Plate tectonics describes the movement of Earth’s rigid outer layer, the lithosphere, which is broken into large, shifting plates. A convergent boundary is a region where two tectonic plates move toward each other, subjecting the crustal material to immense compressive stress. The specific geological features that form at these boundaries depend on the type of lithosphere involved in the collision: oceanic crust, continental crust, or a combination of both.
Oceanic Crust Meeting Continental Crust
When a plate carrying dense oceanic crust collides with a plate carrying buoyant continental crust, subduction occurs. The oceanic lithosphere, which is denser, sinks beneath the less dense continental plate and descends into the mantle. This downward bending creates a deep-sea trench, a narrow topographic low that marks the boundary on the ocean floor.
As the oceanic slab sinks, pressure and temperature cause minerals to release water. This water rises into the overlying mantle rock, lowering its melting temperature and generating partial melting. The resulting magma is less dense and ascends through the continental crust. This buoyant magma ultimately erupts on the surface, forming a chain of volcanoes known as a continental volcanic arc. A classic example is the collision between the Nazca Plate and the South American Plate, which formed the Peru-Chile Trench and the Andes Mountains.
Oceanic Crust Meeting Oceanic Crust
A convergent boundary can form when two plates composed of oceanic crust move toward one another. One oceanic plate will subduct beneath the other; typically, the older, colder plate descends because it is denser than the younger crust. This process creates a deep oceanic trench.
The subducting slab heats up and releases fluids that trigger melting in the overlying mantle wedge. This less dense magma rises through the overriding oceanic plate, eventually erupting on the seafloor. Over millions of years, these eruptions build up a curved chain of volcanoes that rise above sea level, forming a volcanic island arc. Examples include the Mariana Trench and Islands, as well as the Aleutian Trench and Aleutian Islands in Alaska.
Continental Crust Meeting Continental Crust
When two plates carrying continental crust converge, the dynamics change because both plates are relatively low in density. Continental crust is thick and buoyant, making it too light to easily subduct deep into the Earth’s mantle. Instead of sinking, the two continental masses collide and compress one another. This intense compression causes the crust to buckle, fracture, and fold vertically.
The collision results in significant crustal thickening, where the lithosphere is pushed upward and downward. This deformation creates massive, non-volcanic fold mountain ranges characterized by extensive faulting and folding. Magma generated deep below generally cannot reach the surface to form volcanoes due to the crust’s thickness. The most dramatic instance is the ongoing collision between the Indian and Eurasian plates, which created the Himalayan Mountains.