How do the shapes of different costlines support continetal drift?

Answer 1

Simply put, the easiest way to visualize it is to look at the continents as if they were parts of a large jigsaw puzzle. For example, if you take the continental margin of South America it will fit against Africa quite well.

This is what the geographer Wegener noticed in the 1800's. If you have a map of the continents that you can cut apart, it will really help you see what he was talking about as you slide the continents against one another.

He didn't know the cause, but he and others suggested that the continents had drifted apart -- sort of like continental ships adrift in the the sea of crustal material.

If was not until the 1960's that the reality of plate tectonics made it's appearance. At that time it was noted that magnetic anomalies which were found on both sides of the mid-ocean ridges were mirror images of one another. This suggested that the ridges were formed as new material coming from deep within the earth forced the continents apart along these mid-ocean ridges.

Answer 2

When the continents last broke up, the continental margins (not necessarily the currently-visible coastlines) maintained their shape as the breakup occurred. Wegener, in the 19th century, noticed the similarity of the coastlines (in reverse) of South America and Africa (with some of North America thrown in, as well) and proposed they were once joined. So, he was the first to propose the idea that the continents moved, although the actual mechanism for such movement wasn't proposed until the middle of the 20th century.

Other support (unrelated to coastline shape) comes from fossil evidence, and paleoclimatological evidence (from fossils and, in some cases, scratches made in ancient bedrock by glaciers millions of years before the last glacial age). Moreover, rocks that would have been adjacent before the breakup are similar in fossil content and mineralogical makeup.