Continental drift is an early 20th-century hypothesis that proposed Earth's continents move or drift relative to each other over geologic time
. The theory was initially proposed by German naturalist Alexander von Humboldt and was based on the observation that the eastern coast of South America appeared to fit into the western coast of Africa
. The concept of continental drift was later incorporated into the science of plate tectonics, which studies the movement of Earth's continents as they ride on plates of the Earth's lithosphere
. The theory of continental drift was instrumental in developing the theory of plate tectonics, which explains that the movement of large sections of Earth's crust, called tectonic plates, causes changes in Earth's surface
. Today, plate movements are thought to shift Earth's continents about 2 centimeters (or 0.8 inch) per year
