The missing 500 million: Cosmic bombardment melted Earth's first crust
摘要
一项新研究指出，地球早期持续的小行星撞击导致地壳保持高温和薄层状态，从而促进了硅质大陆的形成。地球约45亿年前诞生，但最古老的大陆岩石约40亿年前才出现，其形成机制长期存疑。由于早期地质证据几乎消失，科学家主要依靠推测。科廷大学蒂姆·约翰逊团队认为，太空轰击是大陆形成的关键驱动力。
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,” said Tim Johnson, a geologist at Curtin University in Perth, Australia. “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."
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. In short, the lands we live on are here because of ancient bombardment from space.
Plates and plumes
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 (the earliest era in Earth’s history, spanning the first 500 million years of its existence). 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. So, scientists looking into the origin of continents had to rely largely on educated guesses. “There are huge debates about what was going on in the early Earth, because the data is so scarce,” Johnson said.
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