Oldest Known Ocean Crust Found on Greenland
National Geographic News
|March 22, 2007|
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.
In other locations the movement sends continents careening into each other, creating massive mountain ranges. (Related: "Deadly Java Quake Highlights 'Ring of Fire' Dangers" [June 30, 2006].)
The phenomenon, known only to occur on Earth, is the reason the planet has continents and land instead of being one giant ocean.
But scientists have long debated when the process actually began, since it requires Earth to be cool enough for its surface to become solid and rigid, not liquidlike.
Many experts have argued for a late start, because until now the oldest known ophiolite was a 2.5-billion-year-old formation spotted in north China.
"There is considerable controversy about when plate tectonics started," said Jeffrey Karson, a professor of structural geology and tectonics at Syracuse University in New York state.
"The significance of this discovery is that it pushes the earliest evidence of seafloor spreading back very significantly in time."
But, he pointed out, this find shows only that seafloor spreading was occurring back then—not other features of plate tectonics.
"That is a very important clue as to the nature of the early Earth—but not definitive proof of plate tectonics," he said.
Study leader Furnes, however, suspects—but can't yet prove—that subduction was also going on 3.8 billion years ago.
Origin of Life?
The possibility that early plate tectonics paved the way for life to bloom on Earth brings study leader Furnes and his team full circle.
"The main reason for choosing these particular rocks in Greenland was to sample these oldest known pillow lavas [pillow-shaped formations created during underwater eruptions] in order to look for chemical traces of life," he said.
"At least one group of geoscientists suggests that life may have evolved in close association with oceanic spreading centers," he pointed out.
According to Karson, of Syracuse University, "hydrothermal systems that are focused at mid-ocean ridge spreading centers are places where water-rock chemical reactions and vigorous circulation systems concentrate potential nutrients, such as sulfur and volcanic gases.
"The fractures in oceanic crust," he added, "would be good places for early Earth organisms to survive, despite meteor impacts and other extreme environmental changes affecting the surface of the early Earth."
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