We don’t perceive that the continents we live on are moving. After all, it’s not as if an airplane flight between Europe and Africa takes five hours one year but only three hours the next. But the continents actually are shifting, very slowly, relative to one another. In the early 20th century, a scientific theory called continental drift was proposed about this migration of the continents. That theory was initially ridiculed, but it paved the way for another theory called plate tectonics that scientists have now accepted to explain how Earth’s continents move.

The story begins with Alfred Wegener (1880–1930), a German meteorologist and geophysicist who noticed something curious when he looked at a map of the world. Wegener observed that the continents of South America and Africa looked like they would fit together remarkably well—take away the Atlantic Ocean and these two massive landforms would lock neatly together. He also noted that similar fossils were found on continents separated by oceans, additional evidence that perhaps the landforms had once been joined. He hypothesized that all of the modern-day continents had previously been clumped together in a supercontinent he called Pangaea (from ancient Greek, meaning “all lands” or “all the Earth”). Over millions of years, Wegener suggested, the continents had drifted apart. He did not know what drove this movement, however. Wegener first presented his idea of continental drift in 1912, but it was widely ridiculed and soon, mostly, forgotten. Wegener never lived to see his theory accepted—he died at the age of 50 while on an expedition in Greenland.

Only decades later, in the 1960s, did the idea of continental drift resurface. That’s when technologies adapted from warfare made it possible to more thoroughly study Earth. Those advances included seismometers used to monitor ground shaking caused by nuclear testing and magnetometers to detect submarines. With seismometers, researchers discovered that earthquakes tended to occur in specific places rather than equally all over Earth. And scientists studying the seafloor with magnetometers found evidence of surprising magnetic variations near undersea ridges: alternating stripes of rock recorded a flip-flopping of Earth’s magnetic field.

Together, these observations were consistent with a new theory proposed by researchers who built on Wegener’s original idea of continental drift—the theory of plate tectonics. According to this theory, Earth’s crust is broken into roughly 20 sections called tectonic plates on which the continents ride. When these plates press together and then move suddenly, energy is released in the form of earthquakes. That is why earthquakes do not occur everywhere on Earth—they’re clustered around the boundaries of tectonic plates. Plate tectonics also explains the stripes of rock on the seafloor with alternating magnetic properties: As buoyant, molten rock rises up from deep within Earth, it emerges from the space between spreading tectonic plates and hardens, creating a ridge. Because some minerals within rocks record the orientation of Earth’s magnetic poles and this orientation flips every 100,000 years or so, rocks near ocean ridges exhibit alternating magnetic stripes.

Plate tectonics explains why Earth’s continents are moving; the theory of continental drift did not provide an explanation. Therefore, the theory of plate tectonics is more complete. It has gained widespread acceptance among scientists. This shift from one theory to another is an example of the scientific process: As more observations are made and measurements are collected, scientists revise their theories to be more accurate and consistent with the natural world.

By running computer simulations of how Earth’s tectonic plates are moving, researchers can estimate where the planet's continents will likely be in the future. Because tectonic plates move very slowly—only a few centimeters per year, on average—it takes a long time to observe changes. Scientists have found that the planet’s continents will likely again be joined together in about 250 million years. Researchers have dubbed this future continental configuration “Pangaea Proxima.”

One intriguing aspect of Pangaea Proxima is that it will likely contain a new mountain range with some of the world’s highest mountains. That is because as Africa continues to migrate north it will collide with Europe, a collision that will probably create a Himalaya-scale mountain range. However, Christopher Scotese, one of the scientists who developed these simulations, cautions that it is difficult to predict exactly how the continents will be arranged in millions of years. “We don’t really know the future, obviously,” Scotese told NASA. “All we can do is make predictions of how plate motions will continue, what new things might happen, and where it will all end up.”