Summit troughs wider than 500 m and 30--110 m deep are present along 15--20% of the fast spreading East Pacific Rise (EPR). They occur only along ridge segments with large cross-sectional areas, indicative of a time-averaged robust magma supply. Where available, seismic data confirm that these troughs are underlain by an axial magma chamber (AMC) 1.2--1.6 km below the seafloor. Furthermore, detailed investigation of the large summit troughs which notch the EPR between 17¿56' and 18¿35'S indicates that the vertical relief of the troughs tends to be maximum where the AMC is shallowest. Both of these observations are inconsistent with the predictions from a model in which large summit troughs form by rifting of the brittle upper crust during phases of amagmatic extension. Rather, we propose that they represent elongated collapsed calderas that form when the melt supply to formerly inflated AMCs wanes or nearly ceases. Because seismic studies constrain the melt lens along the EPR to be only 30--80 m thick, the proposed existence of collapsed calderas 40--110 m deep implies that the entire magma reservoir comprising the melt lens and the underlying crystal mush zone deforms and compacts during periods of waning magma supply. In particular, we suggest that a voluminous crystal mush zone will stretch in response to steady state seafloor spreading when the magma supply temporarily decreases. The resulting caldera will further widen with each subsequent dike intrusion. When an abundant melt supply finally resumes, the associated tumescence of the neovolcanic zone and profuse lava flows will combine to smooth out the caldera. ¿ 1999 American Geophysical Union