Here's why:
* Continental rocks are primarily composed of granite, which is rich in silica and aluminum. This makes granite lighter than the denser rocks found in the ocean floor.
* Seafloor rocks are primarily composed of basalt, which is rich in iron and magnesium. These elements make basalt denser than granite.
Think of it like this: Imagine you have a block of wood and a block of metal. The wood will float on water because it's less dense than the water. The metal will sink because it's denser. The same principle applies to continents and the Earth's mantle.
This density difference is a key factor in plate tectonics. The denser oceanic plates subduct (slide) beneath the less dense continental plates, driving the movement of the Earth's crust.
