Earth's Mantle in Overdrive Under Alaska
The mantle under Alaska is moving 20 to 30 times faster than the crust -- reversing the usual order of plate tectonics.
- A new 3-D model shows the mantle flowing "screaming fast" around the Alaska subduction zone.
- The speed comes from the descending slab of crust swirling the mantle.
- Energy used to mix the mantle means less power available to make mega-quakes.
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.
"On plate tectonics timescales that's screaming fast," said Billen.
More typical plate speeds range from one to 10 centimeters per year.
Related Links:
The model, developed by Jadamec, includes 100 million data points and takes 48 hours to run on a supercomputer with 400 processors.
It incorporates the latest in genuine evidence from the Alaska subduction zone. This includes something called seismic anisotropy, which exploits special seismic properties of the mineral olivine in the mantle to detect directions rocks are flowing in the mantle.
Because the mantle rocks are under great pressure, they flow like Silly Putty while remaining solid.
Other seismic pieces of the subduction computational puzzle include the shape of the slab that's now being pushed into the mantle and the viscosity of the mantle, explained Billen. Put the flow direction, viscosity and shape of the slab together with a lot of computer power and you can start to work out the details of what's going on.
"As long as we can put (into the computational model) the right driving force, that's how fast the flow is," Billen explained.
The model also allows researchers to turn up the speed and see how the subduction process might proceed.
"It takes millions of years for the slab (of subducted plate) to descend," said geophysicist Geoff Abers of the Lamont-Doherty Earth Observatory of Columbia University. "These kinds of models allow the slabs to flap around, which is something you could not do and cannot see."
The swirling of the mantle around the sinking slab could have implications for the super powerful megathrust quakes and tsunamis that the subduction zone is capable of generating, said Billen.
Instead of all the force of the colliding plates being bound up in their collision and vented by large quakes, some energy is being transferred into stirring up the mantle.
"This is a good thing," said Billen.
The model could also help to figure out if a chunk of crust is about to fall off into the mantle and stop the subduction dead in its tracks. This, in turn, could help to explain why subduction zones start and then stop in different places said Billen.
http://news.discovery.com/earth/earth-mentle-crust-alaska.html
No comments:
Post a Comment