programs will include several interdisciplinary investigations that incorporate the effects of volcanism within climate modeling, the analysis of different ecosystems, and the investigation of sea-surface/atmosphere interactions.

The energy of a major volcanic eruption is well beyond what can reasonably be expected to be controlled by engineering. Consequently, for the foreseeable future, humanity can best deal with volcanic phenomena by supporting programs aimed at predicting the occurrence and understanding the likely consequences of an eruption.

OCEAN BASIN PROCESSES

During the middle and late 1960s, widespread recognition of the lithosphere's plate structure crowned 20 years of postwar research in the oceans. Since then, research efforts have driven the investigation of the dynamic Earth in two directions: downward, to discover how plate activity relates to the deep mantle, and laterally outward onto the continents, to establish how the continents are involved in today's plate activity and how earth dynamics operated in the past. There have been surprises, such as the discovery of the black smokers, and there have been breakthroughs, such as the establishment of the age of the ocean floor. There have also been new puzzles, such as the significance of the huge oceanic plateaus representing the eruption of vast volumes of basalt over short intervals. The intellectual momentum provided by plate tectonic theory continues to affect the research directions of many earth scientists, including those who study the interior-driven oceanic processes that appear particularly important to the overall system.

FIGURE 2.8 After crystallization of the oceanic ridge magma chamber, tectonics or late magmatic activity breaks open the plated barrier layer at the top, and water actively penetrates into the hot cumulates. This is the stage of high flux geothermal activity, where high-temperatures can be attained even in very permeable rocks. Figure from The Mid-Oceanic Ridge: A Dynamic Global System (1988), National Academy Press.

Ocean Spreading Centers

Ocean basins open along axes where the crust is torn by the force of plates moving away from each other. Basins spread along centers where partial melting of the shallow mantle generates basalt that ascends toward the surface and is solidified as ocean crust (Figure 2.8). New plates are continuously created. On average, spreading centers lie about 2.8 km below the ocean surface, where they can be detected as topographically high, elongated areas, often with valleys at their crest. A wide range of methods has been applied to the study of spreading centers. They include surface and ocean-bottom geophysical techniques, manned and remotely operated submersible observation, and direct sampling by dredging and drilling. Samples of rocks and fluids have been analyzed physically, chemically, and isotopically. The highly specialized biota that characterize ocean spreading centers have also been studied in detail.

Detailed sampling along ocean spreading centers shows that the intensity of melting is inversely proportional to depth below sea level. The depth of a spreading center, and any accompanying ridge, can be taken as an indication of the average mantle temperature beneath it. High-temperature material will produce a higher degree of partial melting and thus a thicker oceanic crust. The relatively narrow range found in the composition of spreading-center volcanic products worldwide strongly suggests that melting beneath an ocean spreading center occurs through a straightforward process of decompression melting. This conclusion is based on a rela-



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