancient rocks, are unpredictable. They come at irregular intervals averaging about 300,000 years; the last one was 780,000 years ago. Are we overdue for another? No one knows.
According to Glatzmaier, the ongoing 10% decline doesn’t mean that a reversal is imminent. “The field is increasing or decreasing all the time,” he says. “We know this from studies of the paleomagnetic record.” Earth’s present-day magnetic field is, in fact, much stronger than normal. The dipole moment, a measure of the intensity of the magnetic field, is now 8 × 1022 amps × m2. That’s twice the million-year average of 4× 1022 amps × m2. To understand what’s happening, says Glatzmaier, we have to take a trip … to the center of the Earth where the magnetic field is produced.
At the heart of our planet lies a solid iron ball, about as hot as the surface of the sun. Researchers call it “the inner core.” It’s really a world within a world. The inner core is 70% as wide as the moon. It spins at its own rate, as much as 0.2° of longitude per year faster than the Earth above it, and it has its own ocean: a very deep layer of liquid iron known as “the outer core.” Earth’s magnetic field comes from this ocean of iron, which is an electrically conducting fluid in constant motion. Sitting atop the hot inner core, the liquid outer core seethes and roils like water in a pan on a hot stove. The outer core also has “hurricanes”–whirlpools powered by the Coriolis forces of Earth’s rotation. These complex motions generate our planet’s magnetism through a process called the dynamo effect.
Using the equations of magnetohydrodynamics, a branch of physics dealing with conducting fluids and magnetic fields, Glatzmaier and colleague Paul Roberts have created a supercomputer model of Earth’s interior. Their software heats the inner core, stirs the metallic ocean above it, then calculates the resulting magnetic field. They run their code for hundreds of thousands of simulated years and watch what happens. What they see mimics the real Earth: The magnetic field waxes and wanes, poles drift and, occasionally, flip. Change is normal, they’ve learned. And no wonder. The source of the field, the outer core, is itself seething, swirling, turbulent. “It’s chaotic down there,” notes Glatzmaier. The changes we detect on our planet’s surface are a sign of that inner chaos.
They’ve also learned what happens during a magnetic flip. Reversals take a few thousand years to complete, and during that time–contrary to popular belief–the magnetic field does not vanish. “It just gets more complicated,” says Glatzmaier. Magnetic lines of force near Earth’s surface become twisted and tangled, and magnetic poles pop up in unaccustomed places. A south magnetic pole might emerge over Africa, for instance, or a north pole over Tahiti. Weird. But it’s still a planetary magnetic field, and it still protects us from space radiation and solar storms. And, as a bonus, Tahiti could be a great place to see the Northern Lights. In such a time, Larry Newitt’s job would be different. Instead of shivering in Resolute Bay, he could enjoy the warm South Pacific, hopping from island to island, hunting for magnetic poles while auroras danced overhead. Sometimes, maybe, a little change can be a good thing.
Magnetic stripes around mid-ocean ridges reveal the history of Earth’s magnetic field for millions of years. The study of Earth’s past magnetism is called paleomagnetism.
Lava flows reveal clues to magnetic field reversals
BY Jill Sakai / Sept. 25, 2008
Ancient lava flows are guiding a better understanding of what generates and controls the Earth’s magnetic field — and what may drive it to occasionally reverse direction. The main magnetic field, generated by turbulent currents within the deep mass of molten iron of the Earth’s outer core, periodically flips its direction, such that a compass needle would point south rather than north. Such polarity reversals have occurred hundreds of times at irregular intervals throughout the planet’s history — most recently about 780,000 years ago — but scientists are still trying to understand how and why.
A new study of ancient volcanic rocks, reported in the Sept. 26 issue of the journal Science, shows that a second magnetic field source may help determine how and whether the main field reverses direction. This second field, which may originate in the shallow core just below the rocky mantle layer of the Earth, becomes important when the main north-south field weakens, as it does prior to reversing, says Brad Singer, a geology professor at the University of Wisconsin-Madison. Singer teamed up with paleomagnetist Kenneth Hoffman, who has been researching field reversals for over 30 years, to analyze ancient lava flows from Tahiti and western Germany in order to study past patterns of the Earth’s magnetic field. The magnetism of iron-rich minerals in molten lava orients along the prevailing field, then becomes locked into place as the lava cools and hardens. “When the lava flows erupt and cool in the Earth’s magnetic field, they acquire a memory of the magnetic field at that time,” says Singer. “It’s very difficult to destroy that in a lava flow once it’s formed. You then have a recording of what the paleofield direction was like on Earth.”
