The oldest minerals in the world have provided researchers with new evidence that plate tectonics, the large-scale movement of the Earth's crust that is responsible for many geological phenomena, began 3.6 billion years ago.
Existing estimates of the age of plate tectonics vary widely because there is little coherent geological evidence from that long ago. The findings, published Friday in Geochemical Perspectives Letters, use ancient stones to tackle this issue, adding another piece to the puzzle of how and when this critical transition occurred.
"Continents and plate tectonics are intimately related to the story of earth, water and the emergence of life," said first author Michael Ackerson, from the National Museum of Natural History at the Smithsonian Institution. "And I think that it's kind of a beautiful geological symbiosis that's happening that enabled the Earth to form this habitable planet that we live on."
Plate tectonics helped life emerge on Earth by regulating the composition of the atmosphere and the oceans, and by forming continental nuclei, the beginnings of the continents we know today.
"Many people think that these continental nuclei served as a kind of a petri dish for the origin of life," Ackerson said.
The start date of plate tectonics is somewhat of a white whale in geology. In 2020, "When, why and how did plate tectonics start?" was listed as the second-highest priority question in Earth sciences by the National Science Foundation, which uses these priority rankings to allocate funding.
The fact that plate tectonics only began to be discovered and accepted in the second half of the 1900s, which is relatively recent for a major scientific theory, only enhances the interest in refining its origin story.
"Plate tectonics itself was a paradigm-shifting discovery of the 20th century in pretty much every single aspect of the geosciences," Ackerson said. "I would liken plate tectonics to the discovery of the double-stranded structure in DNA in terms of its importance for a field. It's absolutely critical to how we interpret Earth systems."
Despite widespread interest in the question of when plate tectonics began, this has proven a difficult question to answer. This is due to, as Ackerson put it, a "scarcity of knowledge" about that period of time. Because plate tectonics involves constant melting and resolidifying of portions of the Earth's crust, it tends to destroy its own history.
Researchers estimating the age of plate tectonics must turn to clues that persist through the activity of tectonic plates. And while the results of these studies have varied widely over the years, modern estimates trend toward 3 billion to 4 billion years ago. One recent estimate of 3.2 billion years was done by using the magnetic properties of rocks, which contain a stable record of the Earth's magnetic activity.
But for the new study, the researchers turned to zircons, some of the oldest rocks on the planet, which have applications in both jewelry and industry. For geologists, zircons are also a treasure trove of information about the deepest past.
The researchers gathered 15 rocks mined from the Jack Hills in Western Australia, each rock just the size of a grapefruit. By grinding them into a fine sand and separating the resulting powder by density, the researchers were able to isolate more than 3,500 tiny individual zircons.
Then came the real challenge.
"Unlocking the secrets held within these minerals is no easy task," Ackerson said. "We analyzed thousands of these crystals to come up with a handful of useful data points, but each sample has the potential to tell us something completely new and reshape how we understand the origins of our planet."
The key to the analysis was using some of the extra elements embedded in the minerals. First the team used uranium to figure out the age of the samples, a well-known method for dating rocks that relies on the known rate of radioactive decay for uranium.
Then, to confirm that their zircons were present during the transition to plate tectonics, the researchers looked for aluminum, which Ackerson explained is a tell-tale sign of plate tectonics because aluminum can only occur within zircons in a limited number of ways.
"It's really hard to get aluminum into zircons because of their chemical bonds," Ackerson said. "You need to have pretty extreme geologic conditions."
These conditions include when sedimentary rock containing aluminum melts and recrystallizes, or when thick continents melt from below. These events only occur through the activity of plate tectonics. This allowed the researchers to use the relative amounts of aluminum in zircons of different years to pinpoint the age of plate tectonics to 3.6 billion years.
The researchers are next planning to look for signs of life in the Jack Hills zircons, because while the findings are expected to have implications for the start date of life, Ackerson warns that it will take further research to establish this connection.
The team is also interested in looking at other ancient rocks to see if its estimate can be reproduced elsewhere, because one group of rocks in one area is unlikely to have all the answers when studying something as complex and dynamic as plate tectonics.
"We are reconstructing how the Earth changed from a molten ball of rock and metal to what we have today," Ackerson said. "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."
The study, "Emergence of peraluminous crustal magmas and implications for the early Earth," published May 14 in Geochemical Perspectives Letters, was authored by Michael Ackerson, National Museum of Natural History; Dustin Trail, University of Rochester; and Jacob Buettner, Los Alamos National Laboratory.