Plate Tectonics

Plate tectonics is the scientific theory that Earth's lithosphere is broken into a number of large tectonic plates, generally seven or eight major ones along with many smaller ones, that have been slowly moving for three to four billion years, riding atop the viscoelastic asthenosphere beneath them. The theory grew out of the earlier idea of continental drift and gained wide acceptance once seafloor spreading was confirmed in the mid to late 1960s; the strongest force driving plate motion appears to be slab pull, the weight of cold, dense plate material sinking into the mantle at ocean trenches, with mantle convection and density differences contributing as well. Where plates meet, their relative motion defines the boundary as convergent, divergent, or transform, and these boundaries produce most of Earth's major geological activity: earthquakes concentrate along oceanic trenches and spreading ridges, convergent boundaries compress, fold, and uplift rock into mountain ranges, most active volcanoes sit along plate boundaries, and divergent boundaries create new oceanic crust through seafloor spreading while subduction zones form trenches and volcanic arcs.