The Earth’s surface is a mosaic of immense, rigid pieces known as tectonic plates. These plates are segments of the lithosphere, the planet’s outermost layer encompassing the crust and the uppermost part of the mantle. Driven by heat and convective currents deep within the mantle, these massive fragments are constantly in slow motion relative to one another. This movement occurs along three main types of boundaries: divergent, transform, and convergent. The complex interactions at these boundaries account for nearly all of the planet’s significant geological activity, including the formation of mountains, earthquakes, and volcanic eruptions.
Convergent Boundaries: The General Term
The specific answer to what it is called when tectonic plates collide is a convergent boundary. This boundary type is defined by the plates moving toward each other, resulting in a zone of intense compression. The immense forces generated along this boundary cause the lithosphere to be destroyed or significantly deformed. The friction and pressure between the converging plates lead to significant rock deformation and are the primary source of the world’s most powerful earthquakes. As plates collide, the region experiences substantial crustal shortening and thickening. The outcomes of this collision vary dramatically depending on the specific type of crust—oceanic or continental—that is involved. The process where one plate slides beneath another is called subduction, and it is the dominant mechanism at most convergent boundaries.
Collision Type One: Oceanic and Continental Plates
When a plate carrying dense oceanic crust collides with a plate carrying lighter continental crust, subduction is clearly demonstrated. Oceanic crust is composed primarily of basaltic rock, making it significantly denser than the granitic rock that forms the continental crust. Because of this difference in buoyancy, the oceanic plate is forced to sink beneath the overriding continental plate and descend into the mantle. This downward flexure creates a deep ocean trench, which marks the surface expression of the subduction zone. As the subducting slab sinks, it carries water and volatile materials into the hotter mantle environment. Heat and pressure cause these compounds to be released from the slab’s minerals. This water then migrates into the overlying mantle material, lowering its melting temperature, a process called flux melting. The resulting magma rises buoyantly through the continental lithosphere. If this magma reaches the surface, it erupts, forming a chain of explosive volcanoes on the edge of the continent, known as a continental volcanic arc. The Andes Mountains serve as the most prominent example.
Collision Type Two: Oceanic and Oceanic Plates
A collision between two oceanic plates also results in a subduction zone. The distinction for which plate sinks is based on subtle density differences: the older of the two oceanic plates subducts beneath the younger plate. Oceanic lithosphere cools and contracts as it moves away from its spreading center, making older sections colder and denser. The subducting plate bends downward, forming an ultra-deep ocean trench on the seafloor, such as the Mariana Trench. As the slab descends, the heat-induced release of water into the overlying mantle wedge causes partial melting, identical to the process at an oceanic-continental boundary. The magma generated rises through the overriding oceanic plate. This volcanic activity builds up on the ocean floor, eventually creating a series of volcanoes that may breach the ocean surface. This results in a curved chain of volcanic islands parallel to the trench, called a volcanic island arc. The Aleutian Islands and Japan are well-known examples.
Collision Type Three: Continental and Continental Plates
When two continental plates converge, the outcome is fundamentally different because both plates are composed of relatively low-density, buoyant continental crust. Neither mass is dense enough to sink deeply into the asthenosphere, meaning true subduction ceases. Instead, the colliding masses resist downward motion and compress, buckle, and deform. The immense compressional forces cause the crust to fold, fault, and thicken vertically and horizontally, pushing the rock upward. This process, known as orogeny, creates the world’s most massive and highest mountain ranges. Since there is no deep subduction of a slab, the mechanism for generating magma is absent, meaning these collision zones are typically non-volcanic. The most spectacular example is the ongoing collision between the Indian Plate and the Eurasian Plate. This impact resulted in the formation of the Himalayas, which continue to rise as the plates press together. The resulting mountain belt is characterized by shallow but powerful earthquakes and a broad zone of highly deformed and uplifted crust.