Different layers of the Earth are made of materials with different physical properties. One of the most important physical properties that cause the formation of layers in the Earth is density. Density is defined as the mass per unit volume of a substance, so you can think of it as how heavy that material is. Layers that are less dense, such as the crust, float on layers that are denser, such as the mantle. Both oceanic crust and continental crust are less dense than the mantle, but oceanic crust is denser than continental crust. This is partly why the continents are at a higher elevation than the ocean floor. Because continental crust is less dense than oceanic crust it floats higher on the mantle, just like a piece of Styrofoam floats higher on water than a piece of wood does. The mantle, oceanic crust and continental crust have different densities because they are made of different kinds of rock with different densities. The continental crust is made mostly of rocks with a composition similar to granite (a light-colored rock you would expect to find in the Sierra Nevada), whereas the oceanic crust is made mostly of rocks with a composition of basalt (a dark- colored rock, like the rocks that make up the Hawaiian volcanoes).

The mantle, oceanic crust, and continental crust all have different compositions due to a process called partial melting. This is where you start to melt a rock, but don´t melt it all the way. When you partially melt a rock, certain chemical elements tend to stay in the solid rock while others tend to go into the melted part. As a result, the rock that forms from that melt is less dense than the original rock. If you then partially melt that rock, you get a rock that is even lighter. The oceanic crust is formed by partial melting of the mantle at mid-ocean ridges. The continental crust is formed even more cycles of partial melting over time, resulting less dense rocks.