By Doug Bennett
Updated Aug 30, 2022
Gary Gray/iStock/GettyImages
The theory of plate tectonics, established in the 1960s, explains how the Earth's lithosphere is divided into at least a dozen distinct plates that drift slowly over the asthenosphere. Their interactions along boundary zones give rise to a range of distinctive surface features—fault lines, trenches, volcanoes, mountain ranges, ridges, and rift valleys.
A transform boundary joins two divergent zones, creating a fault line where two plates slide past one another. The world’s most famous example is the San Andreas Fault, which links the East Pacific Rise to the south with the South Gorda, Juan de Fuca, and Explorer Ridges to the north.
Convergent boundaries generate trenches as the denser plate is forced downward in a subduction zone. The Marianas Trench—formed by the collision of two oceanic plates—is the deepest known trench, with its Challenger Deep plunging over 36,000 feet below sea level, deeper than the height of Mount Everest.
Subduction zones also produce volcanoes. When the descending plate melts, magma rises to the surface. Mount Saint Helens, for example, erupted from an oceanic plate subducting beneath North America. When two oceanic plates converge, a trench and a chain of volcanoes often appear side‑by‑side, as seen along the Mariana Islands that flank the Marianas Trench.
When two continental plates collide, neither plate can subduct, and the immense pressure pushes the crust upward, forming towering ranges. The Himalayas illustrate this process: the Indian Plate pushing against the Eurasian Plate has built the world’s highest peaks.
Opposite to convergent zones, divergent boundaries form ridges where tectonic plates separate and magma rises to create new crust. The Mid‑Atlantic Ridge marks the divergent boundary between the Eurasian and North American plates, carving a continuous underwater mountain chain across the Atlantic.
Divergent boundaries on continental plates give rise to rift valleys—deep, elongated depressions that slowly fill with water and may eventually become new ocean basins. The East African Rift Zone, a triple junction where the Arabian, Nubian, and Somalian plates diverge, is a striking example of this phenomenon.
