resents one of the prime volcanic hazards to human populations.

Important and difficult questions challenge investigators of subduction zones. These questions address the controls over subduction and magma genesis; the fate of subducted slabs; and the proportions of converging plates that remain near the surface, that descend only to reemerge as volcanic material, and that sink far into the mantle to affect the convecting material at great depth. Because of the complex nature of subduction zones, answers to these questions are likely to come slowly and only by integration of a wide variety of data. As with many areas of the earth sciences, understanding the causes and consequences of subduction zone volcanism and the history of the assembly of the continents from volcanic arcs will result from integration of the increasingly detailed data provided by geological, geochemical, and geophysical observations with powerful numerical models.

CONTINENTAL STRUCTURE AND EVOLUTION

The other terrestrial planets lack the surface division into continental and oceanic crust that is a distinctive feature of the Earth. The continents may owe their existence to another distinctive characteristic: the Earth's abundance of free water. As described in preceding sections, oceanic water forms submarine hydrothermal vents at oceanic ridges, where associated hydration of the juvenile ocean-floor basalt forms minerals such as chlorite, serpentine, and amphibole with water bound into their structures. The subsequent dehydration of these minerals when they are subducted provides a flux of solutions into the overlying mantle wedge that initiates arc magmatism and causes the distinctive geochemistry of the magmas that build the island arcs. Parts of these in turn provide the raw material that is built into continents.

Studies of the continental crust and its margins will be a prime focus of geological research in the twenty-first century. Deciphering the complex interplay between tectonism, volcanism, climate change, sedimentary deposition, and geomorphic processes is vital for understanding the nature of global change. The development of this field will have a major influence on the intellectual development of the solid-earth sciences as well as on exploration for material resources. A large variety of investigatory methods will be applied.

Seismic Imaging of the Crust

In the early 1960s an active program of seismic investigation gathered data about the continental crust. As a result, the general characteristics of the continental crust in North America and the relationship between crustal structure and tectonic features were recognized. However, the usefulness of the seismic data was restricted by the limited capabilities of existing instrumentation and the absence of sophisticated processing and modeling techniques for interpretation. A rejuvenation of seismic crustal studies occurred in the 1980s with the availability of modern digital recording instruments and powerful computers.

Seismic information on the structure of the continental crust has been provided by three primary types of experiments—deep seismic reflection profiling, refraction and wide-angle reflection methods, and analyses of earthquake network and array data. A stimulus in understanding the present structure of the continental crust has come from the work of the Consortium for Continental Reflection Profiling (COCORP) and other deep reflection profiling efforts, both in the United States and abroad. Their mission was to extend the methods of seismic reflection profiling, which had been developed with great sophistication for shallow depths in oil exploration, to depths of tens of kilometers. In this way the structure of the continental crust, which is normally between 30 and 50 km thick, could be analyzed. This technique produces images detailing intrusions, shear zones, complex configurations of faults, boundaries between rocks of different composition, and reflectivity associated with variations in physical properties such as porosity. Deep reflection has mapped areas of ancient continental collision and located many dormant rifts, both within continents and along Atlantic-type margins.

The long-range capabilities of refraction and wide-angle reflection profiling techniques make these methods particularly suitable for studies of the deep structure of the continental crust and upper mantle (Figure 2.12). These capabilities have led to the discovery that crustal velocity structure, thickness of the crust, depth to the crust-mantle boundary, and the configuration of reflective interfaces within the crust are most strongly related to the nature and timing of the most recent major tectonic event to have affected the continental crust. Scientists had assumed that these properties were determined by the characteristics of the crust at the time of its origin. For example, profound changes in the seismic structure of the crust are produced by



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