Early Earth was covered with oceanic crust-like surface

Scientists have precisely dated the world's oldest rock at 4.02 billion years old, which suggests that early Earth was largely covered with an oceanic crust-like surface.
"It gives us important information about how the early continents formed," said lead author Jesse Reimink, a post-doctoral fellow at the Carnegie Institute for Science in the US.
"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," said Reimink.
Only three locations worldwide exist with rocks or minerals older than 4 billion years old: one from Northern Quebec, mineral grains from Western Australia, and the rock formation from Canada's Northwest Territories examined in this study.

While it is well known that the oldest rocks formed prior to 4 billion years ago, the unique twist on Reimink's rock is the presence of well-preserved grains of the mineral zircon, leaving no doubt about the date it formed.
The sample in question was found during fieldwork by Reimink's PhD supervisor, Tom Chacko, in an area roughly 300 kilometres north of Yellowknife in Canada.

"Rocks and zircon together give us much more information than either on their own. Zircon retains its chemical signature and records age information that doesn't get reset by later geological events, while the rock itself records chemical information that the zircon grains don't," said Reimink, who completed his PhD at the University of Alberta in Canada.

He noted that the chemistry of the rock itself looks like rocks that are forming today in modern Iceland, which is transitional between oceanic and continental crust.
In fact, Iceland has been hypothesised as an analog for how continental crusts started to form.
"We examined the rock itself to analyse those chemical signatures to explore the way that the magma intrudes into the surrounding rock," Reimink said.
One signature in particular recorded the assimilation step of magma from Earth's crust, researchers said.
"While the magma cooled, it simultaneously heated up and melted the rock around it, and we have evidence for that," Reimink said.

The research appears in the journal Nature Geoscience.