The oldest substantial chunk of Earth’s crust has been found in Greenland, and dates back at least 3.8 billion years. The find is important because it is of a type known as an ophiolite – a signature of plate tectonics – and provides the best evidence yet that plates have been moving across the Earth’s surface for at least a billion years longer than thought.
Plate tectonics would have begun soon after the first solid crust grew dense enough to sink back into the then-molten mantle below it, thereby starting a cycle of crust generation and destruction.
Ophiolites form when a piece of ocean crust is forced up onto a piece of continental crust. Ocean crust is denser than continental crust, and so it usually is forced under, meaning ophiolites are quite rare. They are created when two tectonic plates are moving towards each other, with one being forced under the other. The fragments of the descending – or subducted – oceanic plate can be shaved off and left on top of the other plate, forming an ophiolite.
Until now, the oldest example of an 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 near zone where plates are converging. They form when water released from a subducted plate melts the rocks above and causes magma to punch through the crust above.
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 over 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.
The oldest rock is the 4.04 billion year old Acasta gneiss from Canada, discovered in 1999. Tiny grains of near-indestructible zircon have been dated at 4.4 billion years old, but they have been recycled into other younger rocks.
Journal reference: Science (vol 315, p 1704)