Mid-ocean ridge basalts

Mid-ocean ridge basalts are slowly extruded and build up oceanic ridges so that they or their coarse-grained equivalent make up the bulk of the oceanic crust. The basalts form by the melting of peridotite in the upper mantle. Most of these basalts are characterized by lower potassium than basalts formed in other tectonic environments.

Composition of Basalts

Plate tectonics is the theory that the Earth's crust is divided into seven major plates and many minor plates and microplates created at divergent boundaries on one edge and destroyed by subduction at another edge. Divergent boundaries are ones where basaltic magma is being produced and added as a new oceanic crust to the plate as it moves slowly away from this boundary. Large mountain chains called mid-ocean ridges, which encircle the Earth mostly below the ocean's surface, are formed at the divergent boundaries. The plate gradually moves away from the divergent boundary and is eventually subducted or thrust under another plate.

Basalts are dark, fine-grained to porphyritic rocks that contain calcium-rich plagioclase, pyroxene, olivine, and minor spinel. Basalts or their coarser-grained equivalents, gabbros, are the main rock types produced at oceanic ridges. Basalts and gabbros make up most of the oceanic plate that moves in conveyer-belt fashion out from the oceanic ridges. The basalts at oceanic ridges are reasonably homogeneous. For example, they mostly range from 48 to 52 weight percent silicon dioxide. Most of the basalts at oceanic ridges are in a subgroup called tholeiitic basalts. They have the same composition as other basalts, except they contain orthorhombic pyroxene and generally have low potassium contents.

Tholeiitic basalts are also found in other tectonic environments with other rock types, such as on ocean floors away from plate boundaries (as in the Hawaiian Islands), subduction zones, and continental rifts. They can also occur as flood basalts on continents. The main difference between most oceanic ridge tholeiitic basalts and tholeiites formed in other tectonic environments is that the oceanic ridge tholeiites have lower amounts of potassium and certain trace elements (rubidium, barium, and the light rare-earth elements) than the others.

Variation in Composition: Fast-Moving Versus Slow-Moving Ridges

The variation in composition of mid-ocean ridge basalts has not been studied in the same detail as basalts at the surface since they usually occur several kilometers below the ocean's surface. Detailed sampling in some places, however, suggests interesting variations in composition and style of eruption.

Oceanic ridges vary a great deal in the speed at which they separate. Fast-moving ridges, such as the East Pacific Rise, spread at rates of eight to sixteen centimeters per year, and there is no distinct valley produced at the highest portions of the ridge. Also, the width of the volcanic zone is narrow, suggesting a narrow but large magma chamber below the surface. In contrast, slow-moving ridges, such as the Mid-Atlantic Ridge, move at rates of only one to four centimeters per year, and there is a central valley about eight to twenty kilometers wide and one to two kilometers deep with a wider zone of volcanic activity. This suggests that the magma chambers below the surface are wider than those at fast-moving ridges.

In the case of fast-moving oceanic ridges, sheets of lava are extruded fairly rapidly and continuously along a low-relief ridge. In contrast, slow-moving ridges have lavas slowly extruded at only certain points along the ridge, where the ridge is built to fairly high relief. In addition, fast-spreading ridges have frequent eruptions that are fairly homogeneous in composition, while slow-moving ridges have less frequent eruptions that are more heterogeneous in composition. The fast-moving ridges are believed to have narrowly focused magma chambers where the magma is injected along a narrow region all along the ridge crest, while slow-moving ridges are believed to have only a few, less focused magma chambers that erupt lavas only at different points along the ridge over a wider area.

Slow-moving ridges seem to exhibit regular variations in the composition of basalts from the center of the ridge out to the valley walls. Basalts in the center of the rift have a greater abundance of large olivine crystals relative to plagioclase and monoclinic pyroxene crystals than those closer to the valley walls. The volcanic glasses in the center of the ridge also contain less silica and potassium than those near the valley walls. Spreading of ridges at speeds intermediate between these two extremes has characteristics intermediate between those of the fast- and slow-moving ridges.

Variation in Composition: Eruption Styles

Volcanic eruption rates at undersea ridge crests are not continuous, and they are of low volume compared to eruptions in other tectonic environments. Nevertheless, the continued eruption of the tholeiitic basalts over long periods on the ridge crests results in the largest total volume of basaltic eruptions on Earth.

Basalts formed at mid-ocean ridges likely formed by the partial melting of rocks in the Earth's upper mantle. The melts may be modified in composition by the crystallization of minerals that settle out of the melt, thus changing their composition.

The main rock in the upper mantle is probably peridotite. Up to 30 percent melting of this rock produces tholeiitic basaltic melts similar in composition to many found at oceanic ridges. The peridotite must rise upward from some depth in the mantle as a blob or plume. The peridotite is close to its melting point, so it can begin to melt as the pressure is reduced on rising to shallow depths of about eighty kilometers below the surface. This melting produces a liquid-crystal mush that melts further as it rises to shallower depths. Eventually, a point is reached where most of the melt can leave the crystals behind as the liquid rises into the oceanic crust.

