During the Permian period, about 300 million years ago, all of the Earth’s landmass was joined together into a single continent called Pangaea. What would eventually become the Americas was on the west coast of Pangaea and the tectonic plate that comprised the seabed that lay to the west of that is known as the Farallon Plate by geologists.
When Pangaea began to break up about 200 million years ago, the Farallon Plate slid under North America and into the Earth’s mantle. This caused the land above it to buckle upwards to form the Rocky Mountains. The Farallon Islands, 43 kilometres (27 miles) west of San Francisco, are all that is left of the Farallon Plate, which will slide completely beneath America in another 5 million or so years.