4.3: Plate Boundaries

One of three actions occurs at each plate boundary: two lithospheric plates collide with each other (convergent plate boundary), pull away from each other (divergent plate boundary), or slide past each other (transform plate boundary). Each action deforms the Earth’s crust in a different way, creating characteristic topographic features. The behavior of the Earth’s crust at a plate boundary depends on the type of crust at the edge of each of the adjacent plates and the directions of movement of the plates in relation to each other.

There are three types of convergent plate boundaries. In the first type, the edge of one converging plate is oceanic crust and the edge of the other is continental crust (e.g., the Pacific coast of South America); in the second type, both plate edges have oceanic crust (e.g., the Aleutian and Indonesian island arcs); and in the third type, both plate edges have continental crust (e.g., the Himalaya Mountains that divide India from the rest of Asia). At the first two types, where at least one of the plates has oceanic crust on its converging edge, oceanic crust is pushed under, or subducted, into the mantle (CC3, Fig. 4-6). These are called subduction zones.

Divergent plate boundaries are locations where lithospheric plates are moving apart. There are two types. The first type, called an oceanic ridge, is a plate boundary where both plates have oceanic crust at their edges (e.g., the Mid-Atlantic Ridge and the East Pacific Rise). The second type, called a rift zone, is a location where a continent is splitting apart (e.g., the East African Rift Zone). If the divergence at a rift zone continues long enough, a new ocean will form between the two now separated sections of the continent. The gap that would otherwise be created as two plates move apart at a divergent plate boundary is filled by magma upwelling from below.

Plate boundaries where plates slide past each other are called transform boundaries. There are few long sections of transform plate boundary on the present-day Earth. The best known stretch of transform boundary is on the Pacific coast of North America, primarily California, where the Pacific Plate slides past the North American Plate. Much shorter sections of transform plate boundary occur at intervals along all plate boundaries.

At certain locations along plate boundaries, three plates meet (Fig. 4-7) in areas called “triple junctions.” There are two types of triple junction: stable and unstable. Stable triple junctions can persist for long periods of time, although their locations may migrate. Unstable triple junctions are those in which the relative motions of the plates cannot be sustained over time because of their geometry. The reason for this is a little complicated, but for a triple junction to be stable, the geometry must be such that each plate can continue to move at the same rate and in the same direction as a whole plate (the plate cannot “bend”). However, the rate at which the plate is subducted or added to by oceanic ridge spreading can be different along its boundaries with each of the other two plates it contacts. Triple junctions where three divergent plate boundaries intersect, as they do near Easter Island in the Pacific Ocean, are always stable. Most other triple junctions are unstable. Unstable triple junctions exhibit enhanced and complex tectonic activity, and their locations move as the interacting plates move relative to each other. An example of an unstable triple junction is the junction where three convergent plate boundaries (the Ryukyu, Japan, and Mariana trenches) intersect south of Japan (Fig. 4-11).

Figure 4-11. The locations of hot spots, subduction zones (trenches), and oceanic ridges. Hot spots occur both at plate boundaries and within plates. Subduction zones occur mostly in the Pacific Ocean, and oceanic ridges are interconnected throughout all of the world’s oceans.