Wegener noticed that the coasts of western Africa and eastern South America looked like the edges of interlocking pieces of a jigsaw puzzle. He was not the first to notice this, but he was the first to formally present evidence suggesting that the two continents had once been connected.
Wegener was convinced that the two continents were once part of an enormous, single landmass that had split apart. He knew that the two areas had many geological and biological similarities. For example, fossils of the ancient reptile mesosaurus are only found in southern Africa and South America. Mesosaurus, a freshwater reptile only one meter (3.3 feet) long, could not have swum the Atlantic Ocean. The presence of mesosaurus suggests a single habitat with many lakes and rivers.
Wegener believed that all the continents—not just Africa and South America—had once been joined in a singlesupercontinent. This huge ancient landmass is known asPangaea, which means “all lands” in Greek. Pangaea existed about 240 million years ago. By about 200 million years ago, this supercontinent began breaking up. Over millions of years, Pangaea separated into pieces that moved away from one another. These pieces slowly assumed their present positions as the continents.
At first, other scientists did not accept Wegener’s theory of continental drift. But scientists now know that the continents rest on massive slabs of rock called tectonic plates. The plates are always moving and interacting in a process called plate tectonics. Over time, tectonic activity changes the Earth’s surface, rearranging and reshaping its landmasses.
Today, scientists believe that several supercontinents like Pangaea have formed and broken up over the course of the Earth’s lifespan. These include Pannotia, which formed about 600 million years ago, and Rodinia, which existed more than a billion years ago.
The continents are still moving today. Underwater exploration has revealed seafloor spreading. Seafloor spreading is the process of new crust forming between two plates that are moving apart. Along a network of mountain ranges in the ocean, molten rock rises from within the Earth and adds new seafloor to the edges of the old. As the seafloor grows wider, the continents on opposite sides of the ridges move away from each other.
North America and Europe, for example, are moving away from each other at the rate of about 2.5 centimeters (1 inch) per year. If you could visit the planet in the future, you might find part of California separated from North America, becoming an island in the Pacific Ocean. Africa will eventually split in two along the Great Rift Valley. It is even possible that another supercontinent may form someday.