Jesse Reimink and his colleagues found something extraordinary in these rocks from the Acasta Gneiss formation near Yellowknife, Canada, and published a paper on them in the Nature Geoscience. Even at 4.02 billion years, they’re not quite the oldest ones out there but the oldest known rocks that still hold the tiny crystals of zircon that they had inherited since their formation. Together, the rock composition and the zircon crystals have supposedly answered an old question: when did the continents form? Zircon retains its chemical signature and records age information that doesn’t get reset by later geological events.

Their paper published in the journal on Monday stated that the Acasta Gneiss was once part of a giant plume of molten rock that pushed its way around the young planet before coming to rest and cooling. As it cooled, it interacted with the even older rock around it—an interaction that left its mark inside those crystals of zircon and the rocks in which they are embedded. Those marks are clues about what was in those early formations, the very earliest crust of the Earth.

Scientists have previously concluded there are two types of that early crust. One is associated with land and called continental; the other with water and called oceanic. Oceanic crust is generally older than continental. Interestingly, the Acasta rocks look like they interacted with oceanic crust in the process of turning into continental crust.

This new study suggests that early Earth was largely covered with an oceanic crust-like surface. Only three locations worldwide exist with rocks or minerals older than four billion years: one from Northern Quebec, mineral grains from Western Australia and the rock formation from Canada’s Northwest Territories examined in this study.