The Atlantic Ocean, separating the Americas from Europe and Africa, is a relatively young feature compared to the ancient Pacific. Its existence is a direct consequence of immense geological forces that fractured a global landmass and initiated continental separation. The story of its formation begins hundreds of millions of years ago when its future boundaries were fused together within a single supercontinent.
The Continental Precursor
Before the Atlantic Ocean began to form, all of Earth’s major landmasses were united in the supercontinent Pangea, fully assembled by the early Permian period (around 299 to 273 million years ago). This enormous landmass stretched from pole to pole and was shaped like a giant “C.” There was no Atlantic basin; the coastlines of North America, South America, Europe, and Africa were seamlessly connected.
Pangea was surrounded by a single global ocean known as Panthalassa. The landmasses that would eventually border the Atlantic were interior regions of this supercontinent. The breakup of Pangea, which began approximately 200 million years ago, defined the separation of the continents and the birth of the Atlantic Ocean. This event set the stage for the geological processes that transformed the landmass into the continents and ocean basins of the modern world.
Initiating the Split
The fragmentation of Pangea and the opening of the Atlantic began in the Late Triassic period, around 200 million years ago, driven by plate tectonics. The initial trigger is linked to the emplacement of the Central Atlantic Magmatic Province (CAMP), a voluminous flood basalt event in Earth’s history. This massive outpouring of magma provided the heat and upward pressure that weakened the overlying continental crust.
The separation was driven by a divergent plate boundary, stretching and pulling the crust apart. This extension caused the continental lithosphere to thin, forming deep depressions known as rift valleys. These early rifts were initially filled with freshwater, creating vast Triassic-Jurassic rift lakes, remnants of which exist in sedimentary basins along modern coastlines.
As the crust continued to stretch, it eventually ruptured, forming a nascent, shallow marine basin—the earliest Central Atlantic Ocean. This rupture often created three-armed rift systems: two arms continued to spread, while the third, known as a failed arm or aulacogen, stopped and became a sediment-filled valley. This mechanism of pulling apart and subsequent flood basalt volcanism marked the transition from continental rifting to the first stages of true seafloor spreading.
Sequential Growth of the Ocean Basin
The Atlantic Ocean did not open all at once, but grew sequentially through distinct phases of seafloor spreading over tens of millions of years. The initial major phase began in the Early to Middle Jurassic period (approximately 180 to 175 million years ago), separating North America from Africa. This event created the Central Atlantic, which progressively widened as new oceanic crust was generated.
The second major stage, involving the South Atlantic, occurred much later, primarily during the Cretaceous period. The separation of South America from Africa began around 140 million years ago, with rifting propagating from south to north. The opening was not uniform; the southernmost regions separated first, followed by the more equatorial regions.
The final major phase involved the separation of the northern landmasses, including the opening of the North Atlantic between Europe and Greenland/North America. While precursory rifting occurred earlier, the main phase of spreading in the far North Atlantic, creating the Norwegian-Greenland Sea, began in the Late Paleocene epoch, around 62 to 60 million years ago. This sequential separation resulted in the older oceanic crust being located farthest from the central axis of the ocean. The oldest known Atlantic crust dates back up to 145 million years, situated off the coasts of West Africa and eastern North America.
The Mid-Atlantic Ridge and Ongoing Expansion
The most prominent geological result of the Atlantic’s formation is the Mid-Atlantic Ridge (MAR), an immense underwater mountain range running down the center of the basin for approximately 16,000 kilometers. The MAR represents the active divergent plate boundary where the ocean continuously expands. Here, tectonic plates pull away from each other, allowing molten material from the mantle to rise and solidify, creating new oceanic crust.
This process of seafloor spreading is responsible for the Atlantic’s ongoing widening. The North American and Eurasian plates, and the South American and African plates, are separating at a relatively slow rate. The average spreading speed along the MAR is estimated to be approximately 2.5 centimeters per year. This slow expansion rate contributes to the steep, irregular topography and the prominent central rift valley that characterizes the Mid-Atlantic Ridge.