A section of the mid-Atlantic Ridge where the African and South American Plates are created. A rift valley over a mile (2 km) deep marks the axis of the ridge. Depths range from 1900 (pink) to 4200 meters (dark blue). Image used with permission of Ken Macdonald.
Divergent (or spreading centers) plate boundaries are characterized by features called mid-ocean ridges. Combined, the ridges are nearly 46,000 miles (74,000 km) long. The ridges are home to Earth's highest mountains, deepest canyons, and longest escarpments. The shape of the mid-ocean ridge and the style and rate of volcanism is controlled by the rate the plates move apart.
Growth of tectonic plates at mid-ocean ridges. This photo shows a ropy lava flow erupted in April 1991, on the floor of the axial summit collapse trough of the East Pacific Rise near 9 degrees 50.6'N. Note dead tubeworms. Photography courtesy of Woods Hole Oceanographic Institution and members of the Adventure dive (Principle Investigators: D. Fornari, R. Haymon, K. Von Damm, M. Perfit, M. Lilley, and R. Lutz).
New oceanic plates are created at mid-ocean ridges. About 2.4 cubic miles (10 cubic km) of new oceanic crust is added each year (not all of this magma is erupted by volcanoes). This is about 100 times the volume of lava erupted by Kilauea each year.
Mid-ocean ridges are divided into these groups based on their spreading rates:
Slow: 1-5 cm/yr total opening rate
Medium: 5-10 cm/yr total opening rate
Fast: 10-20 cm/yr total opening rate
Computer-generated topographic map of the east Pacific Rise near 9 degrees north. Blues and greens represent lower elevations. Yellows and reds represent higher elevations. The narrow axis (red linear features) and narrow or absent axial rift, characteristics of fast-spreading mid-ocean ridges, are obvious. The spreading centers overlap near the middle of the image. The cones in the top-right corner of the image are a linear chain of near-axis seamounts on the Pacific Plate. The seamounts are greater than 200 m tall. The Clipperton transform fault offsets the ridge at the top of the image. View is to the north. Compare with map below. Image courtesy of Stacey Tighe, University of Rhode Island. Image from U.S. Geological Survey's This Dynamic Earth.
Fast-spreading mid-ocean ridges move 100-200 mm/yr. The East Pacific Rise is perhaps the best studied fast-spreading mid-ocean ridge. The ridge segment that creates the Nazca and Pacific plates moves up to 5.6 inches (142 mm) each year.
The topography across fast-spreading mid-ocean ridges is less pronounced relative to slow-spreading mid-ocean ridges. Geologists believe the amount of heat and magma plays an important role in defining the morphology and behavior of the ridge. At a slow-spreading mid-ocean ridge magma is supplied at a slow rate and the oceanic plates cool, causing the crest of the ridge to subside. At a fast-spreading mid-ocean ridge the magma supply rate is higher. This keeps the plates warmer and the crest of the rise does not subside. Cross-section of East Pacific Rise at 3 degrees S from Macdonald (1982). Vertical exaggeration = 4x.
In contrast with slow spreading mid-ocean-ridges, where hundreds of seamounts form in the rift along the crest or the ridge, volcanism at fast spreading mid-ocean-ridges is much like the subaerial fissure erupts associated with volcanic rift zones. This photo shows a curtain of fire during an eruption of Mauna Loa in Hawaii. Photo by Robin Holcomb, U.S. Geological Survey's Hawaiian Volcano Observatory, July 5, 1975.
Left: This map shows segments of the East Pacific Rise near the Clipperton Transform. The thick black lines show the location of the spreading axes along the rise. The thick red lines indicate portions of the rises where the presence of an axial magma chamber has been inferred. Stars show the location of active hydrothermal vent areas. Compare to the computer-generated topographic map at the top of this page. Modified from Macdonald and Fox (1988).
