According to a new study published today in AGU Advances, AGU’s magazine for high-impact, open-access research and opinion across the Earth and space sciences, a handful of ancient zircon crystals discovered in South Africa hold the oldest evidence of subduction, a critical feature of plate tectonics.
These unusual time capsules from Earth’s early days allude to a change approximately 3.8 billion years ago from a long-lived, stable rock surface to the active processes that create our planet today, adding to a long-standing controversy over when plate tectonics began.
The Earth’s crust is divided up into stiff plates that move slowly on top of viscous yet moveable lower layers of mantle rock. Volcanoes, earthquakes, and mountain range uplift are all caused by the slow but inexorable motion of heat from Earth’s core.
When this process accelerated and contemporary crust formed, estimates range from around 4 billion years ago to less than 800 million years ago. Because of the surface recycling action of plate tectonics, the geologic record from Earth’s youth is scant, and uncertainty develops. From the Hadean Eon, Earth’s first 500 million years, almost nothing remains.
“The Hadean Earth is this big mystery box,” said Nadja Drabon, a geologist at Harvard University and the lead author of the new study.
The record we have for the earliest Earth is really limited, but just seeing a similar transition in so many different places makes it really feasible that it might have been a global change in crustal processes. Some kind of kind of reorganization was happening on Earth.
Nadja Drabon
Tiny time capsules
Drabon and her colleagues discovered a chronological series of 33 microscopic zircon crystals from a rare, ancient block of crust in the Barberton Greenstone Belt in South Africa, that formed at different times over a critical 800-million-year span from 4.15 to 3.3 billion years ago, in an exciting step forward in solving this mystery.
Zircon is an ubiquitous accessory mineral in the Earth’s crust, but ancient examples from the Hadean Eon, 4 to 4.56 billion years ago, are extremely rare, occurring in only 12 spots on the planet and usually in small groups of three.
The Greenstone Belt zircons contained hafnium isotopes and trace elements, which revealed a story about the environment on Earth at the time they crystallized. Zircons aged 3.8 billion years and younger appeared to have developed in rock subjected to pressures and melting similar to present subduction zones, implying that the crust had begun to move.
“When I say plate tectonics, I’m specifically referring to an arc setting, when one plate goes under another and you have all that volcanism think of the Andes, for example, and the Ring of Fire,” Drabon said, describing a classic example of subduction.
“At 3.8 billion years there is a dramatic shift where the crust is destabilized, we have new rocks forming and we see geochemical signatures becoming more and more similar to what we see in modern plate tectonics,” Drabon said.
The older zircons, on the other hand, showed evidence of a worldwide cap of “protocrust” formed by remelting mantle rock that had remained stable for 600 million years, according to the study.
Signs of global change
A similar transition to conditions matching present subduction was discovered in zircons from other parts of the planet, dating to within 200 million years of the South African zircons.
“We see evidence for a significant change on the Earth around 3.8 to 3.6 billion years ago and evolution toward plate tectonics is one clear possibility.” Drabon said.
While the findings aren’t clear, they do show that a global transformation has begun, maybe starting and halting in several spots before settling into the efficient global engine of constantly moving plates that we see today, according to Drabon.
Plate tectonics influences both the atmosphere and the surface of the Earth. The release of volcanic gasses and the formation of new silicate rock, which consumes large amounts of carbon dioxide from the atmosphere tempered significant temperature swings caused by too much or too little greenhouse gas.
“Without all of the recycling and new crust forming, we might be going back and forth between boiling hot and freezing cold,” Drabon said. “It’s kind of like a thermostat for the climate.”
Plate tectonics has only been detected on Earth so far, but it may be necessary for making a planet livable, according to Drabon, therefore the beginnings of plate motions are of interest in research into the early evolution of life.
“The record we have for the earliest Earth is really limited, but just seeing a similar transition in so many different places makes it really feasible that it might have been a global change in crustal processes,” Drabon said. “Some kind of kind of reorganization was happening on Earth.”
