Is Pangea still a theory, or is it fact now?
It will remain a theory, but, in all likelyhood, it is true.
See Pangea
To make it a fact, one would have to be able to use the “torn newspaper” example in the above link.
Does your definition of “fact” include “reasonable extrapolation from observations”? If so, then the observed details of geology make Pangea an unavoidable and inevitable extrapolation and therefore a fact.
By most people’s definition of “fact,” Pangea cannot possibly be proven a fact. The evidence, however, strongly supports the idea.
Don’t be so literal, guys. I know what she means. She means, “When you come across it, is it still prefaced by the phrase, ‘this is only a theory’, or is it generally being accepted as the way things happened?”
The correct answer is (b), it is now generally accepted as the way things must have happened, although there’s never going to be any way to prove it. When you run across it in non-fiction books about geology (John McPhee, for one), it’s presented as “this is the way we think it must have happened because it seems to fit all the facts we have so far”.
However, you know as well as I do what wild and crazy guys these scientists are, so I wouldn’t bet the farm that Science won’t come up with some other, equally plausible theory that “seems to fit all the facts we have so far”. Remember, not so long ago, James Horner, the “hot-blooded dinosaur” guy, was being dismissed as a crank by the majority of the paleontological community, and now suddenly the theory of hot-blooded dinosaurs “seems to fit all the facts we have so far”. :rolleyes:
And the giant-comet-hits-the-earth-and-wipes-out-the-dinosaurs theory is now generally accepted as fact, ever since the guy proved, more or less, that that big empty spot in Yucatan is where the comet/asteroid/whatever probably hit.
So don’t hold your breath on the Pangaea theory. There may be another one along any minute now.
Actually, if I remember McPhee’s Annals… correctly, Pangea isn’t all that special–the landmasses have probably been lumped together into one block like that a number of times, and probably will be again a number of times before the earth cools.
What we call “Pangea” is just the most recent contiguous blob.
(…bracing myself for the next tectonic pile-up…)
[aside]
True, it’s nitpicking, but as you know, there’s an important scientific distinction between a fact and a theory. A theory does not become a fact. A theory is an explanation. For example, evolution is both. It’s a fact that life evolves and the Theory of Evolution explains how this happens. A theory is never 100% proven, but continues to be studied in order to obtain more supporting or disproving evidence.
Anyway, nitpicking is a major pasttime of the SDMB. 
[/aside]
toadspittle is correct in pointing out that there’s been more than one supercontinent in Earth’s history. (By “supercontinent,” I mean that much, but not necessarily all, of the Earth’s landmass appears to have been collected in one big patch.) Before Pangea there was Gondwana (~550-500 Ma) and Rodinia (~1,000-750 Ma). The existence of these ultra-big landmasses is inferred from a sizable body of evidence, although not all the details are well known - there are limitations to what the data can tell us, as well as what kinds of data we can acquire. The further back in time you go, the more difficult it is to reconstruct past continental positions. As other posters have already noted, we can’t prove that they existed, but the available evidence suggests strongly that they did.
BTW, the body of evidence used to reconstruct continental positions is considerably greater than that used to make arguments about the physiology of dinosaurs, say, so it shouldn’t be terribly surprising that some interpretations are still subject to significant changes. And while the K-T impact is widely thought to be the last straw for T. rex and friends, there is still a considerable amount of debate over the extent to which other factors may have contributed to the dinosaurs’ demise, and the impact’s true environmental effects. Compared to these two issues, the existence of Pangea is pretty much set in stone (pun intended).
When two or more cratons (continental core areas) come together, one has a supercontinent. Gondwanaland was one of two supercontinents around at its time, the other being Laurasia, which consisted of North America (with Greenland) plus Eurasia (without India, which was part of Gondwanaland).
Pangaea is the name given to the last time almost all the masses of land came together in one mega-supercontinent. And, as noted, late precambrian time was characterized by Rodinia, another such mega-merging of continents (and both had significant ice ages, an otherwise rare occurrence, during their existence, FTR). (The western edge of the Adirondacks and the eastern edge of the Canadian Shield represent the fracture zone from when Rodinia broke up, with what’s east of them having been later additions to North America, explaining in part why Cambrian trilobites from east Newfoundland match those in England and Wales, and those from west Newfoundland match those from British Columbia.)
We have one supercontinent around now: Eurasia, which consists of several cratons mooshed together. Europe is more or less one of them, with another including most of the (non-African) Near East, India another, most of Siberia another, and north and south China two others. And of course Africa seems determined to join in the fun, at its own mile-a-millennium-or-so pace.
