Gulf of California
The Gulf of California was made as parts of the planets surface moved the Baja California Peninsula off of the North American Plate.  As part of this process, the East Pacific Rise formed along the middle of the Gulf along the bottom of the sea.  The Gulf would go as far as Indio, California if there wasn't a large river delta made by the Colorado River. This delta blocks the sea from flooding the Mexicali and Imperial Valleys.  Volcanoes can be found along the East Pacific Rise.  The island of Isla Tortuga is one example of the volcanoes found.