In discussions of the movements of the poles of Earth's rotational axis, it is common for geoscientists to distinguish between the long-term drift of one or another pole and the wobble of that pole. Because we are located in the northern hemisphere, we'll examine what is currently known about the drift and wobble of Earth's north pole.
R. Gross and J. Vondrák (1999, Geophys. Research Ltrs., v. 26) analyzed astrometric and space-geodetic observations of polar wander. They conclude that during 1900 to 1992, the Earth's rotation pole has drifted toward the 79.2 ± 0.2° W longitude; that is, roughly toward Hudson Bay. The speed of this drift approximates 33 ft per century and is due, presumably, to “some sort of mass redistribution” within the Earth (Kerr, R., 1987, Science, v. 236, p. 147). Thus, we are currently undergoing pole shift, although at a very slow rate.
It seems counter-intuitive, but it turns out that certain kinds of earthquakes also affect the global drift of Earth's spin axis. In fact, according to L Alfonsi and G. Spada (1998, Jour. Geophys. Research, v. 103, no. B4), these earthquakes “preferentially drive the rotation axis toward a well-defined direction (i.e., ~ 140° E [or toward Tokyo, Japan]) and systematically induce negative variations of the Earth's oblatness.” The authors “also find that the trend of the pole toward 140° E is essentially due to the seismicity which characterizes the western Pacific subduction zones.” Oddly, then, the drift of the North Pole toward Hudson Bay of about 33 ft per century mentioned above was influenced to a certain degree by subduction earthquakes in the western Pacific, which tended - however weakly - to push the drifting pole in roughly the opposite direction.
This brief evaluation of what is known about the drift of the north pole of rotation is made to alert us to the difficulties of extrapolating historical observations of pole drift to the causative mechanisms for that drift and to projections of where pole motion might lead in the future.
The following press release was sent out in July, 2000, by the American Geophysical Union (AGU), a society composed of over 35,000 geophysical scientists from 115 countries.
A Mystery Of Earth's Wobble Solved: It's The Ocean
“WASHINGTON - The century old mystery of Earth's Chandler wobble' has been solved by a scientist at NASA's Jet Propulsion Laboratory in Pasadena, California. The Chandler wobble, named for its 1891 discoverer, Seth Carlo Chandler, Jr., an American businessman turned astronomer, is one of several wobbling motions exhibited by the Earth as it rotates on its axis, much as a top wobbles as it spins.
Scientists have been particularly intrigued by the Chandler wobble, since its cause has remained a mystery even though it has been under observation for over a century. Its period is only around 433 days, or just 1.2 years, meaning that it takes that amount of time to complete one wobble. The amplitude of the wobble amounts to about 20 feet at the North Pole. It has been calculated that the Chandler wobble would be damped down, or reduced to zero, in just 68 years, unless some force were constantly acting to reinvigorate it.
But what is that force, or excitation mechanism? Over the years, various hypotheses have been put forward, such as atmospheric phenomena, continental water storage (changes in snow cover, river runoff, lake levels, or reservoir capacities), interaction at the boundary of Earth's core and its surrounding mantle, and earthquakes.
Writing in the August 1 issue of Geophysical Research Letters, Richard S. Gross of NASA's Jet Propulsion Laboratory reports that the principal cause of the Chandler wobble is fluctuating pressure on the bottom of the ocean, caused by temperature and salinity changes and wind-driven changes in the circulation of the oceans. He determined this by applying numerical models of the oceans, which have only recently become available through the work of other researchers, to data on the Chandler wobble obtained during the years 1985-1995. Gross calculated that two-thirds of the Chandler wobble is caused by ocean-bottom pressure changes and the remaining one-third by fluctuations in atmospheric pressure. He says that the effect of atmospheric winds and ocean currents on the wobble was minor.
Gross credits the wide distribution of the data that underlay his calculations to the creation in 1988 of the International Earth Rotation Service, which is based in Paris, France. Through its various bureaus, he writes, IERS enables the kind of interdisciplinary research that led to his solution of the Chandler wobble mystery. Gross's research was supported by NASA's Office of Earth Science.”
