c. 140 Million BCE

The Atlantic Ocean

The reconstructed history of the relative motions of the Earth’s several dozen major lithospheric plates is based on evidence from the fossil record of plants and animals, as well as the geologic record of mountain-building. That history tells us that the Earth’s continental plates have probably come together into single supercontinents at least several times since the Archean. The last and best known such supercontinent was called Pangea, which started forming around 300 million years ago.

The Earth’s lithospheric plates are dynamic, however, and so it wasn’t too long (only about 100 million years) until the same kinds of internal forces that brought it together began rifting Pangea apart. The fact that the start of the breakup of Pangea occurred right around the same time and in the same region as an episode of massive volcanic eruptions associated with large-scale mantle upwelling is not a coincidence, nor is it a coincidence that the catastrophic End Triassic mass-extinction event also occurred around the same time. The breakup of Pangea was a global, game-changing event.

The result of the creation of a rift zone in the Pangean crust and the pulling apart of the North American and African plates, the African and South American plates, and the North American and Eurasian plates, was the creation of a deep basin into which ocean waters from other parts of the globe could flow. The Atlantic Ocean was born. The mantle plume that first initiated the breakup of Pangea began to spew forth enormous volumes of volcanic rock to create a new mountain belt within the widening basin between the American plates and the African/Eurasian plates. This mid-Atlantic ridge became a divergent plate boundary running roughly north–south, pushing out new oceanic plate volcanic rocks to the east and west. The volcanism and plate motion continues to this day.

The mid-Atlantic ridge is part of the largest chain of mountains on Earth, extending from northern polar latitudes near Iceland to southern polar latitudes near Antarctica. For most of human history, these volcanically active mountains were unknown. It took modern seafaring technology such as sonar to eventually map the seafloor in the 1950s through 1970s in order to reveal these mountains, and in order to finally understand plate tectonics.

Sunrise seagull over the Atlantic Ocean, which formed some 140 million years ago as a rift in the supercontinent of Pangea that separated what are now the North American and African tectonic plates.