The Atlantic Ocean, covering approximately 20 percent of the Earth’s surface, is a relatively young feature in geological history. Its formation is a direct consequence of plate tectonics, the continuous movement of large, rigid slabs of the Earth’s lithosphere. The ocean basin was created by the long-term separation of massive continental landmasses that once formed a single supercontinent. This slow continental drift, driven by heat and motion within the Earth’s mantle, gradually opened a widening gap that filled with water.
The Supercontinent Pangea
Before the Atlantic Ocean began to form, all major landmasses were merged into the supercontinent called Pangea. This unified landmass was fully assembled between 300 and 200 million years ago, during the Permian and Early Triassic periods. North America, South America, Europe, and Africa were connected, forming the core that would eventually border the Atlantic.
The future Atlantic Ocean basin was initially dry continental crust situated within Pangea’s interior. The supercontinent remained intact for about 100 million years before internal stresses initiated its breakup roughly 200 million years ago. This fragmentation was the first step toward the modern arrangement of continents.
The Initial Continental Rifting
The breakup of Pangea began in the Late Triassic period with continental rifting, which pulled the landmasses apart. Rifting involves the stretching and thinning of the continental lithosphere due to forces in the underlying mantle. This tension created a broad zone of weakness, leading to the formation of extensive, deep rift valleys similar to the modern East African Rift Valley.
Within these rift zones, the crust fractured along large-scale normal faults, creating down-dropped blocks called half-grabens where thick layers of sediment accumulated. As the continental crust thinned, pressure on the underlying mantle decreased, causing hot material to rise and induce volcanism. Evidence of this early activity remains in the Newark Group sediments and the basaltic lava flows of the Palisades Sill in New Jersey and New York.
As rifting progressed, the rift floor eventually sank low enough to be invaded by seawater. The resulting narrow, shallow marine basins were restricted, leading to cycles of flooding and evaporation that deposited thick layers of salt and evaporite minerals. This transition marked the point where true oceanic crust formation was imminent. The initial separation between northwestern Africa and North America began in the Early-Middle Jurassic, forming the Central Atlantic.
Formation and Widening of the Ocean Basin
The Atlantic Ocean basin formed when rifting transitioned into full-scale seafloor spreading, primarily beginning in the Jurassic period. This occurred when the stretched continental lithosphere finally ruptured, allowing hot, buoyant mantle material to continuously well up and solidify. This rising magma forms new oceanic crust, establishing a divergent plate boundary that pushes the continents apart.
The Mid-Atlantic Ridge (MAR) is the massive underwater mountain chain that expresses this continuous creation of new crust. The MAR runs for approximately 16,000 kilometers, dividing the Atlantic basin into eastern and western halves. Along this ridge, molten rock erupts and cools rapidly to form pillow basalts, the typical rock of the ocean floor.
The continents bordering the Atlantic continue to move away from the MAR at a relatively slow rate, estimated between 1 and 10 centimeters per year. This continuous motion means the Atlantic Ocean is actively widening, unlike the Pacific Ocean, which is shrinking due to subduction. The opening progressed in phases, with the South Atlantic forming later, around 140 million years ago, as South America separated from Africa.
Geological Confirmation of the Timeline
The history of the Atlantic’s formation is confirmed by two primary lines of physical evidence preserved in the oceanic crust. The first is magnetic striping, which records the Earth’s ancient magnetic field reversals. As new basaltic crust forms at the Mid-Atlantic Ridge, iron-rich minerals within the rock align with the planet’s magnetic polarity at that specific moment.
Because the Earth’s magnetic field reverses periodically, the newly formed crust records a pattern of alternating normal and reversed magnetic stripes parallel to the Mid-Atlantic Ridge. These stripes are perfectly symmetrical on either side of the ridge, demonstrating that new crust is created at the center and pushed outward. By dating the continental rocks corresponding to these reversals, geologists confirm the rate and history of seafloor spreading.
The second form of evidence is the age and thickness of deep-sea sediments found across the ocean floor. If the seafloor spreading model is accurate, the youngest oceanic crust should be at the Mid-Atlantic Ridge, becoming progressively older farther away. Deep-sea drilling confirms this pattern: the oldest sediments are found closest to the continental margins, while the crust near the ridge crest is the youngest and virtually sediment-free.