The Atlantic Ocean is the ocean that is currently expanding. It grows wider by about 2 to 5 centimeters (roughly 1 to 2 inches) every year as new oceanic crust forms along an underwater mountain chain called the Mid-Atlantic Ridge. While that sounds tiny, over millions of years it has pushed North America and Europe apart by thousands of miles.
How the Atlantic Grows Wider
Running down the center of the Atlantic like a spine is the Mid-Atlantic Ridge, part of a global system of underwater ridges stretching over 50,000 kilometers. Along this ridge, hot molten rock rises from deep inside the Earth, breaks through the seafloor, and solidifies into new crust. As fresh crust forms, it pushes the older crust on either side outward, literally widening the ocean basin. This process is called seafloor spreading.
The rift valley at the crest of the Mid-Atlantic Ridge is roughly the depth and width of the Grand Canyon. North America and Europe are currently drifting apart at an average of about 3 centimeters per year, and South America and Africa are separating in the same way. These continents were once joined as part of the supercontinent Pangaea, and the Atlantic Ocean didn’t exist until seafloor spreading began tearing them apart around 180 million years ago.
Why the Pacific Is Shrinking at the Same Time
While the Atlantic expands, the Pacific Ocean is shrinking. This isn’t a coincidence. The Earth’s surface area stays roughly constant, so when one ocean basin grows, another has to give. The Pacific is bordered by subduction zones, areas where the ocean floor dives beneath neighboring tectonic plates and sinks back into the Earth’s interior. This ring of subduction zones is the famous “Ring of Fire,” known for its earthquakes and volcanoes.
Geologist Harry Hess first proposed this relationship in the early 1960s: the Atlantic was expanding while the Pacific was contracting. Decades of measurement have confirmed it. The Pacific remains the largest ocean on Earth, but it is slowly closing as the plates around its edges consume oceanic crust faster than new crust is created within it.
The Life Cycle of an Ocean Basin
Geologists describe the birth, growth, and death of ocean basins through a framework called the Wilson Cycle, which has six stages. The Atlantic and Pacific are at very different points in that cycle.
- Embryonic rift: A continent begins to split apart. The East African Rift is at this stage right now, with tectonic forces slowly tearing the continent along a zone in northeastern Ethiopia where three rift systems meet.
- Young ocean: A narrow seaway forms with an active spreading ridge. The Red Sea is the best modern example. Some scientists believe it represents the early stages of a future ocean that could eventually split Africa apart.
- Mature ocean: The basin is wide, with a well-established spreading ridge. The Atlantic and Indian Oceans are both at this stage.
- Declining ocean: Subduction zones form around the margins, and the basin begins to shrink. The Pacific Ocean is in this stage.
- Terminal ocean: The basin is nearly closed, leaving only restricted seaways. The Mediterranean Sea fits here, a remnant of a much larger ancient ocean.
- Collision: The ocean disappears entirely as continents collide, leaving behind a mountain range. The Himalayas mark the spot where an ancient ocean once separated India from Asia.
Every ocean goes through this cycle. The Atlantic is still in its growth phase, but hundreds of millions of years from now, subduction zones will likely form along its edges and it will begin to close.
A New Ocean Forming in Africa
The Atlantic isn’t the only body of water getting bigger. In the Afar region of northeastern Ethiopia, the Earth’s crust is being pulled apart at the junction of three tectonic rift systems: the Red Sea Rift, the Gulf of Aden Rift, and the Main Ethiopian Rift. This is one of the few places on the planet where continental rifting can be observed on land.
The Red Sea itself is a young ocean in the making, with an active spreading ridge already generating new seafloor. If the rifting continues on its current trajectory over millions of years, a new ocean basin will form as the Horn of Africa separates from the rest of the continent. The process is extremely slow by human standards, but geologically it is well underway.
Where the Continents Are Headed
If the Atlantic keeps expanding and the Pacific keeps shrinking, the continents will eventually drift back together. Projections by geologists suggest that roughly 250 million years from now, the continents could reassemble into a new supercontinent sometimes called Pangaea Proxima. The cycle of supercontinents forming, breaking apart, and reforming has repeated multiple times over billions of years of Earth’s history.
For now, though, the Atlantic continues to widen. The next time you fly from New York to London, the trip is a few centimeters longer than it was the year before.