National Geographic News
Scientists have discovered a 3.8-billion-year-old rock formation in Greenland that they say is the earliest example of oceanic crust ever to be discovered.
The find suggests that some processes of plate tectonics—the slow but steady drifting and collision of giant pieces of Earth's outer shell—may have begun much earlier than previously suspected.
Hidden in plain sight among an oft-studied cluster of ancient rocks, the Greenland formation contains a telltale structure known as a sheeted dike complex (Greenland map).
Because such a complex can form only during continuous crustal spreading, it identifies the Greenland rocks as ophiolites, or pieces of ocean crust that were later stranded on land.
During a process called seafloor spreading, new crust is constantly created by molten rock rising to fill ever widening cracks in the ocean floor. Ophiolites can be formed when this oceanic crust is caught up in a collision of continents, forcing some of the material onto the surface.
"The smoking gun here is the sheeted dike complex and hence the most important component for recognizing these rocks as an ophiolite," said study leader Harald Furnes, a professor of earth science at the University of Bergen in Norway.
"The rock sequence we describe from southwest Greenland we recognized as representing the oldest ophiolite on Earth, and hence the oldest oceanic crust formed by seafloor spreading," he added.
The finding, which appears in tomorrow's issue of the journal Science, may have enormous consequences for understanding the early history of the planet—and its most primitive life.
Many experts suspect the first organisms originated at or near the hydrothermal vents responsible for seafloor spreading, because they may have provided the energy for chemical reactions and helped concentrate vital nutrients. (Related: "World's Oldest Rocks Suggest Early Earth Was Habitable" [November 28, 2005].)
According to the theory of plate tectonics, Earth's upper surface is a jigsaw puzzle of rigid plates that slowly drift over a layer of hot fluidlike rock.
At many places these plates collide violently, causing volcanic activity and triggering massive earthquakes as one plate is subducted, or pushed beneath the other.