[minor point] The presence of multiple cratons (very old pieces of continental crust, generally older than 2 billion years of age) alone does not a supercontinent make. For example, Australia (a continent by modern definition) is home to four: the Yilgarn, Pilbara, Gawler and Curnamona cratons. The cut-off point for calling something a supercontinent vs. a plain old continent, size-wise, is a bit arbitrary, but the general connotation is that the landmass in question consisted of significant portions of multiple modern continents. In terms of modern landmasses, IMHO, Eurasia qualifies, but more for the sheer area involved rather than the number of cratons that exist within it. [/minor point]
I’ve gotta love a man who brings up late Precambrian ice ages unbidden. 
Poly, is there anything you don’t know something about?
My impression was that gigayear-stable segments of crust were terranes, with craton restricted to the largest of these, constituting the hearts of major continents, e.g., Angara, Canadian Shield, Fennoscandian (love that term) Shield, etc.
And to answer your other question:
The distinction between cratons and terranes comes not from their respective sizes, but their geologic setting. Cratons like the Canadian and Fennoscandian shields have been largely unaffected by orogenies (mountain-building collisional events), except around the edges, for a very long time - generally not since 1.5 billion years ago. Their stability made them ideal for accumulating other bits of landmass through a variety of means, which is why they tend to sit now at the hearts of continents, as you so aptly put it.
Terranes, on the other hand, are fault-bounded bits of land that have been accreted (“smeared”) onto the edges of existing continents along subduction zones. Avalonia (one of four that compose a goodly chunk of New England and southeastern Canada) is a good example; it was sutured to Laurentia during the Acadian orogeny in Devonian time, circa 380-360 Ma. Western North America is plastered with terranes, some as young as Mesozoic. A feature common to terranes and not common to cratons is extensive structural deformation, caused by the accretion process.
In response to your response: I bow before you, humbled.
Thanks for the clarification on cratons and terranes, Fillet – and the answer was supposed to indicate that despite having read a lot about a lot of stuff and retained much of it, I was myself (humbly) aware of what I didn’t know – including the important things. 'Least that’s how I read the Proverbs verse. Although if we get into a “I’m humbler than you are” contest… 
( ::: Hearing Zev injecting a “Look who thinks he’s nothing” anecdote here ::: )
So, the entire land mass west of the San Andreas counts as a terrane?
Oh and I hear Alaska is a very large collection of old land masses that smashed onto the endge of North America as well.
Doobieous - Come to think of it, I’ve never heard the land west of the San Andreas referred to as a terrane, just as the leading edge of the Pacific Plate. AFAIK, that might be because that part of California hasn’t been permanently accreted onto the North American continent. Maybe jrepka can shed some more light on that; I think he does field work there.
You are right on the money with Alaska, though. If you continue down from Alaska through Washington State and Oregon into northern California (east of the San Andreas), you’ll find that there’s a 500 to 800-mile-wide swath of terrances, both Paleozoic and Mesozoic in age. Most of Oregon and British Columbia are composed of terranes, in fact.
Polycarp - I know what you were getting at, but my response still stands.
OK, terranes. My picture of the situation is that we’ve had big madagascar size chunks stick onto NA’s pacific coast. But, how long ago did this happen? And shouldn’t we be finding some unusual fossils on the pacific coast? After all, if these were madagascar-like continental islands that were isolated in the pacific we should see some interesting things evolving there. So what’s the scoop? Any lemurs on those old terranes? I’ve never heard of any bizarre fossils, but maybe I haven’t been paying attention.
OK, now that I have a couple of references in front of me I can better address your questions, Lemur866.
There are five or six larger terranes of varying sizes, and a whole bunch of smaller slivers of crust, that have been tacked on to the west coast over the past few hundred million years. The bulk of terrane accretion occurred during the late Permian period and throughout the Triassic period (circa 270-200 Ma).
People had indeed noticed that there was something odd about these bits of land, because their invertebrate fossil assemblages were more closely related to faunas described from other places like Japan and southeast Asia (e.g., Permian Tethyan fusulinid foraminifera). There are other supporting lines of evidence (paleomagnetism, stratigraphy) for the far-traveled nature of these crustal bits as well, but since the terranes are mostly deep marine and/or volcanic in origin there are AFAIK no distinctive vertebrate fossils associated with them. (Side note: the earliest primate fossils seem to Paleocene in age [post 65 Ma], so none of these could have served as index fossils for the terranes.)
If you want a better idea of what some of the terranes might have looked like originally, just look at modern Japan or Indonesia - both are volcanic island arcs perched on the overriding edge of subduction zones. If there were continental landmasses on the plates going down underneath the island arcs, those arcs would eventually get stuck onto the edge of the continent as terranes and subduction would come to a halt. (Australia is currently moving northeastward - not quite the right direction for a head-on collision with Indonesia, so terrane accretion onto Australia would probably be limited at best.)
Thanks, Fillet. So, these are too old for Lemurs…too bad…but I guess forams have their place.