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
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.