Jan. 25, 2000 COLLEGE STATION - Millions of years before Carole King felt the earth move under her feet, the planet rapidly and drastically shifted on its axis, according to research by Texas A&M University oceanographer William Sager.
"The earth pretty much rolled relative to its rotation axis, so that there was a 16- to 21-degree change," Sager said of the "true polar wander" (TPW) shift that occurred approximately 84 million years ago. "It's an odd event."
It's also an event that may offer an explanation other than drifting of the earth's tectonic plates - the dominant theory since the 1960s - for the movement of the continents.
Although the findings by Sager and Anthony Koppers of the Scripps Institution of Oceanography don't eliminate plate tectonics as a factor in the poles' movement, Sager said their research suggests that tectonics alone would not have caused it. Their research appears in the current issue of the journal, Science.
"It's an important piece of a puzzle that, taken with other data, can help change our understanding of how the earth works," Sager said.
"The idea of true polar wander has been around for quite a while but it's a difficult thing to see because you have to separate two competing motions," he said. "On one hand, plate motion makes the axis appear to move, and on the other, the axis may actually shift. To separate the two, you have to somehow subtract out the plate motion and see what's left."
What was left suggests that the shift occurred as a result of rapid redistribution of matter within the planet's mantle layer.
"What we're saying is that the direction and the speed of the shift seems to be much faster than can be explained by plate motion," Sager said.
The TPW shift could have caused rapid relocation of the continental landmasses (sufficient to put Dallas off the coast of Acapulco).
"The rapidity is one important thing," Sager said, adding that plate motion occurs much slower than the event they observed. The planet's plates move about 1 degree per million years.
"Our data set indicates that this polar shift took place at a rate between 5 and 10 degrees per million years," he said. "Essentially it happens within the blink of an eye, in terms of geological time."
The fact that the TPW shift occurred simultaneously with other geologic events - such as changes in plate motion, volcanic eruptions, changes in the earth's magnetic field - all indicate the cause was something linking the planet's deep interior with its surface, Sager added. "That's why we think it was some kind of mantle overturn event."
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