Why are oceanic plates denser than continental plates?

It all has to do with the composition of the plates. Continental plates are made of mostly granite, while Oceanic plates are made of mostly basalt. Basalt is denser than granite, so oceanic plates are denser than continental plates. For more info, read this:

http://scign.jpl.nasa.gov/learn/plate3.htm

Hope it helps!!

Because the Earth acts as a giant iron smelter. The continental crust is the glassy slag floating on top of the less differentiated oceanic crust, which in its turn floats on the even less differentiated mantle.

As denser minerals sink out of the mantle toward the core the less dense minerals rise to the surface and harden as oceanic crust.

As the oceanic crusts subduct at trenches they remelt with further differentiation. The denser minerals sink down into the mantle and the less dense minerals rise to the surface to form volcanic island arcs.

Over the course of billions of years the island arcs are pushed one atop the other, remelting to become the granites of continents.

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Geekmast is correct. The reason continental plates are composed of granite is because they are composed of the lighter minerals built up in continents in the processes of plate tectonics. The heavier minerals tended to pushed into the depths of the earth. The lighter ones rise like ice on water.

Dylan Snee