Gravity plays a significant role in tectonic plate motion, particularly through mechanisms such as C: ridge push and slab pull. Here's how it relates to the options you've mentioned:
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A: Earth internal heating: While internal heating is a factor that drives convection currents in the mantle, which can influence plate motion, it doesn't directly describe gravity's role.
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B: Earth magnetic field reverses: This phenomenon is related to the movement of tectonic plates and the creation of new oceanic crust at mid-ocean ridges, but it is not a mechanism influenced by gravity.
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C: Ridge push and slab pull: This is the best choice. Ridge push occurs as new oceanic crust is formed at mid-ocean ridges, creating a slope due to gravity that encourages the plates to move away from the ridge. Slab pull occurs when the dense, subducting plate (slab) pulls the rest of the plate downward into the mantle due to gravity. Both of these mechanisms are directly influenced by gravitational forces.
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D: Circulating material in the mantle: While convection currents involve the movement of material in the mantle, which can be influenced by gravity, this option does not directly describe gravity's role in the context of tectonic plate motion.
In summary, C: ridge push and slab pull is the most accurate description of how gravity influences tectonic plate motion.