A Penn State study revealed that over billions of years, the Earth’s crust has continued a slow reforming process, in contrast to the rapid slowdown in its growth about 3 billion years ago. The researchers said the new discovery contradicts existing theories that point to the rapid formation of tectonic plates earlier in Earth’s history.
The research was recently published in the journal Geochemical Perspectives Letters.
This work may help answer a fundamental question about our planet and could hold clues about the formation of other planets, according to lead author Jesse Remink, assistant professor of Earth sciences.
“The prevailing theory points to an inflection point about 3 billion years ago, implying that we had a stagnant mantle planet with no tectonic activity before the sudden shift to plate tectonics,” Remink said. “We have shown that this is not the case.”
To map the composition of the Earth’s crust – or the growth curve of the Earth’s crust – researchers turned to more than 600,000 samples comprising a database of Earth’s rock records. Researchers around the world — including Penn State — have analyzed every rock sample in the record to determine geochemical contents and age. The researchers chose rock records over mineral samples, which informed the sudden formation theory, because they said the rock record is more sensitive and less susceptible to bias on those time scales.
Knowing that the reliability of the mineral record decreases over time, the researchers recreated the growth curve of the Earth’s crust using rock records. To do this, they developed a unique method to determine how igneous rocks dating back millions of years rework over time: they experimentally showed how the same rock can change in different ways over time. Rocks can be repaired in many ways, such as weathering in sediments or remelting in the mantle, so the researchers used this experimental data to inform new mathematical tools capable of analyzing rock records and figuring out differences in sample changes.
“We calculated the amount of reworking that occurred by looking at the composition of igneous rocks with a new method that detects the proportion of sediments,” Remink said.
They used these calculations to calibrate the reworking documented in rock records. Next, the researchers calculated the Earth’s crust growth curve using the new understanding of how rocks form. They compared the newly calculated curve to the growth rate derived from mineral records by other experts.
The work of Reimink and his team suggests that the Earth’s crust follows the path of the mantle – the layer on which the crust lies – suggesting a relationship between the two. Remink said this is not the first time that geologists have suggested gradual growth of the Earth’s crust; However, this is the first time a rock record has been used to back it up.
“Our crustal growth curve matches the crustal growth record, so it appears that these two signals overlap in a way that they did not overlap when the mineral record was used to create the crustal growth curve,” Remink said.
Remink cautioned that the research improves what researchers understand, but it is not the be-all and end-all solution for crustal growth research. There are simply too few data points that cannot speak to the vast time and space of the Earth’s crust. However, further analysis of existing data points could help inform investigations of other planets, Reimink said. VenusFor example, it has no tectonic plates and could be a modern example of an early Earth.
“When did Earth and Venus become different?” Remink asked. “And why did they become different? This rate of growth of the Earth’s crust plays a big role. It tells us how, what and why the planets evolved along different paths.”
Reference: “A comprehensive view of continental growth lithosphere” by J.R. Reimink, J.H.F.L. Davies and J.-F. Moen and DJ Pearson, August 3, 2023, Geochemical Perspective Letters.
Joshua Davies, from the University of Quebec in Montreal; Jean-François Moyne, University of Lyon, France; Dr. contributed to this research. Graham Pearson, from the University of Alberta, Canada.
This research was supported in part by the Natural Sciences and Engineering Research Council of Canada.
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