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Continental drift is the theory that the Earth's crust is divided into plates that move relative to each other, with their interactions producing a variety of geological features. Plate tectonics is the study of the causes and effects of these processes.
How continental drift works
Plates consist of a layer of silicon-magnesium ("sima") rock, which may have some less-dense silicon-aluminum ("sial") rock on top of them. Ocean floors have only sima, while continents and continental shelves have sial on top of sima.
Plates separate from each other at ridges in oceans like the Mid-Atlantic Ridge; new plate material is formed at such places. Plates can also slip past each other, as at California's San Andreas Fault, and they can also run into each other. At such places ocean crustal rock is more dense, and will "subduct", going underneath the other plate, making a trench in the ocean floor; the deepest of these trenches are in the western Pacific. As a plate subducts, it may partially melt, producing a line of volcanoes in the overriding plate (volcanic arc). However, if both plates have continental rock on them, that rock will crumple and be forced upward; the collision of the Indian and Eurasian plates produces the Himalaya Mountains. Plates can drift over "hot spots" that produce chains of volcanic islands like the Hawaiian Islands and the Emperor seamounts (underwater mountains). The Hawaiian Islands have a nice progression from the undersea volcano Loihi in the southeast to the Big Island, with its three active volcanoes (Mauna Loa, Mauna Kea, and Kilauea), and further northwestward to smaller and older islands, and ultimately to seamounts, whose above-water parts have eroded away.
The continents have been drifting apart since the Triassic, when nearly all the continents were combined in a supercontinent called "Pangaea". This continent split into two smaller ones, "Laurasia" and "Gondwana", which in turn split into the present-day continents in the Jurassic and Cretaceous.
Pangaea, in turn, was formed by collisions of earlier continents, becoming assembled in the late Paleozoic. Those continents, in turn, were fragments of an earlier supercontinent, Rodinia, which existed between 1100 and 750 million years ago. There is evidence of even earlier supercontinents, but the evidence becomes difficult to interpret. This continual drifting has been described as "continental bumper cars", and some geologists theorize that the periodic accretion and fracturing of continental masses may be a necessary part of a geologic cycle.
Continents have grown over geological time, accreting volcanic arcs and pieces of other continents over the last 2.5 billion years; in the Archean, continents were relatively small.
What drives continental drift is uncertain; the two favorite theories are mantle convection and subduction-zone pulling, perhaps combined with oceanic-ridge pushing.
How was continental drift discovered?
When surveying and cartography efforts became good enough to produce good outlines of the shorelines of the continents, some people noticed that South America and Africa have a remarkably-good jigsaw-puzzle fit. But it was difficult to go much further than this observation for a long time. After Charles Darwin showed the importance of biogeography in understanding evolution, the geologist Eduard Suess in 1885 used some reasoning from the biogeography of fossils like the seed fern Glossopteris to conclude that South America, Africa, Madagascar, India, Australia, and Antarctica were once a single continent, which he named Gondwana ("land of the Gonds" in some Asian Indian language). But to produce the multiple continents of today, he proposed that either the Earth either swelled or parts of Gondwana sank.
Sunken land bridges were a favorite theory in the first half of the twentieth century. That hypothesis is not as far-fetched as it might seem, because some areas, like the Bering Straits, had clearly been high and dry during much of the Pleistocene, They were revealed by oceanic water being sequestered in large continental glaciers during the Pleistocene Ice Ages. But there was a certain lack of evidence for them in ocean-floor surveys, more about which later.
The modern concept of continental drift was first proposed by German meteorologist Alfred Wegener; he pointed out the biogeographical details that Suess had pointed out, and he pointed out that Permian glaciers in the southern continents had flowed from oceans -- unless those continents had been united to form Gondwana back then. However, Wegener had failed to think of a convincing mechanism for it to happen, and his followers, like Alexander du Toit, were no more successful. As a result, mainstream geologists dismissed continental drift because they found it difficult to picture how continents could plow through ocean floor. Sunken land bridges were easier to swallow.
But in the 1950's, surveys of the ocean floors revealed evidence of seafloor spreading, and studies of paleomagnetism revealed that the continents had moved relative to the Earth's poles -- and moved in different trajectories. This led to the revival of continental drift; the continents did not plow through ocean floor, but instead drifted with it in big plates of crust. And the trajectories deduced from paleomagnetism gave the "right" relative motions. Continental drift has even been directly observed, notably by tracking Global Positioning System (GPS) satellites from different continents; the observed drift rates closely match the estimated rates for the last few million years.
Some YEC's accept this, notably Walt Brown, who has proposed a hydroplate theory of superfast continental drift. In it, there was once a layer of subterranean water that had acted as a lubricant, allowing the continents to drift from their Pangaea positions to their present-day positions in only a few thousand years.
No YEC's have been able to provide convincing, if any, evidence for their theories, though.