The Atlantic Ocean is actively increasing in size, growing wider every year as the continents on its shores slowly move apart. This continuous expansion is a direct consequence of the Earth’s global system of plate tectonics, which describes the movement of the planet’s rigid outer shell, known as the lithosphere. While this process is extremely slow from a human perspective, adding only a few centimeters annually, it is a measurable and persistent geological phenomenon that has been reshaping the planet for millions of years.
Seafloor Spreading at the Mid-Atlantic Ridge
The physical mechanism for the Atlantic’s growth is concentrated at a massive underwater mountain chain called the Mid-Atlantic Ridge (MAR). This ridge is a divergent plate boundary where the tectonic plates on either side are pulling away from each other. The MAR is the longest mountain range on Earth, stretching approximately 10,000 miles from the Arctic Ocean to the southern tip of Africa.
As the North American and South American plates separate from the Eurasian and African plates, the pressure on the underlying mantle decreases. This decompression allows molten rock, or magma, to rise from the mantle and fill the void. The magma cools rapidly upon contact with the seawater, solidifying to form new basaltic oceanic crust.
This constant introduction of new material continuously adds fresh seafloor and pushes the older crust, and the continents resting on it, outward. A deep rift valley runs along the center of the ridge, marking the location where crustal separation and magmatic upwelling are most intense. This process of seafloor spreading, where new crust is created and moves away from the ridge axis, is the direct cause of the Atlantic Ocean’s widening.
Plate Tectonics and the Driving Force
The Mid-Atlantic Ridge creates new crust, but the engine driving continental separation lies deeper within the Earth. Plate tectonics is powered by heat transfer within the mantle, a process known as mantle convection. Heat generated by the decay of radioactive elements causes the mantle rock to slowly churn in vast, circular currents, moving hot material upward and allowing cooler material to sink.
The movement of the tectonic plates is primarily driven by two gravitational forces. One is “ridge push,” which occurs because the elevated Mid-Atlantic Ridge stands higher than the surrounding ocean floor. Gravity causes the new, hot, and relatively light lithosphere formed at the ridge to slide away from the crest, exerting a pushing force on the rest of the plate.
The second, and often stronger, force is “slab pull,” which acts on plates elsewhere in the world. This force is generated where cold, dense oceanic crust sinks back into the mantle at subduction zones, pulling the entire attached plate along. These two gravity-driven forces work with the underlying convection to sustain the movement of the four plates bordering the Atlantic, ensuring the ocean continues to expand.
Determining the Rate of Expansion
Scientists have determined the rate of the Atlantic Ocean’s growth through historical geological evidence and modern measurements. One primary piece of evidence comes from paleomagnetism, the study of the Earth’s ancient magnetic field recorded in rocks. As new crust forms at the ridge, iron-rich minerals in the magma align with the planet’s magnetic field before the rock solidifies.
Because the Earth’s magnetic field periodically reverses polarity, the ocean floor exhibits a symmetrical pattern of magnetic “stripes” parallel to the Mid-Atlantic Ridge. By measuring the width of these stripes and knowing the timing of past magnetic reversals, geologists calculate the average rate of spreading over millions of years. This historical evidence is complemented by modern satellite geodesy, which uses tools like the Global Positioning System (GPS) and Very Long Baseline Interferometry (VLBI).
These modern methods involve placing instruments on different continents to measure the distance between them with millimeter accuracy. Direct measurements confirm that the North American and Eurasian continents are separating at a rate between 2 and 5 centimeters (0.8 to 2 inches) per year. This rate varies slightly along the length of the ridge but consistently confirms the Atlantic is widening.
The Atlantic’s Growth in a Global System
Since the Earth’s surface area remains constant, the creation of new crust in the Atlantic must be balanced by the destruction of old crust elsewhere. This global compensation occurs primarily in the Pacific Ocean basin, which is actively shrinking. In the Pacific, old, cold, and dense oceanic crust is forced beneath less dense plates at subduction zones, a process often marked by deep-sea trenches and volcanic arcs.
The Atlantic, by contrast, is mostly surrounded by passive continental margins that lack these destructive subduction zones. This allows the Atlantic to continually expand without recycling its own crust. The current phase of expansion is part of a larger, recurring geological pattern known as the Wilson Cycle.
The Wilson Cycle describes the birth, growth, and eventual closure of ocean basins over hundreds of millions of years. The Atlantic Ocean represents the mature stage of this cycle, following the breakup of the supercontinent Pangea about 180 million years ago. While the Atlantic is currently growing, the cycle implies that eventually, a subduction zone will form, and the ocean will begin to shrink, leading to the formation of a new supercontinent.