The mechanism that operates along the oceanic ridge system to create new seafloor is appropriately called seafloor spreading. Typical rates of spreading average around 5 centimeters (2 inches) per year, roughly the same rate at which human fingernails grow. Compara- tively slow spreading rates of 2 centimeters per year are found along the Mid-Atlantic Ridge, whereas spreading rates exceeding 15 centimeters (6 inches) per year have been measured along sections of the East Pacific Rise. Although these rates of seafloor production are slow on human time scale, they are nevertheless rapid enough to have generated all of Earth’s ocean basins within the past 200 million years.
The primary reason for the elevated position of the oceanic ridge is that newly created oceanic lithosphere
is hot, which means it is less dense than cooler rocks found away from the ridge axis. (Geologists use the term axis to refer to a line that follows the general trend of the ridge crest.) As soon as new lithosphere forms, it is slowly yet continually displaced away from the zone
of upwelling. Thus, it begins to cool and contract, thereby increasing in density. This thermal contraction accounts for the increase in ocean depths away from
the ridge crest. It takes about 80 million years for the temperature of oceanic lithosphere to stabilize and contraction to cease. By this time, rock that was once part of the elevated oceanic ridge system is located
in the deep-ocean basin, where it may be buried by substantial accumulations of sediment.
In addition, as the plate moves away from the ridge, cooling of the underlying astheno- sphere causes it to become increasingly more rigid. Thus, oceanic lithosphere is generated
by cooling of the asthenosphere from the top down. Stated another way, the thickness of oceanic lithosphere is age dependent. The older (cooler) it is, the greater its thickness. Oceanic lithosphere that exceeds 80 million years in
age is about 100 kilometers (60 miles) thick— approximately its maximum thickness.