The Mid-Ocean Ridge
The Mid-Ocean Ridge marks a line along which major plates of the Earth's crust are pulling apart at a rate of 1 to 2 centimeters (0.4 to 0.8 inch) a year to form new ocean floor. As the plates separate, molten lava rises up from below to fill the rifts (gaps), solidifying as it meets the icy-cold seawater. As the oceanic plates move away from the rifts, the valleys between the ridges slowly become filled with sediment until, after millions of years, they become abyssal hill regions.
Where an oceanic plate collides with a continental plate, a subduction zone is formed. At a subduction zone, the dense oceanic plate is forced downward, beneath the lighter continental plate, and begins a slow descent back into the Earth's interior, where it is melted down again. The colliding plates and heat make subduction zones regions of volcanic activity and earthquakes.
Subduction zones are marked by deep trenches hundreds of kilometers in length. A trench is formed when an oceanic plate drags some of the crust downward with it as it dives beneath a continental plate. Most of the world's oceanic trenches are found in the Pacific; the only major one in the Atlantic is the Puerto Rico Trench. This is the deepest point in the Atlantic Ocean: 8,648 meters (28,374 feet) below sea level. The deepest known spot on Earth is the Challenger Deep of the Mariana Trench in the central Pacific Ocean, measured at 11,033 meters (36,198 feet) below sea level.