Plate tectonics theory, formerly known as the theory of continental drift, is well supported in geology, geography and biology. It has the power to explain many phenomena, such as volcanoes and earthquakes. The theory provides a working model for analyses of phenomena that scientists observe. This explanatory power is, itself, strong evidence that the theory is correct.
The theory of plate tectonics posits that the continents and ocean basins on Earth's crust rest on large plates that are pulled continuously along over semi-molten material just beneath them. In places where these plates meet, the dense, heavy sea-floor slides under, or "subducts," the lighter continental plates. At other places, new sea floor is created from frequent volcanic eruptions. These phenomena are observed with earthquakes clustering around the known plate boundaries and fresh crust being pushed up at the mid-ocean ridges.
Another observation that supports the theory is that, in some places, landmasses seem as if they could fit together like puzzle pieces. The classic example of this is the way South America looks like it fits into the gap in Africa's west coast. On inspection, veins of ore have been found that run continuously to the coast in these areas, only to stop at the seashore and reappear on the other side of the ocean. Animal populations, such as Old- and New-World monkeys, are also distributed across these gaps, as if the continents had rifted apart.