Earth's Oldest Chunk of Crust Found

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March 22, 2007 — Volcanic rocks that solidified 3.8 billion years ago show that some very modern processes were under way even on a very young Earth. The rocks from under Greenland's ice sheet are evidence that the recycling of Earth's crust by plate tectonics started early.

The discovery would seem to drive a stake in the heart of theories that consider plate tectonics a latter-day process that's only been happening for the last half of Earth's 4.5-billion-year existence.

The key to the ancient rocks are called sheeted dikes, vertical layers of once-molten rock that are squeezed between previous dikes like books on a shelf. These are sandwiched between deep-sea pillow lavas above and hard crystalline rocks below.

This sequence of rocks matches exactly what’s created at mid-ocean-type volcanic spreading centers — like the Mid-Atlantic Ridge. The technical term for the combination of rocks is an ophiolite.

"These sheeted dikes offer evidence for remnants of oceanic crust formed by sea-floor spreading of the earliest intact rocks on Earth," reported Harald Furnes of the University of Bergen in Norway. Furnes and his international team of coworkers published their discovery in the March 23 issue of Science.

The previous, earliest-known ophiolite was in northern China and found to be 2.5 billion years old.

The 3.8 billion-year-old rocks found in Greenland’s Isua Supracrustal Belt haven’t made it through the ages unscathed, of course. The oceanic crust they were a part of eventually moved to a subduction zone — where it was being shoved under another plate.

The Greenland rocks just happened to be scraped off during the collision of those long-gone plates, leaving a somewhat mangled remnant behind for the ages.

"We have only a very scanty sampling of this period of Earth's history, so it is extremely difficult to reconstruct the dynamics and modes of crustal processes 4 billion years ago," said Nicolas Dauphas, a geologist at the University of Chicago.

Today, plate tectonics controls the opening of oceans and creation of continental land masses, explained Dauphas. But the early Earth is supposed to have been a lot hotter inside than today. And since that heat is believed to drive the plate tectonics engine, it’s difficult to say exactly how plate tectonics worked in the past.

"In these conditions, scientists have no clear idea on how the terrestrial thermal engine would have operated," said Dauphas. "Furnes and co-workers give field and chemical evidence that it may not have been very different from today's world."