When Earth's Continents merged into Pangea, some 250 million years ago, the vast intervening ocean stretched across two oceanic plates, the Pacific and the Farallon; spreading out from a mid-oceanic ridge, the Pacific Plate lied beneath the western half of the sea while the Farallon occupied the eastern half, abutting the future Americas.
Pangea began to break up during the Triassic, as the Tethys Sea opened from east to west. Later, in the Jurassic, the Atlantic began to open, shoving the American Plates to the west. As this occurred (and continues to occur), the Farallon Plate was forced to subduct beneath the American Plates and, in doing so, added a cargo of sub-continents to western North America, crumpled up the Rockies and created a broken chain of subduction volcanoes from the Andes to southeast Alaska. Today, only remnants of the Farallon persist, including the Nazca Plate along the west coast of South America, the Cocos Plate west of Central America and the Juan de Fuca Plate (and a few smaller remnant plates) off the west coast of the Pacific Northwest.
The Pacific Plate, devoid of major continental masses continues to form along its ridge with the remnants of the Farallon and along its younger ridge with the Antarctic Plate. Inching to the northwest, the Pacific Plate scrapes through Southern California along the San Andreas Fault and eventually subducts beneath or overruns the North American, Eurasian, Philippine and Australian Plates in a broad arc, from the Aleutians to New Zealand. For now, only hotspot volcanic islands disrupt its vast expanse of open sea; eventually, this great oceanic plate will also disappear as new oceans open, our current continents rift apart and the Pacific subducts into history. Just like the Farallon and its many other predecessors, the Pacific Plate is a transient feature of Planet Earth.
