Polar Wander

Introduction

Earth spins like a top: the geographic poles are the two points on its surface through which the two ends of its axis of rotation, the invisible line around which it spins, point. The phenomenon called true polar wander (TPW) is the shift of the whole Earth relative to its axis of spin. TPW occurs because of changes in the mass distribution of Earth—how much of its mass is located where. If the poles wander through a large angle, places that were formerly warm will become cold and, likewise, some regions that were not tropical will be moved into the tropics. Large TPW events are rare and very slow by human standards, happening over millions of years, but small amounts of polar wander can be detected on time scales of months or years and can be influenced by natural climate cycles and human-caused climate change.

Historical Background and Scientific Foundations

The concept of polar wander in response to shifting global weight distribution was first proposed about 140 years ago. It was already known to astronomers in the nineteenth century that the precise locations of the geographic poles are continuously drifting or wandering across the Arctic and Antarctic landscapes.

Any process that changes the mass distribution of Earth may cause TPW. For example, the Chandler wobble is a back-and-forth motion of the Pole by about 20 ft (6 m) every 14 months. The Chandler wobble is driven mostly by variations in water pressure on different parts of the ocean floor. These variations in turn are driven by changes in the salinity (saltiness) and temperature of the ocean, both of which affect water density. The Chandler wobble is an example of TPW driven by short-term climate cycles.

WORDS TO KNOW

CHANDLER WOBBLE: Small-scale cyclic wobble described by Earth's axis, typically on a 433-day schedule: caused by changes in water pressure at the ocean floor driven by Earth's climate cycle. Named after American astronomer Seth Carlo Chandler (1846–1913), who discovered the phenomenon in 1891, although its cause was not known until 2000.

CONTINENTAL DRIFT: A theory that explains the relative positions and shapes of the continents, and other geologic phenomena, by lateral movement of the continents. This was the precursor to plate tectonic theory.

GEOMAGNETIC: Related to Earth's magnetic field.

ICE AGE: Period of glacial advance.

PLATE TECTONICS: Geological theory holding that Earth's surface is composed of rigid plates or sections that move about the surface in response to internal pressure, creating the major geographical features such as mountains.

SALINITY: The degree of salt in water. The rise in sea level due to global warming would result in increased salinity of rivers, bays, and aquifers. This would affect drinking water, agriculture, and wildlife.

Small amounts of steady polar wander, in addition to the back-and-forth of the Chandler wobble, are always happening. Satellite observations show that the whole Earth is shifting relative to its axis by about 1 degree per million years. This steady, creeping TPW is caused by continental rebound after the last Ice Age: that is, the northern continents were pushed downward by the weight of the vast ice sheets that covered them, and after the melting of the ice tens of thousands of years ago are still bobbing up slowly, like corks held partly under water and suddenly released.

Much larger amounts of polar wander happened in the geologically distant past during episodes of rapid TPW, when Earth was shifting by 3 to 10 degrees per million years. In 2006, scientists announced evidence from Norwegian sediments of an extreme TPW episode about 800 million years ago. At that time, the evidence indicates, Earth essentially turned on its side, and rivers that had been emptying into the Arctic emptied into the tropics instead. Other geological evidence shows major changes in ocean chemistry at that time, reflecting large global climate changes unexplained by any Ice Age or other known cause.

True polar wander is distinguished from apparent polar wander, the shifting of geomagnetic polar traces across the continents. Paths of apparent polar wander can be traced across the landscape by identifying minerals that preserve in the directionality of their crystals a record of which way Earth's magnetic field was pointing at the time they solidified or were deposited as tiny particles. Apparent polar wander is due to the motion of the land itself (plate tectonics) rather than to true polar wander. Apparent polar wander can be compared to the mark left by a pen on a piece of paper that is being dragged beneath it: the pen (analogous to Earth's axis, in the case of apparent polar wander) leaves a mark without actually moving.

Impacts and Issues

The geological evidence of large-angle ancient TPW is difficult to disentangle from the evidence of apparent polar wander, caused by continental drift. The 2006 Norwegian evidence mentioned earlier was the best to date for large TPW in Earth's remote past. As of 2007, scientists intended to test this interpretation by looking at sediments in Australian rocks.

Thanks to satellite technology, including the Global Positioning System (GPS), the position of Earth can now be measured to millimeters. This allows measurement of TPW caused by shifts in water and ice distribution on the planet's surface, such as those caused by natural and anthropogenic (human-caused) climate change. Although anthropogenic climate change cannot cause disastrously large polar wander, fine measurements of TPW provide a new, sensitive method of seeing where water is being moved by such climate-related changes as melting of the Greenland and Antarctic ice sheets.

Polar wander and sudden reversals of Earth's magnetic field are distinct phenomena with different causes, and should not be confused. Polar wander has been proposed to explain features not only of Earth but of Mars, Pluto, and of large moons in the outer solar system such as Io, Europa, and Triton.

See Also Abrupt Climate Change; Sea Level Rise.

BIBLIOGRAPHY

Books

Cox, John D. Climate Crash: Abrupt Climate Change and What It Means for Our Future. Washington, DC: Joseph Henry Press, 2007.

Periodicals

Gordon, Richard G. “Wandering Why.” Nature 387 (1997): 132-133.

Maloof, Adam C., et al. “Combined Paleomagnetic, Isotopic, and Stratigraphic Evidence for True Polar Wander from the Neoproterozoic Akademikerbreen Group, Svalbard, Norway.” Geological Society of America Bulletin (September/October 2006): 1099-1124.

Mound, Jon E., and Jerry X. Mitrovica. “True Polar Wander as Mechanism for Second-Order Sea-Level Variations.” Science 279 (1998): 534-537.

Sager, William W., and Anthony A. P. Koppers. “Late Cretaceous Polar Wander of the Pacific Plate: Evidence of a Rapid True Polar Wander Event.” Science 287 (2000): 455-459.

Web Sites

“Planet Earth May Have ‘Tilted’ to Keep Its Balance, Scientists Say” (press release). Princeton University, August 25, 2006. <http://www.princeton.edu/ main/news/archive/S15/64/72A37/index.xml? section=newsreleases> (accessed October 10, 2007).

Larry Gilman