Has there been more than one Pangaea? Since Pangea began to separate only 200 million years ago, and the Earth is about 4.5 billion years old, do the land masses periodically merge into a supercontinent and then break apart again?  If so, how many times has this happened? If not, why did it only happen once? At their current trajectories, won't the continents smash back into each other?

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We hypothesize, based on geology, that there were prior supercontinents.

The Earth's crust is constantly in motion, rearranging and reshaping itself. Based on our current understanding of the age of the Earth and the motion of its tectonic plates, we presume that this motion follows a supercontinent cycle. This means that the land which is above sea level cycles between two extremes; a dispersed geographic arrangement, like the one we have today, and a condensed one, with virtually all of the exposed landmasses physically connected.

Direct evidence of ancient supercontinents comes in the form of geologic and paleomagnetic data. This is to say that rocks from two locations are compared; if they are found to have formed at the same time, share the same composition and chemistry, and are aligned to the same magnetic polarities, then we presume that they were physically connected when they formed. We can also look at fossil assemblages, isotopes, and volcanic ash deposits. By following the patterns of these connections, we can reconstruct the relationships that describe the physical layout of the supercontinent.

There are competing theories as to exactly how many supercontinents existed. Scientists generally agree that Pangaea did exist and was a supercontinent, but prior to that, things become a bit more murky. It becomes harder to discern associations between rocks because they've been around for so long, and exposed to so many physical and chemical changes. Our current models are accurate to around 200 million years ago, but we know that rocks and land existed at least as far back as a billion years ago, and earlier.

In one model, the supercontinent cycle was initiated in the Ediacaran Period due to changes in the tectonic nature of the Earth. Prior to the beginning of this cycle, all land was concentrated in a supercontinent called Protopangaea-Paleopangaea. In the second model, supercontinents existed prior to the Ediacaran, called Superia, Sclavia, Vaalbara and Kenorland. Both models posit that subsequent supercontinents formed prior to Pangaea, called Columbia (or Nuna) and Rodinia. The exact relationships and compositions of these supercontinents are not known with certainty.

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It is theorized that approximately 430 million years ago the southern continents were joined as a single land mass known as Gondwana.  In the late Paleozoic it is believed that the ancestor of North America may have collided with the ancestor of Africa.  By about 410 million years ago this land mass collided with the ancestors of Asia and Siberia.  The combination of these ancestral land masses was known as Laurasia.  By approximately 250 million years ago with the migration of Gondwana northward the super continent of Pangaea was formed. (After C. Scotses, R. K. Bambach, R. VandedrVoo, and A. Ziegler)

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As far as I can tell, the current thinking is that there were two supercontinents before Pangaea.  There was Rodinia (the one I posted a link about the last time you asked about this) and one supercontinent prior to that.  That first supercontinent has been called "Columbia" because the best evidence for it can be found near the Columbia River.

Columbia began to assemble 1.8 billion years ago.  It appears to have started breaking up 1.5 billion years ago.  The continents reformed into Rodinia around 1 billion years ago.  Rodinia lasted until about 700 million years ago.

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Scientists believe that you are in fact correct and the land masses have periodically merged into supercontinents.  Pangaea was the most recent one.

One other for which there is evidence is something called Rodinia.  It was a supercontinent that came together about 1.2 billion years ago and lasted for something like 350 million years.

eNotes actually has a good article on this subject -- follow the link.  It talks about other supercontinents that may have existed.

As far as the future goes, I found the "scotese.com" link that shows an animation of what may happen...

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