Answer: The reason for the differences in density is the composition of rock in the plates. When two plates come in contact with each other through plate tectonics, scientists can use the density of the plates to predict what will happen. Whichever plate is more dense will sink, and the less dense plate will float over it.
Explanation:
Hope this helps ( not copied and pasted, this answer was done by me so I don't know if it's good or not)