The angles that characterize the direction of the rotational pole within the Earth are called the polar coordinates, x and y. Variation in the values of these coordinates is called polar motion. The polar coordinates measure the position of the Earth's instantaneous pole of rotation in a reference frame defined by the adopted locations of terrestrial observatories. The coordinate x is measured along the 0° (Greenwich) meridian while the coordinate y is measured along the 90° W meridian. These two coordinates determine the directions on a plane onto which the polar motion is projected.
Polar motion consists largely of two motions, an annual elliptical component and a Chandler circular component with a period of about 435 days. These two motions describe most of the spiral motion of the pole as seen from the Earth.
The animation below was constructed by Webmaster Jonathan Eagle. It shows the position of the Earth's north pole of rotation between January 1, 1998, and the present, as if one were looking directly down from above at the North Pole. The circular motion describes the Chandler wobble for the last 70+ months, with each frame adding 30 days worth of motion to the trace. As the motion of the pole accelerates the blue trace stretches out. As the motion slows down, the trace shortens.
The animation will be updated monthly as a Members' benefit. Meaningful deviations in the wobble will be analyzed and reported as quickly as they have been computed. No significant deviations have been noted to date.
While some other Cayce pole-shift enthusiasts are giving great weight to the apparent motion deviation in the polar motion trace starting around the beginning of 1999, it would be well if they looked more closely at past polar motions to see what other deviations might be apparent. For instance, the trace made by the polar motion from early 1973 to the end of 1975 is particularly startling. As the animations show, in February of 1973 the polar motion does a sharp, almost 90°, turn to the south roughly parallel to the 150° W meridian causing the motion to arc more sharply than normal during the southerly part of it's track. In November and December of that year it displays some more anomalous jogs, but in May 1974 things get really bizarre when all polar motion comes to a complete standstill and then reverses direction! In September the polar motion almost completely reverses once again, this time causing the polar motion to travel 90° to it's normal circular path. In January 1975, when the pole's motion looks as if it is about to bisect it's normal path and head into uncharted territory, it does another hard 90° turn which gets it moving roughly tangentially to it's normal course. Again in April 1975 it performs another 90° turn, continuing to make smaller corrections to it's motion throughout the summer. Finally, in October 1975 it settles down and resumes it normal course.
One thing that is readily apparent from examining all the available polar motion data for the last three decades is that the polar motion has tended to stay confined to a circular area centered approximately at 0.3 arc-seconds (about 30 feet on the charts) west of 90° N and confined to a circular area of about the same radius. At the current time there is no indication that the polar motion, dubbed the Chandler Wobble, is deviating in any way from it's normal course. However, if the poles does start to shift, as is predicted in the Cayce Readings, it will be readily apparent here when our updated Polar Motion trace leaves it's well defined area of activity.
“Berkeley - Millimeter deviations from the expected wobble of the Earth's axis are giving geophysicists clues to what happens 1,800 miles underground, at the boundary between the Earth's mantle and its iron core.
A new theory proposes that iron-rich sediments are floating to the top of the Earth's core and sticking like gum to the bottom of the mantle, creating drag that throws the Earth's wobble off by a millimeter or two over a period of about 18.6 years.
The wobble is explained by metal patches attached to the core-mantle boundary,' explained Raymond Jeanloz, professor of geology and planetary science at the University of California, Berkeley. As the outer core turns, its magnetic field lines are deflected by the patches and the core fluid gets slowed down, just like mountains rubbing against the atmosphere slows the Earth down.'
The theory, first proposed by Bruce A. Buffett of the Department of Earth and Ocean Sciences at the University of British Columbia, also explains a peculiar slowing of seismic waves that ripple along the core-mantle boundary.
Buffett laid out the theory at the December meeting of the American Geophysical Union and in an article with Jeanloz and former UC Berkeley post-doctoral fellow Edward J. Garnero, now at Arizona State University's Department of Geological Sciences in Tempe, in the Nov. 17 issue of Science. Much of the work was done while Buffett was on sabbatical at UC Berkeley.
The wobble values that the theory explains have been adopted by the International Astronomical Union as its standard for calculating the position of the Earth's axis into the past as well as the future.
As the Earth spins on its axis the moon and sun tug on its bulging equator and create a large wobble or precession, producing the precession of the equinoxes with a period of 25,800 years. Other periodic processes in the solar system nudge the Earth, too, creating small wobbles - called nutations - in the wobble. The principal components of the nutation are caused by the Earth's annual circuit of the sun and the 18.6 year precession of the moon's orbit.
