The South African mountains older than continents

Reconstruction of where the plates were in the past is mostly determined by paleomagnetism (subject area of my PhD). When lava erupts and cools, it gets magnetized in the direction of the earth's magnetic field. This can be measured directly in the rocks themselves or seen as magnetic anomalies in the oceans. Since the magnetic field flips directions every 1/2 million years or so (a somewhat random process) the rocks around the ocean ridges that are spreading and erupting continuously all get magnetized in one direction and then another. This produces magnetic anomaly stripes that have been measured all over the world.

Using these ocean stripes, which can be treated like slices in time, it is quite straight forward to reconstruct what Pangea, the most recent supercontinent, looked like and where it was located when it began to break up about 175 million years ago. Almost all of the current ocean floor was created after this breakup, so before that reconstructions become much more difficult.

This a very simplified explanation. The older the rock, the less likely the original magnetization was preserved. Also, the continents have been growing slowly over the 4.5 billion history of the earth. The earth was almost all ocean at the start. The oldest rocks known are about 3.2 billion years old, so before that almost nothing is known about the magnetic field of earth (some older zircons survive in these rocks, but they are unlikely to preserve a record of the magnetic field). The wikipedia entry on plate tectonics is a decent place to start to read more about the process.