The boundary between the Pacific and Juan de Fuca Plates in the northeast Pacific Ocean is marked by a series of spreading centers (Fig. 1) and their connecting fracture zones (transform faults). The longest (490 km) of these, the Juan de Fuca Ridge, is bounded on the south by the Blanco Fracture Zone and on the north by the Sovanco Fracture Zone. Despite its relatively small size compared to other mid-ocean ridges, the Juan de Fuca has played a historic role in the development of plate tectonics, and is still one of the most intensively studied spreading centers in the world. The Juan de Fuca Plate, lying east of the ridge system, forms an actively convergent margin with the North American Plate. The Juan de Fuca spreading center is composed of a series of at least six ridge segments, 50 to 150 km long, which although generally spreading at a total opening rate of 6 cm/yr, display a remarkable diversity of ridge-axis morphology.