The theory of plate tectonics explains how the Earth’s rigid outer layer, the lithosphere, is broken into large, solid slabs called tectonic plates. This lithosphere includes the crust and the uppermost mantle, extending to about 100 kilometers deep. These plates move slowly over the hotter, more ductile asthenosphere beneath, driven by convection currents generated by the Earth’s internal heat. This continuous movement, typically ranging from less than one to 15 centimeters annually, shapes the planet’s surface, creating continents, ocean basins, and major geological features.
Divergent Boundaries and Crust Formation
Divergent boundaries are zones where tectonic plates pull away from each other, creating new crust and expanding the lithosphere. The most significant feature resulting from this separation is the mid-ocean ridge system, a massive, submerged mountain range that wraps around the globe.
At these oceanic spreading centers, hot mantle material rises beneath the thinned crust. As the rock rises, pressure decreases, causing it to partially melt (decompression melting). The resulting basaltic magma erupts onto the seafloor and solidifies, forming new oceanic crust. This continuous injection of material pushes the older crust outward, a mechanism known as seafloor spreading.
On continents, divergent forces create continental rift valleys, such as the East African Rift. The continental lithosphere stretches and thins, producing a deep valley bounded by faults, accompanied by volcanism and shallow earthquakes. If rifting continues over millions of years, the continental block splits, the valley floods, and a new ocean basin with a mid-ocean ridge begins to form.
Convergent Boundaries and Surface Destruction
Convergent boundaries are zones where plates collide, resulting in the destruction of lithosphere and the formation of dramatic surface features. When a denser plate meets a less dense plate, the denser one bends and plunges beneath the other into the mantle, a process called subduction. The resulting geological features depend on the type of crust involved in the collision: oceanic or continental.
Oceanic-Continental Convergence
When an oceanic plate collides with a continental plate, the oceanic lithosphere subducts beneath the continental plate. This downward bend creates an extremely deep depression on the ocean floor known as an ocean trench. As the subducting plate descends, it carries water-rich minerals into the mantle.
At depths of about 100 to 150 kilometers, heat and pressure drive off this water, causing the overlying mantle rock to partially melt. This buoyant magma rises through the continental crust, forming a chain of explosive volcanoes known as a continental volcanic arc. The Andes Mountains along the western edge of South America are a classic example.
Oceanic-Oceanic Convergence
A collision between two oceanic plates results in subduction, where the older, denser plate sinks beneath the younger one. This process creates a deep ocean trench on the seafloor. Water release and partial melting in the overlying mantle wedge occur, generating magma.
The magma rises to the surface of the overriding oceanic plate, forming a curved chain of volcanic islands parallel to the trench, called a volcanic island arc. Examples include the Mariana Islands and the Aleutian Islands off the coast of Alaska.
Continental-Continental Convergence
When two continental plates collide, neither plate is dense enough to subduct easily. Instead, compressional forces cause the crust to buckle, fold, and fracture. The crustal material is scraped up and thrust over itself, resulting in a dramatic thickening of the lithosphere. This stacking of the crust forms the highest non-volcanic mountain ranges on Earth.
The Himalayas, formed by the collision of the Indian and Eurasian Plates, are the premier example. Volcanic activity is absent because the continental crust is too thick for magma to penetrate. The ongoing collision pushes the mountains upward, causing frequent and powerful earthquakes as stress is released along numerous faults.
Transform Boundaries and Lateral Movement
Transform boundaries involve two plates sliding horizontally past one another along a large fracture known as a strike-slip fault. Crust is neither created nor destroyed at these boundaries, and the motion is purely lateral.
The San Andreas Fault in California, where the Pacific Plate grinds past the North American Plate, is the most famous example. This horizontal grinding causes significant friction and a build-up of strain. When this stress overcomes the resistance along the fault, it is released in a sudden slip, generating frequent, shallow, and intense earthquakes.
Most transform faults are found on the ocean floor, where they offset segments of mid-ocean ridges. Since movement is side-to-side, these zones lack volcanic features but are characterized by seismic activity. The primary surface change is the deformation and offset of the landscape along the fault line.