The Atlantic's Great Expansion
The Atlantic Ocean is getting wider by about 2.5 to 4 centimetres each year, roughly the speed your fingernails grow. This expansion is driven by a colossal underwater mountain range called the Mid-Atlantic Ridge. This ridge is a divergent plate boundary,
where the North American and Eurasian plates, and the South American and African plates, are pulling apart. As they separate, magma from deep within the Earth's mantle rises to the seafloor, cools, and forms new oceanic crust. This process, known as seafloor spreading, has been happening for around 150 to 200 million years, ever since the breakup of the supercontinent Pangaea began. In essence, the Atlantic is constantly creating new floor for itself, pushing the continents on either side further apart.
The Pacific's Ring of Fire
In stark contrast, the Pacific Ocean is slowly contracting. It's surrounded by what is famously known as the "Ring of Fire," a nearly continuous series of deep ocean trenches and volcanic arcs. This 40,000-kilometre-long belt is not a place of creation, but one of destruction. It's dominated by subduction zones, where oceanic plates dive beneath continental plates or other, less dense oceanic plates and are recycled back into the Earth's mantle. The Pacific Plate is being pulled under the surrounding plates on almost all sides. This process is responsible for about 90% of the world's earthquakes and 75% of its active volcanoes, hence the fiery name.
A Tale of Two Processes
The fate of an ocean depends on the balance between seafloor creation at spreading ridges and seafloor destruction at subduction zones. The Atlantic has a major spreading centre (the Mid-Atlantic Ridge) but very few subduction zones around its edges. This means it creates far more crust than it destroys, leading to its net expansion. The Pacific, on the other hand, has its own spreading ridge—the East Pacific Rise—but it is ringed by so many powerful subduction zones that the rate of crust destruction far exceeds the rate of creation. While the Pacific loses about an inch of its span each year, the Atlantic gains about the same amount. This geological give-and-take is what drives the slow, relentless change in the size of our planet's largest bodies of water.
A Glimpse into the Far Future
This geological cycle has been happening for billions of years, creating and destroying supercontinents. Based on current plate movements, scientists use supercomputers to model what Earth might look like millions of years from now. One popular theory suggests that the continued shrinking of the Pacific Ocean will eventually lead to the collision of North America with Asia. In about 200 to 300 million years, this could result in the formation of a new supercontinent, whimsically named "Amasia" (a portmanteau of America and Asia). Other models predict the closure of the Atlantic, leading to a different configuration called "Pangaea Proxima." While these are just projections, they highlight the incredible power of plate tectonics to reshape the globe over geological time.
