The width of and along-axis variations within the plate boundary zone of the southern segment of the Endeavour Ridge of the Juan de Fuca Ridge System have been quantified using Sea MARC I side-looking sonar imagery. The sonar imagery was calibrated with video and photographic surveying to provide visual corroboration of the activity, spacing, and offset of faults and fissures. This ridge segment contains an axial high bisected along its length by a summit depression whose width, relief of its rims, depth of its floor, and spacing of faults and fissures vary systematically away from the mid-point of the ridge segment toward its tips. The axial high and its summit depression are likened to an elongated shield volcano that is being disrupted by collapse of its crest into a widening linear caldera. The observed distribution of faults and fissures could be produced by mechanisms which stretch young ocean crust which is characterized by rheological properties which vary with distance from the center of the elongated volcano. ¿ American Geophysical Union 1988