Ivins describes it as similar to a pot of soup placed on the stove.
It is basically the circulation of material in the mantle caused by heat from Earth's core. Mantle convection is responsible for the movement of tectonic plates on Earth's surface. The authors argue that mantle convection makes up the final third. In the new study, which relied heavily on a statistical analysis of such rebound, scientists figured out that glacial rebound is likely to be responsible for only about a third of the polar drift in the 20 th century. As that ice melts, or is removed, the land slowly rises back to its original position. And what is glacial rebound? During the last ice age, heavy glaciers depressed Earth's surface much like a mattress depresses when you sit on it.
Previous studies identified glacial rebound as the key contributor to long-term polar motion. "There is a geometrical effect that if you have a mass that is 45 degrees from the North Pole - which Greenland is - or from the South Pole (like Patagonian glaciers), it will have a bigger impact on shifting Earth's spin axis than a mass that is right near the Pole," said coauthor Eric Ivins, also of JPL. While ice melt is occurring in other places (like Antarctica), Greenland's location makes it a more significant contributor to polar motion. This makes Greenland one of the top contributors of mass being transferred to the oceans, causing sea level to rise and, consequently, a drift in Earth's spin axis. In fact, a total of about 7,500 gigatons - the weight of more than 20 million Empire State Buildings - of Greenland's ice melted into the ocean during this time period. As temperatures increased throughout the 20 th century, Greenland's ice mass decreased. In general, the redistribution of mass on and within Earth - like changes to land, ice sheets, oceans and mantle flow - affects the planet's rotation. We identified not one but three sets of processes that are crucial - and melting of the global cryosphere (especially Greenland) over the course of the 20 th century is one of them." "We assembled models for a suite of processes that are thought to be important for driving the motion of the spin axis. But recently, many researchers have speculated that other processes could have potentially large effects on it as well," said first author Surendra Adhikari of NASA's Jet Propulsion Laboratory in Pasadena, California. "The traditional explanation is that one process, glacial rebound, is responsible for this motion of Earth's spin axis. Using observational and model-based data spanning the entire 20 th century, NASA scientists have for the first time identified three broadly-categorized processes responsible for this drift - contemporary mass loss primarily in Greenland, glacial rebound, and mantle convection. Over the course of a century, that becomes more than 11 yards (10 meters). These spin-axis movements are scientifically referred to as "polar motion." Measurements for the 20 th century show that the spin axis drifted about 4 inches (10 centimeters) per year. When it rotates on its spin axis - an imaginary line that passes through the North and South Poles - it drifts and wobbles. Our actual planet is far less perfect - in both shape and in rotation.Įarth is not a perfect sphere. A typical desk globe is designed to be a geometric sphere and to rotate smoothly when you spin it.
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