The Atlantic Ocean grows about 1.5 inches wider every year.

The Mid-Atlantic Ridge is a long chain of mountains on the ocean floor. It marks the spot where massive pieces of the Earth's crust, called tectonic plates, are moving away from each other. In the 1950s, scientists Marie Tharp and Bruce Heezen mapped this ridge and proved the seafloor was spreading.Heat from deep inside the Earth causes hot, liquid rock called magma to rise toward the surface. When this magma hits the cold ocean water at the ridge, it hardens into a rock called basalt. This new rock pushes the old seafloor outward, moving the Americas away from Europe and Africa.The rocks right at the center of the ridge are the youngest on the ocean floor. The rocks near the coastlines are the oldest, dating back 180 million years to when the supercontinent Pangea broke apart. In an average human life of 80 years, the ocean grows about 10 feet wider.While the Atlantic gets bigger, the Pacific Ocean is actually shrinking. In the Pacific, old ocean floor sinks back down into the Earth's interior at places called subduction zones. This balance keeps the Earth the same size even as the oceans change shape.