The Pacific Ocean is gigantic. It covers about a third of the whole planet's surface, more than all of Earth's land put together. It's hard to imagine anything that big changing size at all. Yet the Pacific is shrinking, little by little, every single year.
The cause is plate tectonics. Earth's outer shell is broken into huge plates that slowly move. Around much of the Pacific, the ocean floor is sliding beneath neighboring plates and sinking back into the planet, a process called subduction. As a result, the Pacific is shrinking by roughly an inch a year on three sides. Meanwhile, the Atlantic Ocean is getting bigger.
An Ocean Surrounded by Trenches
The Pacific is the only ocean that's mostly bordered by subduction zones. These are the places where one plate dives under another. Only a few stretches, including the central part of the North American coast and the coasts of Antarctica and Australia, have no subduction zones nearby.
Subduction also explains the Pacific's famous Ring of Fire. Several hundred active volcanoes sit above these subduction zones around the ocean's edges, making the Ring of Fire the world's leading belt of explosive volcanoes.
The process is also behind the ocean's spectacular depths. The deepest known point on Earth, the Challenger Deep in the Mariana Trench, plunges about 35,853 feet. The Pacific also holds the deepest point in the Southern Hemisphere, in the Tonga Trench.
Why the Atlantic Is Growing
While the Pacific is being squeezed, the Atlantic is being stretched. Running down the middle of the Atlantic floor is the Mid-Atlantic Ridge, a place where plates are pulling apart and new seafloor forms. It separates North America from Europe and Africa in the north, and Africa from South America in the south. It's part of the longest mountain range in the world, almost all of it hidden underwater.
An Ocean With Young Floors
Here's a surprising clue to all this recycling. The Pacific Ocean is ancient, born roughly 750 million years ago. But the oldest Pacific seafloor today is only about 180 million years old. The older crust has already been swallowed back into the Earth through subduction.
For much of its history, this great ocean is known to scientists as Panthalassa. That name is generally used until the breakup of the supercontinent Pangaea, about 200 million years ago.
Is a New Supercontinent Coming?
If the Pacific keeps closing, where does that lead? Some scientists have proposed a possible future supercontinent called Amasia. In this idea, the Americas would eventually merge with Asia as the Pacific closes, while the widening Atlantic pushes North America westward.
The timing is anyone's guess, and estimates vary widely. One study suggested Amasia could form over the North Pole in about 50 to 200 million years. A 2022 model suggested it might take as long as 600 million years. Either way, nobody alive today needs to worry about packing up the beach house.
A Peaceful Name for a Restless Ocean
The Pacific got its name in 1520 from the explorer Ferdinand Magellan. During the Spanish voyage around the world, he found favorable winds on reaching the ocean and called it Mar Pacífico, meaning peaceful sea. Before that, in 1513, the Spanish explorer Vasco Núñez de Balboa had crossed the Isthmus of Panama and named it the Southern Sea.
It's a lovely name, but the ocean beneath the waves is anything but still. So the next time you stand on a Pacific beach, remember that the seafloor out there is slowly sliding away under your feet, one tiny inch at a time.
