THEORY

Plate tectonics is the theory that Earth's outer layer is made up of plates, which have moved throughout Earth's history. The theory explains the how and why behind mountains, volcanoes, and earthquakes, as well as how, long ago, similar animals could have lived at the same time on what are now widely separated continents. You probably wouldn't recognize the Earth if you could see it 225 million years ago. Back then, all the major continents formed one giant supercontinent, called Pangaea.

Exactly what drives plate tectonics is not known. One theory is that convection within the Earth's mantle pushes the plates, in much the same way that air heated by your body rises upward and is deflected sideways when it reaches the ceiling.
Another theory is that gravity is pulling the older, colder, and thus heavier ocean floor with more force than the newer, lighter seafloor.

Whatever drives the movement, plate tectonic activity takes place at four types of boundaries: divergent boundaries, where new crust is formed; convergent boundaries, where crust is consumed; collisional boundaries, where two land masses collide; and transform boundaries, where two plates slide against each other.

DIVERGENT BOUNDARY

The Earth's longest mountain chain isn't the Andes in South America, or the Himalayas in Asia, or even North America's Rockies. It's an underwater chain of mountains 47,000 miles long. The chain runs down the middle of the Atlantic Ocean (surfacing at Iceland), around Africa, through the Indian Ocean, between Australia and Antarctica, and north through the Pacific Ocean.
Running along the top of this chain of mountains is a deep crack, called a rift valley. It is here that new ocean floor is continuously created.

 As the two sides of the mountain move away from each other, magma wells up from the Earth's interior. It then solidifies into rock as it is cooled by the sea, creating new ocean floor.

The speed at which new ocean floor is created varies from one location on the ocean ridge to another. Between North America and Europe, the rate is about 2.2 inches (3.6 cm) per year. At the East Pacific rise, which is pushing a plate into the west coast of South America, the rate is 12.6 inches (32.2 cm) per year.