[tzs]

Is the coastlines of South America and Africa looking like they fit together actually because of plate tectonics, or is it just a coincidence?

The shape we see for the coastlines of South America and Africa is affected by sea level. Depending on when you happened to look over the last say 140 million years sea level would have varied from around 135 meters below current sea level to around 75 meters above current sea level. That is a range of 210 meters.

Surely over that range both costs would change quite a bit, and I can't think of any mechanism that would make those changes complimentary in a way to keep the two coasts looking like they fit together.

[dylan604]

are you playing devil's advocate? perhaps you're just not familiar with Pangea? here's a video to show plate movement:

https://www.youtube.com/watch?v=OGdPqpzYD4o

[tzs]

I'm familiar with that. We see that shortly after a split the edges of the two sides of the split match, as we would expect. As they separate water fills the gap so those matching edges and now also matching coastlines.

Those two edges will continue to match as they get farther and farther apart. The coastlines will always match if the coastline stays at the elevation of the edge.

But as sea level changes the elevation of the coastline should change. For example, suppose sea level rose 300 meters. I don't think there is enough water available for that currently. 200 meters looks like it might be the maximum. But suppose that when Earth was receiving a lot of water from comet bombardment long ago that had been a bit heavier and so we did have enough for 300 meters.

Looking at topographic maps of the east side of South America and the west side of Africa it looks like 300 meters of sea level rise would reshape those coasts in vastly different ways and they would no longer be anywhere the edges of the split and would not match each other.

I couldn't find a good topographic map of the ocean floor to see how much of a sea level drop would be needed to make the coasts no longer match.

What I'm wondering then is if there is something that makes it so the topography of each continent and the limits of possible sea level variation make it so the coastlines long after a split when the two parts are far apart will still be close enough to where the original edges are that the coastlines will keep matching? Or is it just an accident that it has worked out that way on Earth?

[dylan604]

I really do not know what you are getting with all of those words. Put simply, if the continents were puzzle pieces, would you not attempt to put South America and Africa together? QED

[mitthrowaway2]

I think they're wondering whether that's a lucky coincidence, or whether it would still be true with different sea levels (such as during the ice ages, when sea levels were lower).

I guess the point is really it's the continental shelves that should fit together, not the coast lines.