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.
