The mountain ranges on the seafloor that mark the mid-ocean ridge system form the
longest linear feature on the Earth’s surface, with a total
length of approximately 40,000 miles (65,000 km). The ridge
system ranges from about 600 to 2,500 miles (1,000 to 4,000
km) wide, and rises an average of 1–2 miles (2–3 km) above
the surrounding seafloor, or about 1.5–2 miles (2.5 km)
below the sea surface. The ridges are broken into segments by
transform faults that accommodate the differential motion
caused by spreading on offset ridge segments. The amount of
offset on the transforms ranges from a few miles to hundreds
of miles. The sense of motion on the transform faults is opposite
to that of what would be expected if the faults had offset
a previously continuous ridge. This relationship shows that
the transforms accommodate geometric consequences of
spreading on a sphere, and the ridge segments were always
arranged in an offset manner since they formed.
The mid-ocean ridge system is divided into several main
branches located in each of the world’s main oceans. The
Mid-Atlantic Ridge bisects the North and South Atlantic
Oceans and connects in the south with the Antarctic ridge
that surrounds the Antarctic continent. The northern extension
of the Mid-Atlantic Ridge strikes through Iceland (where
it is known as the Reykjanes Ridge) and then continues to
connect with the Arctic Ocean ridge. The East Pacific Rise
branches off the Antarctic ridge between Australia and South
America and in various places separates the Antarctic, Pacific,
Nazca Cocos, and North American plates. The ridge disappears
beneath North America in the Gulf of California
where plate boundary motions are taken up by the San
Andreas Fault system. Remnants of the once-larger East
Pacific rise are located along the North American–Pacific
plate boundary, such as off the coast of British Columbia
where the Juan de Fuca Ridge separates the Pacific and Juan
de Fuca plates. The Indian Ocean ridge branches off the
Antarctic ridge and extends into the Gulf of Aden, where it
continues as the immature Red Sea rift and branches into the
Afar triple junction where incipient spreading is occurring in
the East African rift system.
Morphological studies of these ridge systems has led to
the division of mid-ocean ridges into slow-spreading or
Atlantic-type ridges, and fast-spreading or Pacific-type ridges.
Atlantic-type ridges are characterized by a broad, 900–2,000-
mile (1,500–3,000-km) wide swell in which the seafloor rises
0.5–2 miles (1–3 km) from abyssal plains at 2.5 miles (4.0
km) below sea level to about 1.7 miles (2.8 km) below sea
level along the ridge axis. Slopes on the ridge are generally
less than 1°. Slow-spreading ridges have a median rift, typically
about 19 miles (30 km) wide at the top to 0.5–2.5 miles
(1–4 km) wide at the bottom of the 0.5-mile (1-km) deep
medial rift. Many constructional volcanoes are located along
the base and inner wall of the medial rift. Rugged topography
and many faults forming a strongly block-faulted slope characterize
the central part of Atlantic-type ridges.
Fast-spreading or Pacific-type ridges are generally
1,250–2,500 miles (2,000–4,000 km) wide, and rise 1–2
miles (2–3 km) above the abyssal plains, with 0.1° slopes.
Pacific-type ridges have no median valley but have many
shallow earthquakes, high heat flow, and low gravity in the
center of the ridge, suggesting that magma may be present at
shallow levels beneath the surface. Pacific-type ridges have
much smoother flanks than Atlantic-type ridges.
See also DIVERGENT OR EXTENSIONAL BOUNDARIES.
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