Earth’s Continents Formed by Ancient Bombardment from Space
Earth is the only planet we know of with buoyant, silica-rich continents. But, despite decades of research, geologists still don’t agree on how they formed.
The continents started appearing around about four billion years ago—that’s the oldest continental rock we know about. The Earth is four and a half billion years old, so why they started appearing then is unknown, as is the mechanism to make that continental crust.
Geologist Tim Johnson and his colleagues argue that the formation of continents on Earth was caused largely by an intense, sustained barrage of asteroid impacts that kept the early crust hot and thin enough to make buoyant continents possible.
Challenges in Studying Continental Formation
The problem with studying the formation of continents is that the geological evidence of this process is almost gone. The oldest known continental-type rocks crystallized around 4.03 billion years ago, right at the end of the Hadean eon. Rare basaltic rocks date back about 4.2 billion years, and a handful of the oldest zircon crystals push the record back to 4.4 billion years. Beyond that, there’s hardly anything else.
Scientists looking into the origin of continents had to rely largely on educated guesses.
