Geoscientists use zircon to trace the origin of Earth’s continents

Geoscientists have long known that parts of continents formed deep in the Earth’s past, but the speed with which the earth rose above global seas – and the exact shapes that land masses formed – have so far escaped the experts.

But now, analyzing roughly 600,000 mineral analyzes from a database of roughly 7,700 different rock samples, a team led by Jesse Reimink, assistant professor of geosciences at Penn State, thinks they are getting closer to the answers.

Researchers say that the land masses of the Earth began to slowly rise above sea level about 3 billion years ago. When their interpretation is combined with previous work, including the work of other Penn State researchers, it suggests that the continents took about 500 million years to reach their modern heights, according to results recently published in Science letters of the earth and planets.

To reach this conclusion, the scientists applied a unique statistical analysis to the ages of crystallization from the mineral zircon, which is reliably datable and often found in sedimentary rocks. Although these researchers did not date these samples, the samples were all dated using the uranium and lead decay system. This method measures the amount of lead in a sample and calculates the age of the crystal from the well-established rate of decay of uranium. When zirconium is formed, no lead is incorporated into its structure, so lead comes from the decomposition of uranium.

Minerals found in sedimentary rock samples originally formed in older magmas but, through erosion and transport, traveled into rivers and were eventually deposited in the ocean where they were transformed into sedimentary rock beneath the seafloor’s surface. The age of the zircons recovered from individual rock samples can be used to indicate the type of continent from which they were eroded.

The ages of the cubic zirconia rocks of eastern North America are, for example, different from those of land masses such as Japan, which was formed by much more recent volcanic activity.

“If you look at the Mississippi River, it’s eroding rocks and zircons from all over North America. It’s harvesting mineral grains that have a wide age range from a million years to a few billion years,” Reimink said. “Our analysis suggests that as sediments started forming on Earth, they formed from sedimentary basins with an equally wide age range.”

Sediments are formed from the atmospheric agents of older rocks and bear the signature of the continental mass of the past in time capsules such as zircons. The research does not reveal the overall size of primeval continents, but suggests that modern-scale river basins formed as early as 2.7 billion years ago.

“Our research matches well with the preserved rock record,” Reimink said.

This result is crucial for a few reasons. First, knowing when and how continents formed advances research on the carbon cycle in land, water and atmosphere. Second, it gives us clues to the earliest origins of the Earth. This may come in handy as we learn more about the life and formation of other planets. Earth is a life-sustaining planet, in part, due to how the continental crust affects our atmospheric and oceanic composition. Knowing how and when these processes occurred could hold clues to the creation of life.

“Whenever we are able to determine the processes that led to our existence, it refers to truly profound questions such as: Are we unique? Is the Earth unique in the universe? And there are other Earths out there,” said Reimink . “These findings help guide us on the path to the answers we need on Earth that allow us to compare our planet with others.”