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Scientists discover the oldest known chunk of Earth's crust

These rocks in Greenland are part of the oldest known chunk of Earth's crust.

An international team of scientists has found a group of rocks in Greenland that formed as the sea floor split apart 3.8 billion years ago, which means that this is the oldest known chunk of the Earth's crust.

But, if it formed on the seafloor, what's it doing in Greenland?

The Earth's crust is divided into pieces, or "plates," which fit together like puzzle pieces. At some of the boundaries, the plates move apart from each other, and magma wells up from below to form new crust. These spreading centers occur along the sea floor.

These rocks in Greenland are part of the oldest known chunk of Earth's crust.

At other boundaries, one plate pushes underneath another and eventually dives deep into the Earth, where it melts into the magma. These boundaries are called "subduction zones."

This system, in which new crust is created at the spreading centers and recycled at the subduction zones, is called "plate tectonics."

Sometimes, when a seafloor plate is sinking beneath a continent, the subduction zone can get jammed up and pieces of the seafloor may be pushed onto the continent instead of being recycled below. This is what happened with the crust that is now in Greenland, which was identified by Harald Furnes of the University of Bergen in Bergen, Norway and his colleagues.

Scientists have debated whether plate tectonics occurred early in Earth's history or just in the second half of its 4.5 billion-year existence. The old age of the Greenland rocks suggests that the plate tectonics system arose relatively early.

The Greenland rock sequence is called an "ophiolite." Researchers can recognize the ophiolites because they contain a variety of signature volcanic rocks that are associated with the formation of new crust.

Dr. Furnes and colleagues describe their findings in the 23 March issue of the journal Science.