Categorization of plate boundaries is based off of how two plates move relative to each other. There are essentially three types of plate boundaries: divergent, convergent, and transform.
In the case of divergent plate boundaries, two of earth’s plates move away from each other. We can call these spreading centers, and new ocean crust is created here. An example of a divergent plate boundary is the Mid-Atlantic Ridge. The Mid-Atlantic Ridge is a long underwater mountain range that results from the creation of new oceanic crust. As new crust forms, rocks not only pile up to the form the mountain chain, but the Atlantic Ocean grows larger. Because the movement at the divergent boundary, shallow earthquakes can occur here.
When two plates move towards each other, the boundary is known as a convergent boundary. As previously mentioned, depending on the type of crust involved in this collision, different geographic features are formed. When two oceanic plates converge, the denser plate (usually the older one) will end up sinking below the less dense plate, leading to the formation of an oceanic subduction zone. Whenever a subduction zone is formed, the subducted plate will end up being partially melted by the earth’s internal magma. This melted rock rises, uplifting the crust, eventually developing into a volcano. When this occurs in an ocean, the result is a series of volcanic islands. An example of this can be found in the Marianas Islands, which sit along the Marianas Trench. Trenches are created at convergence zones. Subduction zones are the reason why oceanic crust older than 200 million years old cannot be found. Old, dense crust tends to be subducted back into the earth. When an oceanic plate converges with a continental plate, the oceanic crust will always subduct under the continental crust; this is because oceanic crust is naturally denser. This creates volcanic mountains and trenches along the coast of the continent. An oceanic-continental convergent boundary exists along the Nazca plate and South American Plate. This convergent boundary results in the formation of the Peru-Chile Trench and the Andes Mountains. When two continental crusted plates converge, they eventually collide and end up producing mountains; this was how the Himalayas were created. Neither continental crust will subduct underneath one another because of their similar densities. Convergent boundaries are commonly associated with larger, deeper earthquakes.
The last type of plate boundary is the transform boundary, which is where two plates slide past one another. Unlike the other two types of plate boundaries in which new seafloor is created at divergent boundaries and where old seafloor is subducted at convergent boundaries, transform plate boundaries neither create nor destroy crust. Transform faults are commonly known for creating earthquakes, which can be quite large. One example of a transform fault boundary would be the San Andreas fault.
We will explore plate boundaries in more detail in the next sections.