Although geologists have long argued over when plate tectonics, which refers to the process of the Earth's outer later of tectonic plates grinding and moving underneath each other, first began, a team of scientists from the University of Maryland says that it began approximately 3 billion years ago.
"By linking crustal composition and plate tectonics, we have provided first-order geochemical evidence for the onset of plate tectonics, which is a fundamental Earth science question," Ming Tang, lead author of the study, said in a press release. "Because plate tectonics is necessary for the building of continents, this work also represents a further step in understanding when and how Earth's continents formed."
The team focused on the most unique aspect of the Earth's crust compared to other terrestrial planets such as Mars - its low concentration of magnesium. However, this low concentration only came to be in recent years; early on in the Earth's history it was more similar to the concentrations seen on other planets. The researchers determined that at some point in time, the Earth's crust began to possess more granite, likely due to the start of plate tectonics pushing water underneath its crust, which is one step in the process of granite production.
"You can't have continents without granite, and you can't have granite without taking water deep into the Earth," said Roberta Rudnick, senior author on the study. "So at some point plate tectonics began and started bringing lots of water down into the mantle. The big question is when did that happen?"
Due to the difficulty of measuring magnesium due to its tendency to wash into the ocean, the team looked instead at trace elements that are not water-soluble such as nickel, cobalt, chromium and zinc. They found that higher ratios of nickel to cobalt, as well as chromium to zinc, correlate to higher magnesium levels in the original rock.
"To our knowledge, we are the first to discover this correlation and use this approach," Tang said. "Because the ratios of these trace elements correlate to magnesium, they serve as a very reliable 'fingerprint' of past magnesium content."
The team then compiled data taken from ancient rocks stemming from a time period between four to 2.5 billion years ago and constructed a computer model of the Earth's early geochemical makeup. The results showed that around three billion years ago, the Earth's crust contained 11 percent magnesium oxide, a number that decreased to 4 percent over the course of a half billion years, suggesting that this is when plate tectonics began.
"It's really kind of a radical idea, to suggest that continental crust in Archean had that much magnesium," said Rudnick. "Ming's discovery is powerful because he found that trace insoluble elements correlate with a major element, allowing us to address a long-standing question in Earth history."
The findings were published in the Jan. 22 issue of Science.
