A Canadian scientist named J. Tuzla Wilson observed cracks in the continents similar to those on the ocean floor. According to Wilson the llithosphere is broken into separate sections called plates.
Wilson combined what geologist knew about sea-floor spreading, Earth's plates, and continental drift into a single theory, a scientific theory. The theory of plate tectonics.
Geologist think that movement of convection currents in the mantle is the major force that causes plate motion. As the plates move, they collide, pull apart, or grind past each other.
Plate boundaries extend deep into the lithosphere. Faults—breaks in Earth's crust where rocks have slipped past each other—form along these boundaries. Scientists have used instruments on satellites to measure plate motion very precisely.
The place where two plates move apart, or diverge, is called a divergent boundary. Most occur along the mid-ocean ridges where sea-floor spreading occurs. They also occur on land. When it develops on land, two of Earth's plates slide apart.
Convergent boundaries The place where two plates come together, or converge, is called a convergent boundary. When two plates converge, the result is called a collision. Where two plates carrying oceanic crust meets at a trench, the plate that is more dense sinks under the other plate.