tively small sample—ocean centers and their ridges are not easily accessible for direct study.

Recent studies of the ocean volcanic system show that an assumption of uniform magma production along the axis of a spreading center is too simplistic. Close examination of ocean spreading centers and ridges shows them to be segmented at finer scales. Within each segment is a topographic high that is thought to reflect the center of an isolated magma chamber. These chambers feed magma to the remainder of the segment but not beyond the segment boundaries. The spacing of these magma chambers is quite regular—each is on the order of 50 to 70 km long.

The regular spacing of ocean ridge segments is one case in which theoretical understanding of a phenomenon preceded its observation. Fluid mechanical models for melt separation from a large partially molten zone indicate that the melts are drawn from the host rock to coalesce into local concentrations of magma. These local concentrations begin to rise toward the surface because, compared to the surrounding rock, they are low in density and are buoyant. The rising magma bodies are fed by melt extracted from a much wider area than that represented by the bodies themselves. The regular spacing of the globules, and of the volcanoes they eventually create, can be thought of as the optimal size of the melt feeding zone for the globule. Globules formed too close together would not have enough magma available for them to grow large enough to rise efficiently through the overlying rock. Globules formed too far apart are not capable of extracting magma from the midpoint between the globules. Widely separated globules leave behind a source of magma that eventually will create another globule between the original two.

The observed segmentation of ocean spreading centers conforms to this model and suggests that there is a continuous melt zone at depth feeding the isolated volcanoes of each segment. Researchers are attempting to identify and understand the effects of segmentation. They are especially interested in the cooling and chemical fractionation history of magmas erupted along the spreading centers.

The discovery of black and white smokers—vents of very hot, mineral-laden seawater—in the late 1970s clearly showed that the high heat of volcanically active spreading centers drives hydrothermal circulation of ocean water. This circulation allows extensive chemical exchange between water and newly formed crust, leading to concentration of rare metals, significant modification of seawater composition, and maintenance of the giant clam and tube-worm populations around the vents of hot water (see Plate 6).

FIGURE 2.9 Cross sections showing the shapes of undeformed Cyprus-type and Noranda-type massive sulfide deposits and the massive sulfide mound at the TAG hydrothermal field in the rift valley of the mid-Atlantic ridge. The age of the Cyprus deposits are about 70-million-years and of the Noranda deposits about 2.6-billion-years. Figure courtesy of Peter Rona, NOAA.

Remnants of submarine black smokers were initially observed in submarine massive-sulfide deposits found on the continents. These ancient representatives had been tilted on their sides and planed off by nature, which permitted detailed examination of these deposits in cross section (Figure 2.9). The active black smokers have two features rarely observed in the ancient ones. One is the peculiar biota able to grow in the dark by feeding on sulfur-metabolizing bacteria, and the other is the chimney form, which has since been identified in some ancient examples.

Researchers now model aspects of spreading centers that incorporate the variables of rift propagation, magma chamber size, spreading rate, and rock flow. They can also run models experimenting with variables of (1) chemical composition, (2) temperature of partial melting, (3) depth of equilibration, and (4) isotopically distinct sources. All of these computer-aided techniques should contribute to successful explanations of variation along the 40,000-km length of the ocean spreading centers.

Intraplate Volcanism: Hot Spots and Oceanic Plateaus

Enormous amounts of volcanic material erupt at hot-spots such as Hawaii. Unlike that at spreading centers, however, the rise of mantle beneath hot-spots is not a passive response to movement of the overlying plate. Rather, hot-spot volcanism appears

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