Hoffman, of both California Polytechnic State University at San Luis Obispo and UW-Madison, and Singer are focusing on rocks that contain evidence of times that the main north-south field has weakened, which is one sign that the polarity may flip direction. By carefully determining the ages of these lava flows, they have mapped out the shallow core field during multiple “reversal attempts” when the main field has weakened during the past million years. During those periods of time, weakening of the main field reveals “virtual poles,” regions of strong magnetism within the shallow core field. For example, Singer says, “If you were on Tahiti when those eruptions were taking place, your compass needle would point to not the North Pole, not the South Pole, but Australia.” The scientists believe the shallow core field may play a role in determining whether the main field polarity flips while weakened or whether it recovers its strength without reversing. “Mapping this field during transitional states may hold the key to understanding what happens in Earth’s core when the field weakens to a point where it can actually reverse,” Hoffman says.
Current evidence suggests we are now approaching one of these transitional states because the main magnetic field is relatively weak and rapidly decreasing, he says. While the last polarity reversal occurred several hundred thousand years ago, the next might come within only a few thousand years. “Right now, historic records show that the strength of the magnetic field is declining very rapidly. From a quick back-of-the-envelope prediction, in 1,500 years the field will be as weak as it’s ever been and we could go into a state of polarity reversal,” says Singer. “One broad goal of our research is to provide some predictive capability for what could happen and what could be the signs of the next reversal.”
email : bsinger [at] geology.wisc [dot] edu
email : khoffman [at] geology.wisc [dot] edu
SHIPS’ LOGS + MAGNETIC VARIATION
Ships’ logs give clues to Earth’s magnetic decline
BY Patrick Barry / 11 May 2006
The voyages of Captain Cook have just yielded a new discovery: the gradual weakening of Earth’s magnetic field is a relatively recent phenomenon. The discovery has led experts to question whether the Earth is on track towards a polarity reversal. By sifting through ships’ logs recorded by Cook and other mariners dating back to 1590, researchers have greatly extended the period over which the behaviour of the magnetic field can be studied. The data show that the current decline in Earth’s magnetism was virtually negligible before 1860, but has accelerated since then. Until now, scientists had only been able to trace the magnetic field’s behaviour back to 1837, when Carl Friedrich Gauss invented the first device for measuring the field directly.
The field’s strength is now declining at a rate that suggests it could virtually disappear in about 2000 years. Researchers have speculated that this ongoing change may be the prelude to a magnetic reversal, during which the north and south magnetic pole swap places. But the weakening trend could also be explained by a growing magnetic anomaly in the southern Atlantic Ocean, and may not be the sign of a large scale polarity reversal, the researchers suggest.
David Gubbins, an expert in geomagnetism at the University of Leeds, UK, led the study which began scouring old ships’ logs in the 1980s, gathering log entries recording the direction of Earth’s magnetic field. It was common practice for captains in the 17th and 18th centuries to calibrate their ship’s compasses relative to true north and, less often, to measure the steepness at which magnetic field lines entered the Earth’s surface. Even as far back as 1590, these measurements were typically very accurate – to within half a degree. “Their lives depended on it,” Gubbins explains. Such ship-log records may not be adequate for reconstructing the planet’s past magnetic fields in fine detail, but the data can estimate large-scale features quite well. “In that regard, I think it’s a very solid result,” says Catherine Constable, an expert in palaeomagnetism at the University of California, San Diego, US, who was not involved in the study.
Using the locations of the ships at the time of measurement, these records allowed Gubbins to construct a map of the relative strength of Earth’s magnetic field between 1590 and 1840, which was published in 2003. The data was combined with 315 estimates of the field’s overall strength during that period, based on indirect clues, such as mineral evidence in bricks from old human settlements or volcanic rock. Gubbins showed that the overall strength of the planet’s magnetic field was virtually unchanged between 1590 and 1840. Since then, the field has declined at a rate of roughly 5% per 100 years. Every 300,000 years on average, the north and south poles of the Earth’s magnetic field swap places. The field must weaken and go to zero before it can reverse itself. The last such reversal occurred roughly 780,000 years ago, so we are long overdue for another magnetic flip. Once it begins, the process of reversing takes less than 5000 years, experts believe.
