for National Geographic News
Someday not so soon Washington, D.C., may find itself about where San Francisco is now. According to a recent study, Earth's surface may be slipping slowly westward, dragged by the same lunar forces that produce tides.
The Earth's crust is divided into vast plates that slowly shift, producing earthquakes, mountains, and rifts where they collide or separate. Most earth scientists believe that this movement is the result of rising and falling currents of magma deep below the surface.
In addition to being jostled every which way from below, the planet's plates are sedately sliding toward the sunset, says Carlo Doglioni of the earth science department at Rome's La Sapienza university.
In a study published in the January-February issue of the Geological Society of America's journal Bulletin, Doglioni and a team of Italian and U.S. scientists argue that the westward motion is due to the tidal attraction of the moon.
As the Earth spins eastward beneath the moon, they say, the moon's gravity ever so slightly holds the Earth's surface layer back. This "lunar drag" causes the crust to slip slowly backward, like a loose handgrip on a bicycle handlebar.
No Easy Task
Proving the theory is no easy task.
After all, if everything on Earth is spinning in the same direction at about the same rate, what do you use as a reference point? For Doglioni the answer is to use the volcanic hot spots beneath places such as the Hawaii Islands.
Most geophysicists, including Doglioni, believe that such chains result from heat plumes rising from beneath the Earth's surface, as if from volcanic blowtorches.
As the crust drifts over these hot spots like a conveyor belt, the heat punches through the surface, producing chains of volcanoes, the thinking goes. (Build a virtual volcano online.)
The locations of these islands indicate the crust's motion above them.
For example, Kauai is much farther from the Hawaiian Islands' hot spot than its younger siblings. Kauai has been drifting for millions more years than its neighbor, Oahu. The Big Island, farther east, is currently over the hot spot, as evidenced by its volcanic activity.
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