— A sheeted dike complex with sharp, well-defined chilled margins. These are signature traits of an ophiolite – a piece of oceanic crust embedded within a continent. In a sheeted dike complex, younger magma intrusions, or “dikes” cut across older ones, forming a rock made up exclusively of dikes (Image: Science)
The rock complex was found in Greenland (Image: Science)
The oldest known chunk of Earths crust has been found in Greenland, and dates back at least 3.8 billion years. The substantial rock is an important find, because it is a type known as ophiolite a signature of tectonic plate movement and provides the best evidence yet that continental drift had been going on for at least a billion years longer than thought.
Plate tectonics probably began soon after the first solid crust grew heavy enough to sink into the then-molten mantle below it, researchers say.
Ophiolite is a distinctive sequence of rocks that forms when two continental plates crash into each other. The thinner of the two crustal plates gets forced below the other since it is usually the denser and is called the subducting plate. During the processes, bits of the subducting crust get shaved off and forced onto or into the other plate. Ophiolite is this deposited rock.
Until now, the oldest example of ophiolite was an example in China dated to 2.5 billion years ago, and its identification had been questioned. Now Harald Furnes at the University of Bergen in Norway and colleagues have analysed an ophiolite fragment embedded in the Isua supracrustal belt in south-west Greenland.
The fragment was part of a distinctive layered deposit called "sheeted dikes" that are a key component of ophiolites, measuring about 30 by 50 metres.
"The rock association, together with the chemistry of the volcanic rocks, indicates it was an island arc environment, like the Mariana region in the western Pacific," Furnes told New Scientist.
Island arcs consist of a row of volcanoes that form when heat and pressure causes the molten subducted crust to release spurts of magma through the continental crust. These island arcs are indicative of a subduction zone at a plate margin.
The Isuo rocks are only a small fragment of an ophiolite, but they are "very good evidence of plate tectonics, says Timothy Kusky at Saint Louis University in Missouri, US. He identified the 2.5-billion-year-old ophiolites in China, which extend 60 kilometres.
However, Kusky says "it's very rare to have the ophiolite complex preserved" for so long because plate tectonics fragments them by crashing island arcs into continents and crunching continents together.
In 2001, researchers analysed the most ancient examples of rock ever discovered . The tiny crystals of near-indestructible zircon were dated at 4.4 billion years old.
Journal reference: Science (vol 315, p 1704)