The magma that rises at the Mid-Atlantic Ridge obviously originates in the upper mantle. Its composition differs considerably from that of the mantle, however. Magma that cools at ocean ridges forms a common kind of rock known as basalt. But researchers have found that seismic waves travel through the upper mantle at a rate of more than eight kilometers per second, far faster than they would pass through basalt.
One material that could possibly allow such a high velocity of sound is a type of dense, dark-green rock called peridotite. Peridotite consists mostly of three silicon-based minerals: olivine, a dense silicate containing magnesium and iron; orthopyroxene, a similar but less dense mineral; and clinopyroxene, which incorporates some aluminum and is more than 20 percent calcium. Peridotites also have small quantities of spinel, an oxide of chromium, aluminum, magnesium and iron.
How can basaltic magma be produced from a mantle made of peridotite? More than 30 years ago experimental petrologists such as Alfred E. Ringwood and David H. Green of the Australian National University exposed samples of peridotite to elevated temperatures (1,200 to 1,300 degrees C) and high pressures (more than 10,000 atmospheres). These values duplicate the temperature and pressure that exist in the suboceanic upper mantle roughly 100 kilometers below the seafloor. This research showed that gradual decompression of peridotite at those high temperatures melts up to 25 percent of the rock. The melt had a basaltic composition similar to that of mid-ocean ridge basalts.
These experiments support the view that hot, peridotitic mantle material rises under mid-ocean ridges from depths exceeding 100 kilometers below the seafloor. As the material moves upward, the mantle peridotite decompresses and partially melts. The melted part takes on the composition of a basaltic magma, rising rapidly toward the surface and separating from the peridotite that did not melt. Part of the melt erupts on the seafloor along the crest of the mid-ocean ridge, where it cools and solidifies and adds to the ridge crest. The remainder cools and solidifies slowly below the surface, giving rise to new oceanic crust. The thickness of the oceanic crust depends on the amount of melt that is extracted from the mantle.
The ridge crest's depth below sea level marks an equilibrium level determined by the temperature and initial composition of the upper mantle upwelling below the ridge. If the temperature and composition of the mantle were constant all along the ridge, the summit of the ridge would be at the same depth below sea level all along its length.
In the real world such consistency is unlikely. Small variations in mantle temperature or composition along the ridge would cause the summit to settle at varying elevations. Regions of suboceanic mantle where temperatures are higher have lower densities. In addition, a hotter mantle would melt more and produce a thicker basaltic crust. As a result, the ridge summits there will be higher.
The summit of the Mid-Atlantic Ridge shows just such variations in depth below sea level. For instance, along the ridge between about 35 and 45 degrees north latitude lies an area of abnormally high topography. Earth-orbiting satellites have detected in the same region an upward swell in the level of the geoid (the equilibrium level of Earths surface, roughly equivalent to the average sea level).
Researchers generally attribute this swell to the influence of a so-called hot spot centered on the Azores island group. Hot spots are zones that have high topography and excess volcanism. They are generally interpreted as the surface expression of a "mantle plume"--that is, of a rising column of unusually hot mantle material. Most oceanic islands, including the Hawaiian Islands and Iceland, are thought to be the surface expressions of mantle plumes. The source of the heat is thought to lie in the boundary zones deep inside Earth, even as deep as the core-mantle boundary [see "The Core-Mantle Boundary," on page 36].