While these nutations have been known for many years, extremely precise geodetic measurements of the pointing direction of the Earth's axis have turned up unexplained deviations from the predicted nutation.
An annual deviation that lagged behind the tidal pull of the sun first suggested to Buffett 10 years ago that strange processes may be going on at the boundary between the mantle, made up of viscous rock that extends 1,800 miles below the crust, and the outer core, which is thought to be liquid iron with the consistency of water. The inner core, made of very pure, solid iron, rotates along with the outer core, dragging the Earth's magnetic field with them.
The Earth is getting pulled and tugged at regular periods, but we observe a difference in the way the Earth responds to these tugs and pulls and what we predict,' Buffett said. ?One of the ways you could explain that is by having some dissipation in the vicinity of the core-mantle boundary as the fluid moves back and forth relative to the mantle. But the viscosity of the fluid core is comparable to water, and having water slosh back and forth relative to a rigid mantle wasn't going to produce the kinds of dissipation we needed to see.'
He hit on another way the rotating core could dissipate energy: via electrical drag.
Based on experiments Jeanloz had performed on the chemistry of rocks at the high temperatures and pressures characteristic of the core-mantle boundary, Buffett suggested that silicon-containing minerals would float to the top of the liquid outer core, carrying iron with it. Together they would form an iron-rich, porous sediment at the mantle boundary that would stick to the mantle, settling into depressions.
Because the Earth's core rotates about a slightly different axis than the mantle (due to the tug of the Sun and Moon), the core's magnetic field is dragged through the mantle, passing unhindered because the mantle does not conduct electricity. The porous, iron-containing sediment stuck to the mantle, however, would resist the rotation of the magnetic field, creating just enough tug to perturb the Earth's rotation.
As the core rotates it sweeps the magnetic field with it, which easily slips through the mantle with no resistance,' said Buffett. ?But if the bottom of the mantle has conductivity, then it's not so easy to slip the magnetic field lines through the mantle. The magnetic field tends to stretch and shear or pull out right across the interface. That generates currents, and those currents damp out the motion and create the kind of dissipation we need to explain this lag in response.'
The sediment layer would have to be less than a kilometer thick (about half a mile) in order to have the observed effect, and would probably cover only patches of the outer core.
Support for the idea that a thin layer of iron-rich silicates may be plastered to the underside of the mantle came from the work of Arizona State University's Garnero and his colleagues, who use seismic waves to probe the mantle and core. They had observed very thin layers at the core-mantle boundary in which seismic waves slow to a crawl. Using Buffett's ideas, Garnero modeled what a thin silicate layer would do to seismic waves and found agreement with the data.
The team subsequently predicted where these patches are located, based on where seismic waves slow down substantially and where they do not.
Think of it as a fuzzy boundary between the mantle and the core, with patches perhaps 10 to 20 kilometers across and up to a thousand meters thick,' Jeanloz said.
The rising sediment eventually would squeeze out the iron, leaving the silicate sediments tucked to the bottom of the mantle as the iron falls toward the solid iron inner core. The rising of the silicate contaminants and the subsequent fall of metallic iron would create a convection in the outer core consistent with what geologists think to be the source of the core's magnetic field. Thus, the rising sediments and falling iron could rev up the Earth's dynamo.
In one of the popular models, created by Gary Glatzmaier and Paul Roberts, the dynamo is powered mainly by the growth of the inner core as light elements get excluded and float up through liquid iron, driving convection that powers the dynamo,' Buffett said. ?If this idea about sediments is right, the sediments would add a component to drive flow from the top down. This is going to have a pretty important effect on the style of fluid motions in the core, and even in the way in which the magnetic field gets generated.'
The silicates stuck to the mantle also might be caught up in mantle convection and carried to the surface, accounting for reports of core material in lava erupting from hot spot plumes like that under Hawaii.
Though Buffett first proposed his theory 10 years ago in his PhD thesis, the data to prove it were not available. In particular, long-term measurements were needed to accurately determine an out-of-phase anomaly in the 18.6 period wobble.
Now, with more than 20 years of data, we can confirm that the discrepancy is there and is explained very nicely by the Earth's magnetic field causing friction at the bottom of the mantle,' Jeanloz said.
The work was supported by the National Science Foundation, the University of California Institute of Geophysics and the Natural Sciences and Engineering Research Council of Canada.