The Great Rift Valley represents the surface of a divergent plate boundary, where one part of the African plate is tearing away from the other. In the future, it will become a sea.

Alfred Wegener's Continental Drift Theory

In 1915, Wegner proposed the continental drift theory, suggesting that the continents had once been joined to form a single supercontinent. Wegner's theory was supported by four major types of evidence, including matching fossils, rock types, ancient climate, and the continents' resemblance to a jigsaw puzzle.

However, Wegner's hypothesis was rejected by most scientists because it could not describe a main mechanism capable of moving continents. Wegner proposed that the tidal influence of the Moon was the reason the continents were moving.

Development of Plate Tectonics Theory

In 1967, new technologies, such as extensive data on earthquakes and the earth's magnetic field, allowed the plate tectonics theory to develop. Sonar, a system that uses sound waves to calculate the distance to an object, played a significant role in finding deep ocean trenches, mid-ocean ridges, and the composition of the ocean floor. Scientists found the thinnest sediments on the bottom of the ocean floor at mid-ocean ridges.

Seafloor Spreading Process

Harry Hess proposed the hypothesis for seafloor spreading, suggesting that new ocean floor forms along the earth's mid-ocean ridges and slowly moves outward across ocean basins. Along a mid-ocean ridge, eruptions and evidence of volcanic activity have been found. The rate of seafloor spreading is approximately 5 cm per year. Trenches are associated with subduction, the process of the ocean floor returning to the mantle as it sinks.

Plate Tectonics and Plate Boundaries

In the theory of plate tectonics, Earth's lithospheric plates, which are broken pieces of the lithosphere, move slowly relative to each other driven by convection currents in the mantle. Convection currents are the mechanism that drives plate tectonics. There are three types of plate boundaries: divergent boundaries, where two plates move apart; convergent boundaries, where two plates move together; and transform fault boundaries, where two plates grind past each other.

At constructive plate margins, new lithosphere is produced, while at destructive plate margins, the lithosphere is destroyed. The three types of convergent boundaries are oceanic-continental, oceanic-oceanic, and continental-continental. Continental volcanic arcs are formed by the subduction of oceanic lithosphere, and the Himalayas were formed from the collision of India and Asia.

At a transform plate boundary, plates grind past each other without destroying the lithosphere.