Divergent boundaries are typically marked by the movement of tectonic plates away from each other. This type of crust involves oceanic crust, which is usually younger and thinner compared to continental crust.
As the tectonic plates separate, magma from the mantle rises to fill the gap, creating new oceanic crust. This process is most famously illustrated by mid-ocean ridges, such as the Mid-Atlantic Ridge, where new crust forms as the plates diverge.
In summary, divergent boundaries primarily involve oceanic crust due to the processes of seafloor spreading, resulting in the formation of new crust as the plates pull apart.