London, June 28 (IANS): Continents may have first risen above the oceans about three billion years ago -- about a billion years earlier than geoscientists thought -- reveals a study.

"Earth's surface is continually being reworked by tectonics and agents of erosion, so what may have formed long ago may no longer be present," said geologist Cin-Ty Lee at Rice University in Houston, US, Live Science reported.

Since the continents are very thick and buoyant, they are less likely to get dragged downward.

Bruno Dhuime, an isotope geochemist at the University of Bristol and his colleagues analysed over 13,000 samples of rocks from the continental crust.

Some of these samples were more than four billion years old.

The team found that modern, silica-rich continental crust first appeared about three billion years ago.

The thick, buoyant nature of these chunks of crust would have made them rise high above what became the seafloor.

"They are showing when continents actually emerged from the oceans," Lee said.

"Continents certainly existed early in Earth's history, but perhaps many were submerged," Lee added.

It remains uncertain why continental crust appeared first about three billion years ago.

One possibility may be the onset of plate tectonics when the plates of rock making up the planet's exterior began moving slowly over the Earth's mantle layer.

The study was outlined in the journal Nature Geoscience.