There are many misconceptions about geological concepts. There is a list here, developed by Kent Kirby of the University of Minnesota. This post is to debunk one of those misconceptions. There will be others. Find them here.
Misconception: The edge of a continent is the same thing as a plate boundary.
There is a natural sensibility to Geology’s Theory of Plate Tectonics. Obviously, continents have moved, because of the puzzle-like fit of the continent edges across the Atlantic Ocean.
The animation below shows the process of the opening of the Atlantic Ocean pretty nicely.
Since the continents were clearly once all together, then broke apart, then the boundary of the individual tectonic plates must be along the edges of the continents.
Only that’s not true. Let’s look at a map of the Earth’s known tectonic plates.
If we focus in on Africa and South America, we see that despite the fact that the two continents seem to fit perfectly together, the plates that contain the continents actually extend all the way into the center of the Atlantic Ocean.
It is the mid-Atlantic ridge, a volcanic under-water mountain range that extends from north to south down the middle of the entire Atlantic Ocean that forms the actual plate boundary.
So here’s the deal (I’m only going to focus on South America and Africa here, but realize this applies all the way up and down what is now the Atlantic Ocean basin):
When South America and Africa were connected in Pangaea, there was no plate boundary between them. Then rifting started. The rift developed into a plate boundary (the mid-Atlantic ridge). This rift was essentially along the eastern edge of modern South America and the western edge of modern Africa, even though they were right next to each other.
The rift opened more fully, and ocean crust began to form, pushing South America away from Africa. The ocean crust is denser than the continental crust, and (as its name belies) was rapidly covered by water – the natal Atlantic Ocean.
The mid-Atlantic ridge is the remnants of that original rift. It hasn’t gone anywhere, but all the while ocean crust has been forming along it and shoving the continents further and further apart. We call this ‘sea-floor spreading.’
Where ever rifting and sea-floor spreading have taken place and pushed continents apart, we know that those continent edges are not plate boundaries. We call them passive continental margins.
The drawing above illustrates the process of rifting a continent and forming passive continental margins.
Sometimes the edges of continents are plate boundaries. Western North America is an excellent example of this, and it explains why the state of California is so prone to earthquakes.
Edges of continents that are actually plate boundaries are called active continental margins. The entire rim of the Pacific Ocean is formed by active boundaries, due to the fact that the ocean crust underlying the Pacific Ocean is being dragged under (or subducted) below the continents.
This has to happen, if you think about it. If the Atlantic Ocean is getting bigger and bigger, by pushing the continents apart, some other part of the Earth, another ocean basin, probably, has to be getting smaller. Otherwise the planet would just get bigger and bigger.
Now that doesn’t make sense. The planet would have popped by now!