Have you ever looked at a map and noticed that South America and Africa look like they fit together like puzzle pieces? You’re not the first to notice this.

This is a story about the changing face of Earth. Over the last 4 billion years, our planet has changed a lot. Mountains rise. Landmasses sink beneath the sea. Whole continents crash together. But these changes take millions of years. On the scale of a human life—or even on the scale of all human history—it’s hard to see the huge, slow changes that move continents. The story of how we came to see these changes and understand what they mean is the story of four scholars and over 500 years of shared knowledge.

Ortelius and Wegener: Continental drift

The idea that the continents moved was first proposed in the late 1500s by a Dutch mapmaker named Abraham Ortelius. In the 1500s, not long after Christopher Columbus reached the Americas, the first full world maps were being made. Ortelius looked at these, and he thought that it looked like the Americas had broken away from Europe and Africa. He claimed it had happened because of earthquakes and floods. Ortelius’s theory was forgotten until the early 1900s, when a German astronomer named Alfred Wegener improved on it.

In 1910, Wegener noticed on a world map that the east coast of South America fits against the west coast of Africa. He thought maybe they had once been joined together. Wegener found evidence to support this idea, and he claimed that about 300 million years ago, the Earth’s continents had all been joined together in a single continent. He called this single continent Pangaea, a Greek word meaning “whole Earth.”

To support his claim, Wegener used evidence from different scientific disciplines:

• He found the fossils of ancient tropical plants on an island in the Arctic Ocean.
• He noted that the geology of the Appalachian Mountains in the United States is similar to that of the Scottish Highlands.
• He located rock layers (strata) in South Africa that matched those in Brazil.

At the time, geologists believed that mountains formed like wrinkles on the skin of a drying apple. Wegner said that this was wrong. He claimed that mountains formed when the edges of moving continents collided and crumpled.

Many geologists laughed at Wegener’s ideas. He was not a geologist, and his ideas challenged their theories. Wegener couldn’t explain what caused continents to plow through Earth’s crust. It would have required immense force, like ice-breaking ships cutting through frozen sheets of ice, but vastly more powerful. It would take the discoveries of two more scholars to convince the world that continental drift—which is what Wegener called the movement of continents—was real.

Harry Hess and Marie Tharp: Seafloor spreading and the mid-ocean rift

Even though most geologists didn’t accept the idea of continental drift, some scientists kept talking about it. During World War II (1938–1945), sonar technology allowed scientists to see what the seafloor looked like. Sonar machines on navy ships bounced sound waves off the seafloor and used the echo to reveal the oceans depth and what the seafloor looked like.

During the war, a geologist named Harry Hess was put in charge of an American navy ship in the Pacific Ocean. Hess wanted to continue his scientific investigations even while at war. Ship commanders usually turned on sonar machines to navigate when docking. Hess, however, left his ship’s sonar on all the time. With the images made by his ship’s sonar, Hess saw that the bottom of the sea was not smooth as many scientists expected it would be. Instead, it was full of canyons, trenches, and volcanoes.

After the war, during the 1950s, another geologist, Marie Tharp, was working to map the ocean floor. She used sonar data collected by her colleagues on ships in the Atlantic Ocean. Tharp’s work showed that a huge underwater mountain range called the Mid-Atlantic Ridge ran all the way from the Arctic Ocean to Antarctica. And most important, this ridge had a split in it—the seafloor was pulling apart. At first, her male research partners didn’t believe her. They thought it sounded too much like continental drift. They called her claims “girl talk.” But Tharp suggested they compare her map with evidence from earthquake data. This confirmed that a rift ran the entire length of the Atlantic Ocean floor.

Tharp’s mapping discovery gave Hess the evidence he needed to claim that the Earth’s crust had been moving apart on either side of oceanic ridges. These ridges ran down the Atlantic and Pacific Oceans. They were long—and had lots of volcanoes. He published his theory in 1962. It came to be called seafloor spreading.

In the early 1960s, samples were taken from deep in the ocean’s floor and dated. They showed that the ocean floor was younger at the Mid-Atlantic Ridge. The ocean floor became older and older in either direction. This confirmed that the seafloor was truly spreading, with new floor being pushed up by volcanic activity in the rift. By 1963, scientists realized that Earth’s magnetic field had reversed polarity many times. Each reversal lasted fewer than 200,000 years. When this happened, the North Pole and South Pole swapped magnetization. Rocks on the seafloor show the magnetic polarity at the time that that part of the crust formed. Sure enough, rocks were found in the Mid-Atlantic Ridge that had a pattern of alternating stripes of polarity. It’s almost like a zebra’s coat. That sealed the argument for most geologists. Continental drift was real.

Plate tectonics and the future of Earth

By the 1970s, geologists had agreed to use the term plate tectonics. They already knew that continents move, but they’d also found evidence that so do whole plates of the Earth’s crust. A plate might include a continent or parts of a continent. Even portions of the Earth’s crust that are deep underwater can form plates.

Geologists today know that the Earth’s crust is broken up into 8 to 12 large plates and about 20 smaller ones. These plates move in different directions and at different speeds. Their sizes don’t match the landmasses on top of them. For example, the North American plate is much larger than the North American continent. Iceland is split down the middle. It belongs to two different plates.

The continents have come together into one large mass—and then split apart again—more than once. Over the last 500 million years, it may have happened as many as three times.

Subduction: When one tectonic plate is forced under another, becoming part of the Earth’s mantle

Scientists think that heat currents in the Earth’s mantle are what powers plate tectonics. The mantle is the area below the Earth’s crust. It separates the Earth’s molten core from the hard crust. The mantle is solid in the short term, but over longer periods, the mantle flows very slowly. Tectonic plates float on top of the mantle. Pockets of hot liquid magma in the mantle ooze up along mountain ridges deep under the water.

Where the edges of the plates meet, a few different things can happen, depending on how they collide. Continents are lighter than the ocean floor. So, if both plates carry continents, they may crash head-on, causing mountains to rise up where they meet. If one plate is heavier, it may go under the other, a process known as subduction. The part of the plate that gets subducted becomes part of the mantle.

Some plates grind together as they pass each other. Cracks, or faults, appear in the plates when this happens. Wherever the plate edges meet, earthquakes take place. If you look at a global map of earthquake zones, the outlines of the plates are clearly visible.

Plate tectonics is reshaping our world right now, although very slowly. The European and North American plates are moving apart at the speed a fingernail grows. In a human lifetime, these plates move apart about six feet. Millions of years in the future, parts of California and Mexico will separate from North America and become an island. Most of Africa is pushing toward Europe. Eventually, Africa will squeeze out the Mediterranean Sea. When it reaches Europe, it will cause high mountains to form along the southern coast of Europe. The eastern portion of Africa will split off at the Great Rift Valley and float off into the Indian Ocean. In slow geologic time, the Earth’s plates are always moving.