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I recently had an argument with a conspiracy theory type who had a bunch of strange ideas about a number of pretty well established scientific phenomenon. One that I had never heard before, nor can I find anything to back up his claim, though I suspect it's nonsense, was that the Pangaea supercontinent could have never have existed because it would be impossible for all the earth's land masses to exist on one side of the planet and the oceans on the other side.

My answer to him was that this was nonsensical since continental land is the same as ocean floor, except that it happens to be poking out from the water. But does anyone have any information/research to back up or repudiate this claim?

Thanks.

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    because it would be impossible for all the earth's land masses to exist on one side of the planet and the oceans on the other side What is the idea behind this assumption? A difference of weight which could disturb the Earth symmetry? Can you please ask to this person?
    – A.L
    Commented Apr 4, 2014 at 16:16
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    The current earth is significantly unbalanced in its distribution of continental mass. Just look at the size of the pacific ocean.
    – matt_black
    Commented Sep 21, 2015 at 10:27

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No, continental rocks are not the same as ocean floor.

Continents are mostly made up of minerals like granite, which are lighter than ocean floor minerals like basalt (sima, short for Silicon/Magnesium). Continental material is mostly sial (short for Silicon/Aluminium) which is lighter. Partially molten mantle material is pushed up at mid-ocean ridges, which then causes the tectonic plates (ocean plates) to move apart. As the mantle material is not fully molten, the composition of the melt is different from that of the mantle, which in turn is different from the continents.

As the continents are lighter they effectively float on the oceanic floor, and travel along with the spreading seafloor. There is abundant evidence for the movement of tectonic plates. The first discovery was the striped magnetism of the seafloor, caused by the effect of earth's reversing magnetic field on solidifying rock. Earthquakes are also far more prevalent at plate boundaries than elsewhere, and maps show quite clearly that earthquake follow the boundaries.

Continental movement has been measured directly with GPS satellites, and also by bouncing lasers off the reflectors that the Apollo astronauts left on the moon. Of course, conspiracy theorists may not believe those mirrors are actually there... At least they would have to admit that GPS works.

EDIT

Continental drift does not necessarily mean that all the continents ever were joined together. However, there is a lot of evidence for that, too. Start with the way the continents fit together. There is also the distribution of related fossils across different continents (leading, for example, to marsupials in Australia and the Americas only), which would otherwise be very hard to explain. And, there is the fact that, if you join the continents together, geological features line up. Again it would be a funny coincidence if this happened without them having been joined while those features were formed.

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    +1 for confounding me. I read this and thought "Lighter? What nonsense. That contradicts what I was taught in high-school, and what about sea-shells found in mountains?! Moon reflector used to measure continental drift? Nonsense! I expect more from a user with 1,000 rep." Then I looked at the references. Oh. OH! Wow! Thanks.
    – Oddthinking ♦
    Commented Apr 2, 2014 at 23:50
  • 'continents are lighter they effectively float on the oceanic floor' this doesn't sound right to me, perhaps you mean 'float on the mantle'?
    – Kvothe
    Commented Apr 3, 2014 at 12:34
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    @Kvothe Perhaps less dense is the more correct terminology. The continental crust is less dense than the ocean crust. It is carried along by the oceanic crust and the continents stay on top, crashing into each other, while the oceanic crust is created at expansion zones and recycled at subduction zones like a conveyor belt. Continental crust less readily recycled this way. The oldest oceanic crust is about 200 million years old. Continental crust is on average 2 billion years old, with some being 3.7-4.3 billion years old.
    – user5582
    Commented Apr 3, 2014 at 14:38
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In addition to @hdhondt's answer (+1)

The evidence for the existence of Pangea lies in the existence of matches between fossil records and rock formations in continents that are now thousands of miles apart, in addition to Wegener's observation that the (continental shelves) of the existing continents match quite closely like a jigsaw. If Pangea did not exist, it becomes hard to explain how the same fossil species evolved independently on continents that were not contiguous at some point.

This picture from Wikipedia illustrates the fossil evidence for Pangea quite nicely: enter image description here

As I understand it, Pangea apparently formed as a result of a mantle plume producing very high rates of production of ocean sea floor, which in turn pushed all of the continents together, until the plume subsided and divergent faults within Pangea pushed the continents apart again.

There seem to be a fair few papers about this sort of thing, e.g. here and here, but the geology is well beyond me.

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