Here's why:

* Oceanic crust is primarily composed of basalt and gabbro, which are rich in dark-colored minerals like pyroxene and olivine. These minerals are denser than the minerals found in continental crust.

* Continental crust is predominantly made up of granite and other light-colored, silica-rich rocks.

The density difference is a key factor in plate tectonics. Oceanic crust is denser and therefore subducts (sinks) beneath the less dense continental crust at convergent plate boundaries. This process drives many geological phenomena, including volcanic activity and mountain formation.