Earth & Space Science: Session 4
A Closer Look: Plate Boundaries
What are plate boundaries?
A central concept of plate tectonics is that large portions of the Earth's lithosphere — the crust and the rigid part of the upper mantle — move relative to one another. This relative movement of the plates creates varying conditions at the plate edges, or boundaries. Geologists describe three basic types of plate boundaries: divergent boundaries, convergent boundaries, and transform boundaries.
Divergent boundaries exist where two plates are moving apart. This occurs most commonly at mid-ocean spreading ridges. At such ridges, plates move apart and the mantle exposed is melted by the decrease in pressure, becoming magma. That magma then rises up to “fill in” as new oceanic crust. As the plates separate, the geologic feature known as a “rift valley” is created. Iceland, which sits on the Mid-Atlantic ridge, is splitting apart on the divergent boundary between the North American and Eurasian Plates.
Convergent boundaries exist where two plates are moving toward each other.
Convergent boundaries can be of three types:
Ocean-Ocean Convergence: If the plates moving toward each other are both made of oceanic crust, one of the plates will move downward, or subduct, under the other plate. This is called a subduction zone. A deep trench forms on the ocean floor at the location where one oceanic plate subducts under another. Also, a volcanic arc (a chain of volcanoes parallel to the trench) typically develops above the subduction zone. These volcanoes are generated as water brought down on the subducting plate melts the overlying mantle, causing magma to rise through the mantle and crust, erupting at the surface. The Mariana trench and volcanic arc mark where the Pacific and Philippine plates converge. The Aleutians, Japan, and the Philippines are other examples of volcanic arcs that exist as a chain of islands.
Ocean-Continent Convergence: If one plate topped by oceanic crust moves toward another topped by continental crust, the more dense, oceanic plate will subduct under the less dense, continental plate and a trench will develop off the shore of the continent. Off the coast of South America, along the Peru-Chile trench, the oceanic Nazca Plate is being subducted beneath the continental South American Plate. As a result, the Andes, a mountainous volcanic arc, have developed in South America. The Cascade Range, where Mt. St. Helens is found, is another example of a volcanic arc formed by the convergence of oceanic and continental plates.
Continent-Continent Convergence: If both converging plates carry continental crust, neither of the plates fully subduct. Continental rocks have a relatively low density and, like two colliding icebergs, usually resist downward motion. During a collision, the crust is compressed and subjected to very high temperatures and pressures. The lithosphere thickens and crustal rocks are folded and faulted. Large amounts of uplift push rock high into the sky, forming mountain ranges such as the Alps or the Himalayas.
Transform Boundaries: Transform boundaries exist where two plates slide past each other. Here, no lithosphere is created or destroyed. Often these large faults, or fracture zones, connect divergent or convergent plate boundaries. Most transform faults are found on the ocean floor. Examples include the Alpine fault in New Zealand, which forms the boundary between the Australian and Pacific plates, the Dead Sea fault, which forms the boundary between the African and Arabian plates, and the San Andreas fault, which lies between the Pacific and North American plates in California.
Is it that simple?
Not all plate boundaries are as simple as the main types discussed above. For example, there are several places on the Earth where three plate boundaries intersect. These are referred to as “triple junctions.” An example of a triple junction exists where the African, Australian, and Antarctic plates intersect. There, three spreading ridges intersect. What does that tell us about this area of the African, Australian, and Antarctic plates? — the plates are moving apart in this region.
In some parts of the world, plate boundaries are not well defined. These regions, called “plate-boundary zones,” are large areas where the effects of plate interactions are unclear. Plate boundary zones involve at least two large plates and one or more microplates (small plate fragments) caught up between the larger plates. The geology of these areas can be very complex. The Mediterranean-Alpine region between the Eurasian and African plates is an example of a plate boundary zone.