The large differences in composition among tholeiitic basalts formed in different regions may be explained by differences in the composition of the peridotite source and the degree of melting of the peridotite. For example, the relatively high potassium, rubidium, barium, and light rare-earth element concentrations along the Mid-Atlantic Ridge near Iceland can be explained by the melting of previously unmelted peridotite with high concentrations of these elements. Most peridotite in the upper mantle below oceanic rises appears to have melted at least once before. Melting of peridotite causes it to be depleted in potassium, rubidium, barium, and light rare-earth elements as these elements move into the melt. Thus, melting this peridotite a second time depletes the abundant tholeiitic basalts in these elements.

The smaller differences in concentrations of elements across the valley at the top of the mid-ocean ridge may be explained by the crystallization and settling out of minerals such as olivine, pyroxene, and plagioclase from the melt. These minerals have different compositions from those of the melt, so that as they settle out of the melt or float, the composition of the melt slowly changes. For example, as olivine crystallizes, it picks up less silica than does the melt, so the melt will gradually increase in silica as the olivine settles out. The melt and some suspended crystals may periodically squirt upward along fractures as crystallization takes place. Some melt and crystals may reach the surface of the ridge as a lava flow, or some may crystallize completely within the fractures below the surface.

Over long periods, such processes result in the zonation of the oceanic crust. Depths of about four to eight kilometers below sea level contain mostly coarse-grained gabbros with varied amounts of olivine, pyroxene, and plagioclase. Depths of two to four kilometers below sea level contain mostly fine-grained basalts produced by periodic extrusions of lava on the seafloor ridge. Between the gabbros and basalts, there are a lot of injections of intermediate-grain-size gabbros along fractures. Sometimes, these rocks are so abundant that it is difficult to tell the composition of the original rock.

Interaction of Basalt with Seawater

Some basalts sampled on the sea floor appear to have had sodium added to them and calcium, iron, and silica removed. These basalts likely came into contact with hot seawater enriched in sodium. Hot springs have been observed to exist along the East Pacific Rise. The waters emitted at the hot springs have temperatures of up to 350 degrees Celsius. Some hot springs have a black or gray, smoky appearance. The dark color results from the precipitation of tiny sulfide and oxide minerals from the hot water as it interacts with cold seawater. As a result of this activity, large columns of sulfide minerals are built up, and a unique ecosystem with giant worms up to two meters long, clams, and crabs occurs near the vents of hot water. They obtain their energy from the heat of the water.

These gray and black smokers appear to be the surface expression of an elaborate hot-water plumbing system in which seawater percolates down fractures in an area several kilometers wide along the ridge axis. The seawater likely percolates downward to depths of at least two to three kilometers and is gradually warmed to temperatures of 400 to 450 degrees Celsius. The seawater then runs out of fractures and eventually percolates back up toward the surface, and some is liberated at the ridge axis as smokers. During its circulation through the fractures in the rocks, the hot water likely reacts with the basalts and forms new minerals.

Significance

Some geologists study the potential ore deposits in association with mid-ocean ridge basalts to learn about their origin. In some places, for example, copper sulfide deposits are formed as a precipitate around the vents of hot springs. The island of Cyprus has abundant copper sulfide deposits that now occur at the surface that were originally formed this way. These deposits have been mined since ancient Greek times.

Manganese is also released by these hot waters, but not much of it is deposited around the vents. Instead, the manganese slowly precipitates out of seawater in manganese nodules on large portions of the ocean floor. Although not yet mined, these nodules are a potentially abundant source of manganese and other metals. Other ore deposits associated with mid-ocean ridge basalts are related to the precipitation of minerals out of the basaltic melts below the rise. In some cases, chromite (an ore of chromium) is one of the first minerals to crystallize out of the magma. Since this mineral is dense, it settles out of the melt as pods and layers. Some pods are small, but others may contain up to several million tons of chromium. One such deposit is exposed at the surface in Oman.

Geologists also study the mid-ocean ridge basalts to build theories about how they form and evolve. Much evidence must be accumulated to develop such theories. For example, experiments in furnaces that approximate the conditions in the upper mantle suggest that the main rock in the upper mantle, peridotite, is the only rock that can melt to produce basaltic melts. Also, certain isotopic and trace element concentrations in the basaltic melts are consistent with their origin by melting of peridotite. Estimates of temperatures and pressures in the upper mantle suggest that peridotite is close to its melting point in many places. Experiments in furnaces suggest that the release of pressure on peridotites under these conditions as they rise toward the surface will cause them to melt.

Principal Terms

basalt: a dark-colored, fine-grained to porphyritic igneous rock

gabbro: the coarser-grained equivalent of basalt

magma: molten rock material or melt, mixed with crystals, that occurs below the Earth's surface

peridotite: a coarse-grained rock that consists of olivine, pyroxene, and garnet; it is likely the main rock in the upper mantle of the Earth

porphyritic rock: igneous rock with large mineral crystals embedded in much finer-grained minerals

tholeiitic basalt: a type of basalt with calcium-rich plagioclase, monoclinic pyroxene, orthorhombic pyroxene, and olivine or quartz; it has low potassium concentrations compared to the more alkali-rich basalts

upper mantle: the mantle occurs between the core and the crust of the Earth; portions of the upper part of the mantle are believed to have a zone of partial melting that may produce melts that crystallize to basalts

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