Plate Boundaries

There are three categories of plate boundaries: convergent, divergent, and transform. At each boundary, the resulting landforms depend on whether the plates involved are oceanic, continental, or both types.

Figure 1: The three categories of plate boundaries: convergent, divergent, and transform.

Convergent Boundary

Convergent boundaries occur when two plates move towards each other (converge). While the motion of the plates is the same, the resulting landforms can be very different depending on the type of plates involved.

  • Oceanic-oceanic convergent boundary: Converging oceanic plates form trenches in the subduction zone, the point where the denser plate is subducted. On the overriding plate, a volcanic island arc (a chain of off-shore volcanoes) forms parallel to the trench. Deep earthquakes can form at the subduction zone.

  • Oceanic-continental convergent boundary: When a continental and an oceanic plate converge, the oceanic plate is always subducted. This is because oceanic crust is much denser than continental crust. A trench forms on the ocean floor at the subduction zone, and a volcanic arc (a volcanic mountain range) forms on the continental plate parallel to the trench. Deep earthquakes can form in the subduction zone.

  • Continental-continental convergent boundary: When both converging plates are continental neither is subducted, as continental crust is buoyant in the mantle. The only exception is if ancient oceanic crust is still attached to the continental crust, then the oceanic crust will be subducted. As the continental plates converge mountain ranges are formed.

Divergent Boundary

Divergent boundaries occur when two plates move away from each other (diverge), creating a long, narrow depression called a rift valley.

  • Continental-continental divergent boundary: In most cases, diverging continental plates are found where a continental plate begins to break apart. Rift valleys form, resembling a narrow area of lowland with highland on either side.

  • Oceanic-oceanic divergent boundary: When two oceanic plates diverge, magma rises through the rift valley, creating mid-ocean ridges that appear like volcanic mountain ranges on the ocean floor. This is where seafloor spreading takes place and new oceanic crust is formed.

Oceanic-continental divergent boundaries don't usually occur in nature, as there is unlikely to be a force that pulls apart oceanic and continental crust exactly at their boundary.

Transform Boundary

Two plates moving horizontally past each other form a transform fault. In this process, the crust is fractured and deformed, but it is not destroyed (as it is in convergent boundaries) or created (as it is in divergent boundaries). This action often causes shallow earthquakes. Transform boundaries are very common around mid-ocean ridges, but also occur in continental crust.