A newly discovered 4-billion-year-old rock offers clues to Earth's earliest crust and how the planet's first continents formed.

 The rock was discovered by Jesse Reimink, a University of Alberta geochemistry student who has been collecting and studying ancient rock samples from the Acasta Gneiss Complex in the Northwest Territories for the last three years.

Acasta Gneiss Complex was discovered in the 1980s and found to contain some of Earth's oldest rocks, between 3.6 and four billion years old. Owing to the extreme age, it is very difficult to study the geochemistry of most rocks found in this region. Fortunately, Reimink was able to collect some "well persevered" rocks, among which was the four-billion-year-old chunk of an ancient proto-continent.

According to Reimink, the newly discovered rock provides useful insight into how Earth's first continents formed.  Popular theories state that continents form when one tectonic plate shifts beneath another into Earth's mantle and causes magma to rise to the surface. This process is known as subduction. However, until now, there has been a lack of evidence suggesting plate tectonics existed 2.5 billion to four billion years ago.

"The timing and mode of continental crust formation throughout Earth's history is a controversial topic in early Earth sciences," said Reimink, lead author of the new study in a press statement.

Previous studies speculate that the first continent formed in the ocean when liquid magma rose from Earth's mantle before cooling and solidifying into a crust. Iceland formed in a similar manner when magma from the mantle rose to shallow levels, incorporating previously formed volcanic rocks. For this reasons, Iceland is considered to be a theoretical analogue on early Earth continental crust formation. The discovery of the rock confirmed that a process similar to subduction was responsible for the formation of Earth's first continents.

"This provides the first physical evidence that a setting similar to modern Iceland was present on the early Earth," he said.

The samples of rocks discovered by Reimink from Acasta Gneiss Complex are said to be among the oldest samples of protocontinental crust ever found. They may have helped to initiate the formation of many continental crusts.

Findings of the study were published online in Nature Geoscience.