Tectonic slab window

This animation of the tectonic evolution of western North America shows the birth and lengthening of the San Andreas fault as the triple junction moves north over the past 40 million years. Over time, the Farallon plate is consumed by subduction beneath North America and the divergent boundary (a.k.a. ridge) separating the Farallon and Pacific plates is eventually subducted. Subduction of this ridge segment creates a slab window, which is a place where an opening in the subducting plate or plates allows heat from the mantle to rise and contact the base of the overriding plate. Animation created by Tanya Atwater, https://emvc.geol.ucsb.edu