The surface of the Earth is made of interlocking tectonic plates. The tectonic plates are always moving in relation to each other. When two plates pull away from each other, the seafloor spreads along the boundary of the two plates. At the same time, it contracts in another area.
The Continental Drift Theory
Until 1912, most scientists accepted the contraction theory about the origins of the continents. According to this theory, the continents were formed by the cracking of the Earth's surface as it cooled from its original molten state. The weakness in this theory was that the Earth's mountains should all have formed at about the same time. This was not the case, so there was clearly something missing from the theory. In 1912, scientist Alfred Wegener proposed that the continents actually rested on huge plates that drifted over time, pulling away from each other or colliding together. Wegener's opinions were controversial at first, but later evidence confirmed this theory of continental drift.
When molten rock, or magma, rises up from far below the surface of the Earth, it can split a continental plate in two. This process is called "rifting." The short-term result of rifting is volcanic and earthquake activity, with magma pouring out to the surface along the fault line. The long-term result is that the plate breaks up into two plates, which begin to drift apart from each other as the magma cools and creates new ground. As the two plates push away from each other, a "rift valley" is formed.
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Spreading of the Seafloor
Wegener's hypothesis of continental drift was not embraced when he first proposed it because he was unable to explain what caused the process. In the 1960s, a geologist named Harry Hess was able to show how the seafloor spread when magma rose to the surface. He demonstrated that the ridges in the middle of the great oceans were the result of magma breaking through, creating a "divergent boundary" where the seafloor spread apart. Magma builds up along the edges of the boundary and forms the ocean ridges.
The force that pushes the magma to the surface of the Earth is called convection. Radiation decaying below the surface releases heat. Because heat rises, the hot molten rock below the crust of the Earth tends to rise to the top. Convection forms into currents that drive the tectonic plates either together or apart. The seafloor spreads along diverging boundaries, but it also contracts along the converging boundaries as seafloor is pushed below the surface by two plates in collision with each other. Seafloor is constantly being built in some places and destroyed in others.