This image is a cross section through the Earth showing the convection cells of the mantle. Ridge push happens at spreading centers where plates are moving apart. Slab pull happens at subduction zones where one plate is pulled down into the mantle.
Windows Original after Northcott
How Do Plates Move?
Earth’s center, or core, is very hot, about 9000 degrees F. This heat causes molten rock deep within the mantle layer to move. Warm material rises, cools, and eventually sinks down. As the cool material sinks towards the Earth’s core it is warmed and rises to the top again. This type of movement is called a convection cell.
We used to assume that Earth’s plates just surfed on top of the mantle’s giant convection cells, but now scientists believe that the plates themselves play an important role in the convection process.
How can plates play an active part in the convention process instead of just surfing along? Just like convection cells, plates have warmer, thinner parts that are more likely to rise, and colder, denser parts that are more likely to sink.
New parts of a plate rise because they are warm and thin. As hot magma rises to the surface at spreading ridges and forms new crust, the new crust may actively push the rest of a plate out of its way. This is called ridge push.
Old parts of a plate are likely to sink down into the mantle at subduction zones because they are colder, thicker and denser than the warm mantle material underneath them. This is called slab pull.
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