So what happens to existing continents when new supercontinent forms?

[swampwiz]

I had always known about Pangaea, but that always begged the question of what was before; since there didn't seem to be anything before, I just presumed that the crust was monolithic (i.e., as a general adjective, not "one rock", even though that it was it is, LOL), and under the sea, and a gigantic volcano just spit out all the magma (which I presume is simply the mantle that gets spit out) that cooled into Pangaea. And indeed, this fit with the evolution of lungfish (like Tiktaalik?) 400 Mya.

But this Wikipedia article is "Supercontinent cycle" - https://en.wikipedia.org/wiki/Supercontinent_cycle - so obviously this happens with some regularity (i.e., according to one theory), and thus this begs the question of what happens to the old continents. Do they just sink into the seabed? I suppose some of the magma for the new supercontinent gets spit up on top of some of the old continents. This also begs the question that if magma is being spit up, somehow that volume of the mantle has to contract, which perhaps means that the old continents and seabed falls relative to the geocenter? I suppose that this also means that the total crust grows with each new supercontinent adding to the crust.

 

[willem2]

The continents mostly just get rearranged to form a supercontinent. Continental crust is lighter than oceanic crust and rarely subducts. Some of the crust in Canada and austrlia is older than 4 billion years, ad must have formed right after the formation of the moon.