|Themes > Science > Earth Sciences > Oceanography > Ocean Regions > Ocean Floor > Mid-Ocean Ridge|
The mid-ocean ridge is two chains of mountains separated by a large depression (or rift valley) that form at a spreading center (or where two plates are drifting apart). The mountain ranges can have peaks as high as 12,000 feet (2,500 meters meters) and some even reach above the ocean's surface. Iceland, along the mid-Atlantic Ridge, is an example of this.
In the rift valley, which can be 15 to 30 miles (24 to 48 kilometers) wide, new oceanic crust is being made, which means lots of seismic activity is happening. Hydrothermal vents were discovered in rift valleys.
The plates are spreading at a rate of 2.5 centimeters a year. This means that every thousand years or so the plates spread and grow about 25 meters.
Most seamounts began life as volcanoes formed over hot spots in the ocean floor. After the crust moves off the hot spot, the volcanic activity stops.
Seamounts are usually 25 miles (40 kilometers) in diameter and can be 10,000 to 15,000 feet (3000 to 4500 meters) tall. In fact, some are so tall that their peaks pierce the ocean surface forming a volcanic island or, if there are more than one seamount, a volcanic island chain (think of the Hawaiian Islands).
Seamounts whose peaks have eroded and become a flat surface are called guyots.
Coral reefs sometimes grow around seamounts that rise above the ocean waters. As the seamount sinks or its peak erodes, the seamount will disappear beneath the water leaving the coral ring. This is called an atoll.