Magma chambers extend tens of kilometers along the axes of fast spreading mid-ocean-ridges. These magma chambers feed the fissure eruptions. Some magma chambers are only little more than half a mile (one kilometer) beneath the seafloor. This cross-section of the oceanic crust along the northern East Pacific Rise is based on a model proposed by Sinton and Detrick (1992) and modified by Perfit and others (1994). The cross-section is 5 miles (8 km) across and 3 miles (5 km) deep.
In addition to fissure eruptions, another style of volcanism occurs at fast-spreading mid-ocean ridges. Central-vent eruptions away from the axis of the spreading center form near-axis seamounts (Scheirer and Macdonald, 1995). These volcanoes range in height from 150-7,500 feet (50-2500 m). Small seamounts are much more common than large ones with the characteristic height of seamounts in the Pacific ranging from 180-900 feet (60-300 m). The summits of the volcanoes can be flat or include craters. The flanks of the volcanoes have gentle slopes between 5 and 25 degrees. This image is constructed from side scan data and shows the P9 degrees 05 seamount chain. Image courtesy of Ken Macdonald (see Journal of Geophysical Research, v. 100, no. B2, p. 2239-2259).
Fresh lava at the summits of these volcanoes indicates they continue to erupt up to 50 miles (80 km) from the spreading center. Most of the off-axis seamounts form linear chains away from the spreading center with the most recent activity closer to the rise. The origin of near-axis seamounts, volcanoes that form away from the axis of the spreading center is controversial. Some geologist suggests "mini-hotspots" feed these volcanoes. Other geologists have postulated that broad mantle upwellings beneath the ridge feed magma to the volcanoes. This type of volcanism produces about 1.5-2 percent of new oceanic crust (Shen and others, 1993).
The volume of lava erupted at near-axis seamounts is small relative to hot spot volcanoes like Hawaii. This diagram compares the volumes erupted at seamounts along the northern East Pacific Rise (NEPR), the southern East Pacific Rise (SEPR), and the Hawaiian hot spot in the last 2 million years. The Hawaiian hot spot has produced about 20 times more lava, relative to the northern East Pacific Rise, during this period. The volume of lava erupted at the southern East Pacific Rise is about 3 times greater than the northern East Pacific Rise. Volume estimates from Scheirer and Macdonald (1995).
Mid-Ocean Ridges with medium spreading rates are transitional in character between slow- and fast-spreading ridges. The rift valley of a mid-ocean ridge with a medium spreading (5-10 cm/yr) is only about 50-200 m deep. The rift is only 5 km across. Cross-section of East Pacific Rise at 21 degrees N from Macdonald (1982). Vertical exaggeration = 4x.
The Juan de Fuca Ridge, only about 200 miles (300 km) west of the state of Washington, is perhaps the most studied ridge with a medium spreading rate. The Juan de Fuca Ridge creates part of the Pacific plate and all of the Juan de Fuca plate. Map of the northeast Pacific from Johnston and Embley (1990). Note the presence of linear seamount chains associated with hot spots.
One of the most interesting features of the Juan de Fuca Ridge is the Axial volcano. This active volcano is located at the intersection of the Cobb-Eikelberg hotspot and the Juan de Fuca Ridge. The hot spot has produced the Cobb-Eikelberg chain, a broad band of seamounts which extend over 900 miles (1,500 km) to the northwest. Image courtesy of U.S. Dept. of Commerce / NOAA / OAR / ERL / PMEL / VENTS Program.
Axial seamount is a broad volcano with a summit caldera and two rift zones. The large size of the volcano (roughly 12 by 18 miles; 20 x 30 km) is related to the input from the hotspot. The presence of the caldera suggests eruption from a shallow magma chamber and subsequent collapse. The summit of the volcano is less than a mile (1.4 km) below sea level. Recent lava flows and active hydrothermal systems suggest the volcano is active. This cross-section of the volcano shows pillow lavas (black ovals) which are older than 50,000 years. Sheet flows that range in age from 9,000-5,000 years drape the flanks of the volcano. Sheet flows less than 5,000 years old fill the floor of the caldera. From Zonenshain and others (1989).
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).