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FIGURE 1 Cartoon of the solid Earth plate tectonic cycle, divided into three major tectonic components: plate creation at mid-ocean spreading centers, modification of the plate as it traverses the mantle, and subduction of the plate and creation of new crust at convergent margins. The initiation of an ocean basin through continental rifting is not shown in the figure. Source: FUMAGES (1998), p. 8.

ment into the mantle. The subduction factory is the current mode of continental growth and modification, and over time may have been the principal process leading to the creation of the continents. Ultimately the continental crust again will rift, leading to the formation of a new ocean basin, and the cycle repeats.

Each aspect of the plate tectonic cycle has a host of scientific questions that remain to be answered. These questions naturally divide according to the diverse provinces of the cycle—extending from creation of crust at ridges, to the mid-plate region, to the convergent margin. Figures 1 and 2 present these domains and some of the processes that take place in them, and the specific sections of the FUMAGES report discuss some of the outstanding questions that provide a basis for fruitful new directions for research.

There is an important additional aspect of the evolution of the solid Earth, however, that is not fully represented by Figure 1. Figure 1 is as an instantaneous view of the overall process; Figure 2 conveys the notion that not only is this view one of a continuing and repeating cycle, but also that this cycle may lead to and be influenced by the long-term geochemical and tectonic evolution of the solid Earth system. The Earth's current state is not necessarily typical of all tectonic regimes in the past. One obvious aspect of these changes is reflected in the very different apparent state of the ocean floor during the Cretaceous, when large igneous plateaus were present over much of the seafloor. There may have been major changes that have yet to be discovered, such as, perhaps, changes in the mode of mantle convection. Therefore, the study of the evolution of the solid Earth cycle through time—in all settings—emerges as or e of the clear frontiers of the science over the next decade. Old ocean floor contains one of the best records of this history.

The solid Earth cycle and its evolution through time are driven by fundamental processes that result in mass transport across the boundaries between the asthenosphere, lithosphere, hydrosphere, and atmosphere. These fundamental processes include the generation and segregation of magma, brittle and ductile lithospheric deformation, l he scales and patterns of asthenospheric flow, and the influence of fluid flow on rheology and chemical exchange between the solid and fluid Earth. In addition to its intrinsic scientific interest, investigation of these processes can eventually lead to an understanding of the causes of great earthquakes or volcanic explosions, of the generation and concentration of mineral resources, of cataclysmic events in Earth's history that have modified Earth's climate, and even of the origins of life itself.

An aim in investigations of these processors is the development of quantitative, unifying principles that govern the



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