The East Pacific Rise is a large tectonic spreading center, part of the Mid-Ocean Ridge system that winds around the world like the seam on a baseball.
As the name suggests, it lies in the eastern Pacific Ocean and forms the eastern extremity of the Pacific Plate, separating it from the Nazca Plate in the south and the Cocos Plate (as well as a tiny fragment, the Rivera Plate) in the north. At its northern extremity, the East Pacific Rise ends at a north-northwest-trending transform fault which runs beneath the center of the Sea of Cortez (aka the Gulf of California) and eventually appears on land as the San Andreas Fault.
When the Pacific Ocean was the Panthalassa Ocean, the East Pacific Rise was the western edge of the Farallon Plate and extended all the way to the Arctic. However, so much of that plate was subducted beneath the North American Plate that it broke into the plates mentioned above, as well as the Juan de Fuca Plate further north, and the EPR was truncated to a point off the coast of Mexico. However, fragments of the larger spreading center survive as the ridges separating the Juan de Fuca Plate from the Pacific Plate.
The East Pacific Rise is home to three geological oddities: The Easter Microplate, the Juan Fernandez Microplate, and the Galapagos Microplate, all of which formed as chunks broken off of the larger plates, but which lie trapped within the East Pacific Rise, rotating like ball bearings.
A bit of geological history happened on the East Pacific Rise: In 1979, Bill Normark and Thierry Juteauin dove off Baja California in the United States Navy research vessel Alvin, and discovered the hydrothermal vents now known as "black smokers". The metal-rich hot water exuding from black smokers explained the formation of certain ore deposits. More importantly, however, they discoverd whole communities of life which survived on no sunlight at all, revolutionizing deep-sea biology.