By virtue of its proximity to the coastline of North America and to numerous oceanographic institutions, the Juan de Fuca Ridge has been the focus of a large number of marine geological, geochemical, and geophysical investigations. Systematic studies began in the early 1960's with the geophysical survey of A. D. Raff and R. G. Mason, which provided much of the foundation for the development of the extraordinarily successful paradigms of sea-floor spreading and plate tectonics. Subsequent systematic and detailed studies of the plates and plate boundaries of the area by investigators from many academic, industrial, and government agencies, including the Geological Survey of Canada, have provided the basis for much of the fundamental understanding we now have of global plate motions and the processes that are involved in the creation of new oceanic crust at sea-floor spreading centres. Much of the success of these studies can be attributed to the geological diversity found along the Juan de Fuca Ridge. Clear examples are present of '' normal '' volcanically robust ridge segments, deep extensional rift valleys, stable and evolving transform faults, nontransform ridge offsets, propagating rifts, and off-axis seamount chains. Much has been learned about the nature of hydrothermal circulation through intensive studies of the many active hydrothermal systems and mature hydrothermal deposits that occur in both unsedimented and sedimented environments along the ridge. Better understanding of the way that oceanic crust chemically and physically ''ages'' is emerging from studies on the ridge and ridge flank. A clear history of the evolution of the ridge and of plate motions is provided by the magnetic anomalies mapped over the ridge and adjacent plates. From this history, lessons have been learned about the causes and consequences of plate motions, fragmentation, and internal deformation. Some of the success of these studies can be attributed to the rapidly evolving geophysical tools which provide ever increasing efficiency of operation and resolution. A new phase of study most recently begun involves the deployment of sea-floor geophysical ''observatories'' that provide a means by which temporal variations and events can be monitored over extended periods of time. These new studies are expected to yield yet another level of understanding of the processes that have produced two thirds of the Earth's surface as well as many important geologic formations in terrestrial settings.
