Business · Ars Technica
The missing 500 million: Cosmic bombardment melted Earth's first crust
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Earth is the only planet they know of with buoyant, silica-rich continents.
Key facts
- The models suggest it was less than 5 kilometers thick, with widespread partial melting starting 2 to 3 kilometers below the surface
- 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
- Between 3.9 and 3.5 billion years ago, it had dropped enough that internal heat sources took over as the dominant influence on the crust
- The oldest known continental-type rocks crystallized around 4.03 billion years ago, right at the end of the Hadean eon (the earliest era in Earth’s history, spanning the first 500 million years
Summary
Johnson and his colleagues are now arguing 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. The problem with studying the formation of continents is that the geological evidence of this process is almost gone. 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. One dominant idea holds that plate tectonics, much like today’s, was already running in the Hadean, with continental crust forming above subduction zones—areas where tectonic plates collide. The issue with both these ideas, though, was that Earth, based on most models, appeared too cold for all this to happen.