How did Earth's earliest continental crust form? The world's oldest rock offers new clues, but raises more questions than answers.

Scientists at the University of Alberta have precisely dated the world's oldest rock unit at 4.02 billion years old. The ancient rock's composition is different than scientists expected, suggesting the planet's earliest continental crust was oceanic in structure and composition.

"It gives us important information about how the early continents formed," researcher Jesse Reimink explained in a news release. "Because it's so far back in time, we have to grasp at every piece of evidence we can. We have very few data points with which to evaluate what was happening on earth at this time."

Scientists have now uncovered rocks older than 4 billion years at three different locations. The latest rock unit hails from Canada's Northwest Territories. Researchers have previously unearthed similarly ancient mineral grains in Western Australia, as well as a 4-billion-year-old rock in Northern Quebec.

Researchers say the newest ancient rock -- described in the journal Nature Geoscience -- is special because it contains grains of zircon.

"Zircons lock in not only the age but also other geochemical information that we've exploited in this paper," Reimink said. "Rocks and zircon together give us much more information than either on their own."

The chemical composition of the rock recalls that of rocks forming today in Iceland -- in an intermediary zone between oceanic and continental crust.

While some have posited Iceland as an ideal model for how magma was first incorporated into Earth's continental crust, scientists didn't expect the findings from the latest study. Though the world's oldest rock offers valuable insights into the nature of early crustal formation, it also raises more questions than answers.

The quest for understanding continues, as Reimink and his colleagues continue to look for clues among imperfect geological artifacts.

"Earth is constantly recycling itself, the crust is being deformed or melted, and pre-history is being erased," said Reimink.

If the latest rock is proof of anything, Reimink says, it's that Earth's geology is an ever-changing reflection of complex processing happening both above and below Earth's surface.

"There are constant feedback loops between chemistry and geology," he said. "Though there are still a lot of unknowns, this is just one example that everything on earth is intertwined."