The nature and morphological characteristics of axial summit troughs on fast (~90--130 mm/yr-1 full spreading rate) and superfast spreading (>130 mm/yr-1) mid-ocean ridge crests reflect the time-integrated effects of long-term magmatic cycles, short-term volcanic episodicity, and the tensional stress regime imposed on young ocean crust. Two principal types of axial trough morphology have been identified and associated with distinct volcanic and tectonic processes occurring at fast and superfast spreading mid-ocean ridge crests. (1) Narrow axial troughs, ~300--2000 m wide and ~30--100 m deep) on the East Pacific Rise crest are classified as axial summit graben. The dimensions of axial summit graben, as well as the morphological and structural character of their walls and floors, suggest a primary tectonic origin. An axial summit graben may contain a nested axial summit collapse trough, implying that processes responsible for these endemic features may be linked. Near-bottom, side-looking sonar and observational data collected using the towed vehicle Argo I and submersible Alvin have been used to characterize the axial summit trough of the fast spreading East Pacific Rise between 9¿ and 10 ¿N. A four-stage model is presented for the evolution of this axial summit collapse trough, as well as for other well-studied portions of the East Pacific Rise crest from 21 ¿N to ~20 ¿S. We propose that the transition from a narrow, surface collapse-dominated axial trough to a broader, fault-bounded graben is controlled by the relative importance of diking, volcanism, hydrothermal cooling, and tectonism along a ridge segment over time periods <104 years. ¿ 1998 American Geophysical Union