Where did the continents come from?

Question

In animations that show the location / drift of the continents over the aeons, it seems that the continents (or cratons?) are just floating / drifting around, sometimes fusing or splitting, but having almost non-changing cumulative surface area over time.

This rises the question how that asymmetry between continents and oceans originally formed. Of course continental crust is less dense than oceanic crust, and once formed the lighter one would just float around on the denser one.

Are there scientific theories how and when this originally started? Like in liquid, hot, early Earth, the lighter stuff would just form a less-dense layer covering the whole planet.

And then? Was it a smooth process of accumulating / clumping continental stuff over time, and after accumulation they just keep drifting around? Or was it some cataclysmic event like early Earth being hit by a Mars-sized object that formed the Moon, and that impact lead to a since-then persisting asymmetry in continents vs. oceans because some of the layers where ripped off locally?

note: Answers to What do continents "lay" on? do not answer my question because they don't even mention continent genesis.

Answer 1

The answer is that we don't know. I could give you a list of all the considerations that go into evaluating that question, but it will probably be incomplete and outdated very soon. There were several important papers published in the recent years, including some very readable reviews that you might be interested in (1 2 3). But, there isn't a consensus. A friend just had a paper accepted in which she shows Yet-Another-Breakthrough™ in this field.

However, the general idea is that most of the continental lithosphere formed early on (Hadean to Archean) and since then the rate has slowed down.

Continental crust forms, as you said, by formation of "lighter stuff". Melting ultramafic mantle rocks leads to formation of basalt, and further melting of basalt leads to granites (and other related rocks) which are indeed lighter in colour, and also lighter in density so they end up at the top, and then "float" as the continental lithosphere on top of the mantle. The problem is that you can also remelt over and over again the continental lithosphere, recycling the already buoyant rocks and not increasing the net volume of the continents. It's this balance which is unknown. How much lithosphere is "juvenile" (i.e. formed by melting of mantle) and how much is recycled.

To make matters worse, continental lithosphere can also be destroyed. The most obvious case is erosion from the continents which then settles as a thin sedimentary cover on the oceanic crust and then lost to the mantle during subduction. Another process is delamination. Some rocks such as basalt are normally less dense than the mantle rocks. However, when metamorphosed at high pressure to become eclogite, they paradoxically become denser than the mantle, leading to the bottom part of the lithosphere to be "ripped off" and sink down into the mantle, thus destroying the continental lithosphere from the bottom. Here's a nice visualisation of this process:

Source for the image.