Origin of the continents

Question

Visiting from WorldBuilding SE. Someone recently asked a question that dusted off an old theory I had once had, so I started digging. Sadly I could find little more on the topic than the first time I had the thought so I am coming here.

I remember learning about plate tectonics in school, books, TV, magazine articles, etc. All mention the origin of the continents was quietly skipped. If it were mentioned at all it was just noted as a mystery.

Fast forward a few more years and I learn of another theory, about the formation of the moon. I read about the Theia Impact theory and saw this animation where another planetary body (Theia) impacts the Proto-Earth, forming the moon:

The moment I saw it I knew here was the origin of the continents. Perhaps not the crustal plates but the contents, or rather the cratons that float atop the mantle and form the continents we are familiar with.

Everyone knows of Gondwana and Pangaea, supercontinents of the past. More rarely known is there may have been as many as 10 supercontinents. Ur and VaalBara being among the first, billions of years before Pangaea.

Question: Have there been any publications or studies to prove my novice theory that the moon formation had something to do with the origin of the continents? I was able to find a couple references from Harvard papers, but honestly and obviously it would take a degree to decipher 90 percent of it. In which case I would be answering my own question. Even here the closest answer is that they formed in the early Hadean.

So is this an accepted theory I just haven't ran across, or has it been considered much before?

Answer 1

The impact which created the moon occurred quite early into the Solar System's lifespan. Bodies of that size would have both still been mostly molten at the time of impact; so as far as I am aware, your theory isn't part of mainstream geologic thought, though that shouldn't discourage you from coming up with these ideas!

The current widely-accepted paradigm is that the continents grew gradually over geologic time and, and only rarely are very small fragments of them 'recycled' back into the mantle. There are several other, more fringe theories that favour a static ratio of continental to oceanic crust through geologic time, arguing for a much larger fraction of continental recycling. The latter camp is arguing against quite a large body of radiometric age evidence and a good deal of correlation between those ages and processes ongoing along tectonically active continental margins.

To understand why the continents came to be, you need to understand the process of 'differentiation'. In more recent geological contexts (about anything that happened post-3.7 billion years ago), differentiation explains why igneous rocks (lavas and magmas) attain their various chemical compositions: certain elements are more thermodynamically stable than others in a solid phase, while others prefer to hang out in the liquid (magma) for a lot longer. This means that certain elements are concentrated in early solids (rocks) while others are more abundant in the later formed rocks. It so happens that those in the latter group - sodium, potassium, aluminum, and (because of its abundance) silicon - tend to form much lower density minerals, and those minerals together make for lower density rocks.

If you apply the concept of differentiation to a planet-spanning magma ocean - like the one on Earth in the Hadean - you concentrate the heavy elements deep in the Earth, which is why the core and mantle are rich in elements like iron, nickel, and chromium. Over time, further differentiation operating on a much smaller scale will produce low-density rocks that are too light to be shoved back into the mantle, effectively permanently stranding them near the surface - and thus creating the beginnings of a continent. Repeating this process over billions of years worth of magmatism is what is generally accepted to have led to the gradual (and more or less permanent) growth of the continents.