One thing we can say for certain is that Earth is an ocean world. But how did it end up with as much continental crust as it did? The most prevailing thought started to look really good once scientists decided that plate tectonics was a thing. (Beth wants me to say [checks notes], “Justice for Alfred Wegener!”) After all, plates dive down under one another in subduction zones, causing the underlying rock to melt and rise through the crust to form volcanoes that create more crust. And as those plates come together, others rift apart and create zones where molten rock rises and again, forms new crust. See Iceland.

Of course, the flipside to the subduction zones is that the rocks that submerge and get heated are also destroyed, so we’re not actually seeing a net gain in the continental crust there or from the rift zones. Additionally, plate tectonics hasn’t always worked this way, which really defies a basic precept of geology that’s what’s past is prologue – meaning we can trace back how things worked in the past by looking at how they work now. It’s just not fair when we ignore a basic principle, in my opinion, but that’s how this story goes.

So plate tectonics started gradually about three billion years ago, more than a quarter of the way through Earth’s long history. This means continents had to come from somewhere else; however, with all the oldest rocks being destroyed in the mantle through subduction, scientists had to turn to the tiniest of crystals to get a glimpse of the past. And when we say tiniest of crystals, we mean microscopic bits of zircon, which is nigh indestructible, leftover in larger more recently created crystals.

In particular, research published last year in Nature Communications provides an analysis of microscopic zircon grains found in the archived stream sediment samples taken in a region in the west of Greenland. Using lasers and isotope analysis, scientists learned that “the zircon crystals varied widely in age, from 1.8 billion to 3.9 billion years old—a much broader range than what’s typically observed in Earth’s ancient crust.”

This result proved to be intriguing and set the stage for further analysis of more samples from around the world to compare. And all the samples began to say the same thing: …these large data sets all showed evidence of repeated injection of mantle melts into much more ancient crust. Ancient crust seemed to be a prerequisite for growing new crust.

And these injections all occurred around the same time, about 3.2 billion and 3.0 billion years ago, which coincides with the period when Earth’s mantle temperatures are thought to have been at their peak. So just like hot flashes happen with people, they apparently happen with planets, and this particular hot flash provided such a huge amount of mantle melt to largely create the volume of continental crust we see today. Plate tectonics has destroyed and created lots of rocks, but it wasn’t enough to give us the current amount of rocks. And now we know.

More Information

Earth’s Continents Share an Ancient Crustal Ancestor (Eos)

“Widespread reworking of Hadean-to-Eoarchean continents during Earth’s thermal peak,” C. L. Kirkland et al., 2021 January 12, Nature Communications