The Earth’s crust is unique among the known rocky worlds of the solar system because it is split into continental plates that shift and interact over eons. This feature is believed to be key to life, and there are many unknowns on how and when it began.
Scientists might have been getting a better idea of the answers to these questions. Research over the last several years has suggested an earlier onset for plate tectonics. New work seems to agree with that and has pushed the date even further back to 3.6 billion years ago.
Evidence supporting this scenario, published in Geochemical Perspectives Letters, comes from elements found in some of the oldest zircons in the world. The team collected 3,500 tiny zircons from the Jack Hills of Western Australia, which are aged between 4.3 billion years ago to about 3 billion years ago.
These tiny gems contain a little bit of uranium. This element is radioactive and slowly decays into other elements. By measuring how much there is, the team can work out how old these minerals are.
"We are reconstructing how the Earth changed from a molten ball of rock and metal to what we have today," study author Michael Ackerson said in a statement. "None of the other planets have continents or liquid oceans or life. In a way, we are trying to answer the question of why Earth is unique, and we can answer that to an extent with these zircons."
Having a database of minerals across such a long period of Earth’s history can allow researchers to probe changes to our planet. And the scientists in this group found something really interesting: Around 3.6 billion years ago, the amount of aluminum trapped in the zircons increases.
One way for this to happen is for these zircons to have formed deeper beneath the Earth’s surface. This could suggest that the Earth’s crust was thickening and that tectonic plates had kicked off.
"It's really hard to get aluminum into zircons because of their chemical bonds," Ackerson said. "You need to have pretty extreme geologic conditions."
"This compositional shift likely marks the onset of modern-style plate tectonics and potentially could signal the emergence of life on Earth," Ackerson said. "But we will need to do a lot more research to determine this geologic shift's connections to the origins of life."
The thickening of the Earth’s crust around the same time is also suggested from the Acasta Gneiss rocks in northern Canada, but more samples from different places across the world are necessary to understand the formative years of our planet and how that might have impacted the emergence of life.