A computer generated model shows Alaskan mantle rock swirling around the plunging slab of crust (in gray) like water around a paddle dipped in a stream. Click to enlarge this image. Margarete Jadamec

A new 3-D model of the mega-quake and tsunami-launching subduction zone in Alaska has uncovered a big surprise: The Earth’s mantle there is moving a whopping 20 to 30 times faster than the crust.

So instead of being dragged along for the ride as a slab of crust is pushed under another, the solid rock mantle rock is swirling around the plunging slab like water around a paddle dipped in a stream.

What the models predict are flows up to 90 centimeters per year around the descending slab of crust, said geologist Magali Billen of the University of California at Davis. Billen co-authored a report on the new model with former graduate student and lead author Margarete Jadamec in this week’s issue of the journal Nature.

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