A large-scale reversal might indeed be underway, Gubbins says, but the acceleration of the magnetic decline since the mid-1800s is probably due to a local aberration of the magnetic field called the South Atlantic Anomaly. “It looks like that’s responsible for most of the fall we’re seeing,” he says. This patch of reversed magnetic field lines covering much of South America first appeared in about 1800, according to the ship-log data. It slowly grew in strength, and by about 1860 it was large enough to affect the overall strength of the planet’s magnetic field, Gubbins says. If the field does flip 2000 years from now, the Northern Lights will be visible all over the planet during the transition, and solar radiation at ground level will be much more intense, with no field to deflect it. There is no need to worry, though, argues Gubbins, as our ancestors have lived through quite a few of these transitions already.
email : d.gubbins [at] earth.leeds.ac [dot] uk
BUILD YR OWN
Building a Baby Earth to Test Its Magnetic Field
BY David Kestenbaum / June 2, 2008
The compass has been around since at least the 12th century, but scientists still don’t know exactly how the Earth generates the magnetic field that keeps a compass needle pointing north. But geophysicist Dan Lathrop is trying to find out — by building his own planet. His latest effort at the University of Maryland towers over him, a massive stainless steel sphere that looks like a prop from some old science fiction movie. Later this year he plans to fill it with molten metal and set the whole 26-ton ball spinning. At top speed the equator will whirl by at 80 miles an hour. “It was a little scary the first time we spun it up,” he says. If all goes well, the planet will generate its own magnetic field.
Big Aspirations … and Getting Bigger
Lathrop figures it can’t be too hard to get a magnetic field — after all, most planets in our solar system have one. But while nature has an easy time making magnetic fields, scientists do not. This is Lathrop’s third attempt. “Planets have an advantage because of their size,” he says, “and they’re much more rapidly rotating.” That’s why his quest has taken on gargantuan proportions. Lathrop’s first “planet” was the size of a desktop globe. The second one you could wrap your arms around. The newest one had to be ordered from a company in Ohio that makes heavy-duty items for industry, and the sphere barely fit through the door of the lab when it arrived.
Scientists believe the Earth’s field comes from molten metal churning deep inside its core. If you could dig a deep hole, about 2,000 miles down, you would hit the outer core, which is probably made of liquid iron. That iron can conduct electricity. And if it flows in the right way, it can turn the Earth into what scientists call a dynamo, generating a self-sustaining magnetic field — in Earth’s case, producing one pole up in Canada and another down in Antarctica. Iron only melts at high temperatures, though, so Lathrop’s team will fill his sphere with a different metal — sodium. Sodium becomes liquid at stovetop temperatures and conducts electricity well, but it’s flammable. A sodium fire can’t just be put out with water. Water can actually make things worse — Lathrop’s team has disabled the sprinkler system.
Earth’s Internal Turbulence
If all this seems like a potentially hazardous way to answer an apparently simple question, Lathrop says his team doesn’t have a lot of choice. It’s simply impossible to drill down into the Earth and see what’s going on there. “The conditions of the core are more hostile than the surface of the sun,” he says. “It’s as hot as the surface of the sun but under extremely high pressures. So there’s no way to probe it, no imaginable technique to directly probe the core.” It’s unlikely the iron in the core is flowing in a nice little circle. Lathrop expects a kind of organized turbulence, small eddies and whirls with some overarching larger-scale order and patterns. “The Earth’s magnetic field is anything but simple,” he says, pointing out that the field has flipped directions many times over the ages. Seen over long stretches of time it’s a writhing, spitting thing. The north pole currently wanders around in Canada, and the field’s strength is waning, he says, as if it were readying for a flip. “Every time we go to measure it, it’s different than it was before,” lathrop says. Computer models can’t capture all the complexity of the flow. The whole system likely feeds back on itself — the flowing metal creates a magnetic field, but that field can then exert forces on the iron, changing its flow. This, in turn, can change the field, which will change the flow. Other planet scientists have built physical models that create magnetic fields, but with the help of pipes or other devices to steer the flow. Lathrop wants to let the metal slosh around freely as it does in the Earth.
After some safety checks, the metal planet is ready for a test drive, without the sodium metal. “Shall we spin?” Lathrop shouts across the lab. One of his colleagues throws a switch and the sphere begins to rotate, slowly at first like a subway car gathering momentum. Then it’s whirling, quietly and stately, its ribbed features sliding by again and again in merry-go-round fashion. Lathrop says this model matches the Earth in some important ways, so he’s hopeful it will create a field. If it doesn’t? “Well,” he says, “then it will teach us something about when planets do and don’t make a magnetic field.” He’s not sure he would make a another larger one, though, if this doesn’t work. “I’m a patient man,” he says, “but not infinitely patient.” Lathrop’s team hopes to get the planet up and running for real later this year.
Daniel Perry Lathrop
email : lathrop [at] umd [dot] edu