Plate tectonics is the single process that explains why Earth looks the way it does: why oceans open, why mountain ranges rise, and why nearly all earthquakes and volcanoes line up along the edges of its shifting crustal plates. The engine is internal heat. Tremendous heat and pressure within the Earth's mantle set hot, molten rock flowing in convection currents, and those currents—scientists' leading explanation for the phenomenon—are what drive the plates above. Everything else in the story follows from where two plates meet: they pull apart, collide, or slide past one another, and each arrangement produces its own signature of valleys, ridges, trenches, volcanoes, and quakes.
Earth's Dynamic Engine: Mantle Convection and Plate Movement
The theory holds that Earth's solid outer crust is broken into individual pieces called tectonic plates, and that both the continents and the ocean floor are part of this crust. As the University of Hawaiʻi's Exploring Our Fluid Earth curriculum explains, the movement of these plates is likely caused by convection currents in the molten rock of the mantle beneath them. The same source distinguishes the timescales of the result: earthquakes and volcanoes are the short-term expressions of tectonic movement, while the long-term result is the migration of entire continents over millions of years.
The pace is slow but relentless. The U.S. Geological Survey notes that average plate motion rates range from less than 1 to more than 15 centimeters per year, in the past as well as the present. Most movement is concentrated along narrow zones between plates, which is precisely where the forces at work are most visible—and where the consequences are felt. With some notable exceptions, nearly all the world's earthquake and volcanic activity occurs along or near these boundaries, according to the USGS.
