Tiny Crystals Point to Date Plate Tectonics Began
Dustin Trail, University of Rochester
Media use of these photos in relation to this study is permitted with attribution.
(Inside Science) -- Scientists examining billions-of-years-old crystals from Australia say they've uncovered new evidence that helps pin the start of plate tectonics at roughly 3.6 billion years ago.
Plate tectonics is the process responsible for continental drift, most earthquakes and volcanoes, and potentially even the long-term habitability of the Earth (thanks to the process's role in regulating our atmosphere). It's also, as far as we know, unique to the Earth -- no other planet is known to have plate tectonics. But exactly how, when and why the young Earth started exhibiting plate tectonics is still unknown and answering this question is of top priority to geologists.
One of the ways scientists peer into the distant geological past is through tiny, time capsulelike crystals called zircons. These zircons form in magma or metamorphic rock and then crystallize, essentially freezing in time and providing a chemical snapshot of whatever elements were around them at the time.
In this new work a team of scientists led by Michael Ackerson, a research geologist at the Smithsonian's National Museum of Natural History, obtained a sample of a couple hundred zircons from the Jack Hills of Australia, with ages that ranged from about 4.3 billion to 3 billion years old (the Earth as a whole is roughly 4.5 billion years old).
By zapping them with lasers, the researchers were able to release and measure the quantities of elements like uranium or lead trapped in the crystals. However, in zircons dated to about 3.6 billion years ago and younger, the team also started to find increased traces of aluminum.
"The interesting thing about aluminum is that it's very difficult for aluminum to be incorporated into the zircon's structure," said Ackerson. Getting the element into a zircon likely requires one of a handful of very specific circumstances to occur, all of which point to something akin to active plate tectonics.
This work, which was published today in the journal Geochemical Perspectives Letters, lines up with other pieces of evidence, such as work done in northern Canada, that suggest something important was happening on Earth about 3.6 billion years ago.
This result may also have significance for scientists looking to explain the evolution of early life, as plate tectonics may have helped shape conditions on the early Earth.
"There does seem to be a consistent story emerging that something was happening around 3.6 billion years ago. And most of the signals point to this being something akin to the onset of plate tectonics," said Ackerson.