A team of researchers has overturned common theories about the formation of Earth's crust, proving that it continues to slowly re-evolve, rather than slowing down rapidly 3 billion years ago. By analyzing more than 600,000 rock samples from around the world, they proposed a new way to map the growth of the Earth's crust, suggesting that it was more gradual and shedding light on the formation and divergence of planets, particularly Venus.
A Penn State-led study reveals that Earth's crust has continued a slow process of re-evolution for billions of years, rather than rapidly slowing its growth about 3 billion years ago. Researchers say the new discovery contradicts existing theories that crustal plates formed rapidly early in Earth's history.
The research was recently published in the journal Geochemical Perspectives Letters. Lead author Jesse Reimink, assistant professor of earth sciences, said the work may help answer a fundamental question about our planet and may provide clues to the formation of other planets.
"The leading theory points to an inflection point about 3 billion years ago, meaning our Earth was a stagnant cap planet with no tectonic activity before the plates suddenly turned," Reimink said. "We have shown that this is not the case."
To map the formation process of the Earth's crust, or crustal growth curve, the researchers used more than 600,000 samples from a database of Earth's rock record. Researchers around the world, including Penn State University, analyzed every rock sample in the record to determine geochemical content and age. The researchers chose rock records rather than mineral samples because rock records are more sensitive on these time scales and less prone to bias.
Knowing that the reliability of the mineral record decreases over time, the researchers used the rock record to redraw the crustal growth curve. To do this, they developed a unique method to determine how igneous rocks dating back millions of years have been reworked and transformed over time: experimentally demonstrating how the same rock changes differently over time. Rocks can be modified in many ways, such as weathering into sediments or being remelted in the Earth's mantle, so the researchers used these experimental data to inform new mathematical tools that can analyze the rock record and calculate differences in changes in the samples.
"We calculated in a new way how much reprocessing occurred by looking at the composition of the igneous rocks and figuring out the proportions of sediments," Reimink said.
They used these calculations to calibrate reprocessing in the rock record. The researchers then used their new understanding of how rocks are reprocessed to calculate crustal growth curves. They compared the newly calculated curves with growth rates other experts had gleaned from mineral records.
Reimink and his team's work shows that the Earth's crust follows the path of the Earth's mantle, the layer in which the Earth's crust rests, suggesting a connection. This isn't the first time geoscientists have proposed the idea that Earth's crust grew more gradually; however, it is the first time the rock record has been used to support this idea. "Our crustal growth curve matches the mantle's growth record, so the two signals appear to overlap to some extent, whereas the two signals do not overlap when using mineral records to create the crustal growth curve," he said.
The study raises researchers' awareness, but it's not the be all and end all of crustal growth research. There are simply too few data points to account for the vastness of space and time in the Earth's crust. However, further analysis of existing data points may help inform studies of other planets. Venus, for example, has no tectonic plates and may be a modern example of early Earth.
"When did Earth and Venus become different?" Reimink asked. "Why did they become different? The growth rate of the Earth's crust has a big influence on this. It tells us how the planets evolved on different trajectories, what and why."