Continental drift refers to the gradual movement of continents across the surface of the Earth. Although the ocean floor is also in a state of gradual but continuous change, the term "continental drift" is used because the continents appear to move, or drift, across the surface of the oceans. The theory of continental drift was first developed in the early 1900s by Alfred Wegener, and today a modified version, known as plate tectonics, is the basis for the study of geology.
Geologists now realize that the Earth's surface consists a number of massive rock formations called tectonic plates. According to Cal Tech, these plates move at up to two inches per year. Because of this gradual but steady movement, the regions where plates collide can be subject to earthquakes and volcanoes as the rock layers slide together unevenly. The fault lines along the western coast of North America, for instance, occur where the Pacific Plate and the North American Plate meet. Earthquakes are also common on the western side of the Pacific, such as in Japan, where plates also meet.
Over much longer time scales, in tens and hundreds of millions of years, the effects are even more striking. On these time scales, the tectonic plates are constantly in motion, sometimes colliding together to form new and larger continents, sometimes drifting apart and thus rupturing entire continents. The University of California, Berkeley, has reconstructed what this motion looks like over hundreds of millions of years as a series of animated GIFs, showing continental drift as it is believed to have occurred over a 750-million-year period.
It is possible to reconstruct what the continents looked like in the past by measuring the rates and directions at which they are drifting now, and studying the history of ancient rock formations which have not been disrupted or destroyed for hundreds of millions or even billions of years. The Canadian Shield formation which stretches through central Canada and the upper midwest of the United States, for instance, is composed of extremely old rock, some of it nearly four billion years old.
Using this data, it is possible to work backward through geological history, tracing continental drift over time. According to the Exploratorium, the present continents are a product of the past 200 million years, during which time a former supercontinent called Pangaea split up into two smaller supercontinents, Laurasia and Gondwanaland, and these these ruptured into smaller pieces, as well.
Antarctica, Africa, Australia, South America and the Indian subcontinent were once combined as Gondwanaland, while North America, Europe and Asia combined to form Laurasia. Pangaea, in turn, came into being perhaps 300 million years ago from what had previously been separate continents, known hypothetically to geologists as Laurentia, Siberia, and Baltica. This process of supercontinents forming and breaking apart can theoretically be traced back to the origins of Earth's plates billions of years ago, although the farther back one goes, the less rock formations survive and can be studied. As a result, science's knowledge about the first continents is largely speculative.
The theory that the continents are in motion, first stated by Alfred Wegener in 1912, began as an inference from the fact that the coastlines of some continents appear to match precisely, like puzzle pieces, suggesting that they broke off from one another. The most obvious visible example of this today are the west coast of Africa and the east coast of South America, but an even larger formation can be created by lining up these two continents together with Antarctica, Australia and India. According to Anne Weil at Berkeley, Wegener had difficulty persuading others of his theory because, while the pieces did seem to fit together, he was unable to identify a mechanism which would cause the continents to move. Without being able to explain how the continents moved the way he claimed they could have, Wegener lacked credibility.
As a result, it was not until the 1950s and 1960s, after Wegener had died, that geologists returned to the idea in earnest, this time labelling it the theory of plate tectonics. In the meantime, important advances had been made in understanding the seafloor, which undergoes a continuous "spreading" process for similar reasons while the continents drift, as well as in modelling the molten interior layers of the Earth. The long-term geological history of the continents described above comes from modern plate tectonics. The chief difference is that, whereas Wegener only imagined the continents moving through the ocean, geologists now realize that the seafloor is slowly changing as well, driven by volcanic activity known as seafloor spreading.
Some evidence of tectonic activity has been observed on Saturn's moon Titan, and some scientists believe that the surfaces of Venus and Mars were tectonically active in the distant past, as well. However, for the particular dynamics of continental drift that occurs on Earth, water probably plays an important role by helping weaken the crust. Earth's processes of continental drift appear to be uncommon or rare among planets.