How do the plates move?
How do the plates move? This was a question that plagued many scientists for decades.
Old Hypothesis: Plates are dragged atop a convecting mantle.
The core is much hotter than the mantle, so heat is transferred to the bottom of the mantle. This heated area begins to rise up until it reaches the transition zone. Here it cools and starts to descend back down to the bottom of the mantle. This is called a convection cell. A similar process happens in the upper mantle (the asthenosphere). Hot mantle material rises up to beneath the lithosphere, where it cools. Because the asthenosphere is ductile, as the convecting material moves beneath the lithosphere it moves the lithosphere with it, causing the plates to move.
However, plate motion is much too complex for this to be the only reason.
New: Push-Pull Theory
Scientists believe that there are two other forces drive plate motions: Ridge push and Slab Pull.
- Ridge-push – Hot mantle material is brought to the surface at mid ocean ridges, where it is pushed aside by new rising mantle material.
- Slab-pull – As the ridges push the plates apart, they are eventually going to bump into something and form a subduction zone. Once one plate is pushed under the other, gravity takes over and pulls the subducting plate downward.
- Mantle convection does occur in the asthenosphere, helping the plates to move.
As the subducting plate descends into the mantle, it pushes aside the mantle material. This causes the asthenosphere to move away from this area. Where does it go? As mantle convection moves upward, this additional material joins with it and rises up at spreading centers or hot spots, which helps to spread the plates apart.
Heavy items like ice sheets and mountains push aside the rocks in the asthenosphere, as do the subducting oceanic plates. The rocks flow along with mantle convection to spreading centers and help them to push the plates apart. So, the pushing and pulling of the plates and mantle convection work together to move the plates around.
How can we tell the plates are moving?
Method #1: Absolute Plate Velocities
This is done by measuring the distance between were a spot was in the past to where it is today. Example: We know where the Hawai'l hot spot is now. We also know that the island of Kaua'i used to be on the same hot spot 5 million years ago. Kaua'l is now 519 km from the hot spot, so the plate has moved 519km/5ma, or 10.4 cm/yr.
Method #2: GPS or Real-time motion
GPS and similar technology can show scientists how fast the plates are moving in real-time, not how fast they were moving, like with the absolute velocity method. The results are similar, however.
Note how the North American and Eurasian plates are moving in one direction on one side, and in a slightly different direction on the other. This can cause stress in the middle areas.