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Slow-spreading mid-ocean ridges, like the Mid-Atlantic Ridge, are broad and have a deep central rift valley. Some rifts are 6 miles (10 km) wide and 2 miles (3 km) deep. Faults are numerous and create rough topography. Cross-section of Mid-Atlantic Ridge at 37 degrees N from Macdonald (1982). Vertical exaggeration = 4x.
Active volcanic centers at slow-spreading mid-ocean ridges are discontinuous and consist of very small coalesced seamounts. Smith and Cann (1990) found a minimum of 481 seamounts along a 500 mile (800 km) segment of the Mid-Atlantic Ridge. The seamounts range from 150-1,800 feet (50-600 m) in height. Most are about 180 feet (60 m high). Smith and Cann (1992) used bathymetric data to construct this map of volcanoes on the Mid-Atlantic Ridge. Seamounts range in shape from pointy to flat-topped cones. Older volcanoes and flows shown in darker grays. Fissures not shown.
Smith and others (1997) compared seamounts and submarine hummocks and hummocky ridges along the Mid-Atlantic Ridge to the Laki fissure and associated cones in Iceland. The cone in the foreground is about 600 feet (200 m) in diameter and 90 feet (30 m) high, comparable in size to volcanic features along the Mid-Atlantic Ridge. Photo by Thor Thordarson.
Smith and Cann's detailed estimate of the number of volcanoes along the mid-ocean ridges is ten-times greater than earlier studies. Extrapolating their results for all of the North Atlantic suggests there are as many as 85 million seamounts on the ocean floor. 2.5 million of these are over 600 feet (200 m) tall. The style in which new oceanic crust is created is different at slow- and intermediate-spreading ridges, relative to fast-spreading ridges, because large numbers of small seamounts form the crust along the axis of the ridge. The amount of magma is not enough to generate the large fissure eruptions which occur at fast-spreading mid-ocean ridges like the East Pacific Rise. This cross-section from Smith and Cann (1992) shows the crustal structure of the Mid-Atlantic Ridge. The crust is made of seamounts and fissure-fed flows (area above magma chamber). Normal faults bound the edges of the ridge's inner valley. Small separate magma bodies (gray ovals) feed individual volcanoes. The solidified magma bodies make the lower oceanic crust.
Super-slow mid-ocean ridges move at rates of less than 25 mm/year and are considered to be the most common type of ridge (when measured by the length of ridge). They are found in the Southwest Indian Ridge and the Arctic. Study of this type of spreading center is only beginning because of the complicated logistics of working in the Southwest Indian Ocean (trying to work under the Arctic ice would be to expensive and extremely difficult). Researchers hope to use what they learn from super-slow mid-ocean ridges to test their models of how slow and fast mid-ocean ridges behave. The largest volume of oceanic crust is generated at the fast-spreading mid-ocean ridges. Very slow-spreading ridges, like the Southwest Indian Ridge, constitute the largest single class of mid-ocean ridges (Solomon, 1989).