The Earth’s surface is fragmented into enormous, rigid pieces called tectonic plates. These plates move slowly across the planet’s hotter, more ductile mantle layer, a process defined by the theory of plate tectonics. Tectonic plate movement is typically measured in mere centimeters per year, yet over millions of years, this motion creates significant geological features. Mountains are the result of plates moving toward each other at boundaries known as convergent zones.
The specific features that form at these boundaries depend on the type of crust involved: oceanic crust, which is thin and dense, or continental crust, which is thick and buoyant. There are three fundamental ways these crustal types interact at convergent boundaries to build mountain ranges or volcanic arcs. The density difference between the two types of crust determines which plate will sink (subduct) and which will override the other, dictating the resulting geological landscape.
Collision Between Oceanic and Continental Plates
When an oceanic plate converges with a continental plate, the denser oceanic lithosphere is forced to slide beneath the lighter, thicker continental plate in a process called subduction. The point where the oceanic plate begins its descent is marked by the formation of a deep ocean trench, running parallel to the edge of the overriding continent. The intense friction and pressure from this descent cause the overriding continental crust to crumple, fold, and uplift, forming a coastal mountain range.
As the subducting oceanic plate descends deeper into the mantle, it carries water-rich minerals that begin to release fluids due to increasing temperature and pressure. This water then percolates into the overlying mantle wedge, lowering its melting point in a process known as flux melting. The resulting molten material, or magma, is less dense than the surrounding rock and rises toward the surface through the continental crust.
This rising magma eventually erupts, creating a chain of volcanoes on the continental plate, parallel to the trench and the coastal mountain range. These features are known as continental volcanic arcs, which contribute substantially to the overall height and mass of the mountain belt. Examples include the Andes Mountains in South America, formed by the subduction of the Nazca Plate beneath the South American Plate, and the Cascade Range in North America.
Subduction of Two Oceanic Plates
The convergence of two oceanic plates also results in subduction, but here the distinction is based on age and temperature, which govern density. The older, colder oceanic plate is denser and sinks beneath the younger, warmer oceanic plate into the mantle. This action creates a deep ocean trench in the seafloor, often deeper than the trenches formed at oceanic-continental boundaries.
Similar to the previous collision type, the subducting plate releases water into the overlying mantle wedge as it descends. This flux melting generates magma that rises through the overriding oceanic plate. The magma then erupts on the ocean floor, eventually building a chain of volcanoes that rise above sea level.
This configuration forms an arc-shaped chain of volcanic islands, known as a volcanic island arc. These island arcs are essentially underwater mountain ranges capped by volcanoes. The Mariana Trench and the parallel Mariana Islands in the western Pacific Ocean exemplify this process. The Aleutian Islands, which arc across the northern Pacific near Alaska, are another well-known example.
Collision of Two Continental Plates
When two continental plates converge, the dynamics change because both crustal masses are composed of low-density, buoyant material. Since neither plate is dense enough to sink deep into the mantle, the process of subduction stalls. Instead of one plate sliding beneath the other, the two continental masses smash directly into each other.
This head-on collision subjects the crust to extreme compression, leading to massive folding, faulting, and stacking of rock layers. The crust is shortened horizontally and thickened vertically, which forces the land upward to create the world’s highest and most extensive non-volcanic mountain ranges. This process is known as crustal thickening or stacking.
Because the continental crust is too buoyant to trigger flux melting, this type of mountain building lacks volcanic activity. The resulting mountain belts are instead dominated by metamorphic and sedimentary rocks that have been intensely deformed by the compressional forces. The most famous example is the ongoing convergence between the Indian Plate and the Eurasian Plate, which continues to build the Himalayas. The Alps and the Appalachian Mountains are other examples created by ancient collisions.