When the continental land masses merged into Pangea at the end of the Paleozoic Era, some 250 million years ago (MYA), that supercontinent was surrounded by the Panthalassic Ocean, a global ocean that was already 500 million years old; indeed, it had torn apart the supercontinent of Rodinia near the end of the Precambrian Period, about 750 MYA.
Before Pangea began to rift apart, the Panthalassic Ocean was composed of three major oceanic plates, separated by a network of oceanic ridges. The northern, relatively small plate was the Kula Plate; the Farallon Plate stretched eastward from the central, north-south ridge while the present-day Pacific is the remnant of the plate west of that ridge. Beginning in the Triassic Period (200 MYA) and continuing today, the opening of other oceans, including the Tethys, Atlantic and Indian Oceans, forced segments of the Panthalassic Ocean to subduct beneath the North American, South American, Eurasian and Australian Plates. The Kula Plate has all but disappeared beneath the northwestern segment of the North American Plate while most of the Farallon Plate has subducted beneath the western edge of the North American, Caribbean and South American Plates; the Juan de Fuca Plate of the Pacific Northwest, the Cocos Plate off the west coast of Central America and the Nazca Plate off the western coast of South America are remnants of the once massive Farallon Plate.
In effect, the opening of the Atlantic Ocean (which began about 160 MYA) has pushed the American Plates westward and they now overlie the eastern half of the ancient Panthallasic Ocean, leaving (for now) the Pacific Plate and small remnants of the Kula and Farallon Plates. The residual portion of the central oceanic ridge, known today as the East Pacific Rise, remains an active spreading zone, forcing the Pacific Plate to the northwest and the Farallon remnants to the east. Like all past dominant oceans, the Pacific will eventually disappear, subducting entirely as younger oceans become the primary engines of continental drift. Perhaps 200 million years from now the East African Rift will have produced the largest ocean on Earth.