The geological forces that shape our planet, including earthquakes, mountain formation, and continental drift, might have started much earlier than previously thought. New research suggests that plate tectonics, the driving force behind these phenomena, began as early as 4 billion years ago, during Earth’s Hadean eon. This discovery pushes back the timeline for the initiation of plate tectonics significantly, as scientists previously believed it began later in Earth’s history.
The Hadean eon, a period lasting from Earth’s formation 4.5 billion years ago to 4 billion years ago, was a tumultuous time. Earth was still cooling down from its initial formation, with a scorching hot surface and an atmosphere composed of ammonia and methane. Yet, even in this harsh environment, Earth’s surface had cooled enough to form a solid outer crust. The familiar pattern of plate tectonics, where massive plates of Earth’s crust move and interact with each other, began to take shape during this period. However, the exact timing of this geological shift has been a topic of debate among scientists.
The challenge in pinpointing the onset of plate tectonics lies in the lack of surviving rocks from the Hadean eon. The oldest rocks on Earth date back to about 4 billion years ago, leaving scientists to rely on indirect clues to reconstruct Earth’s early history. One such clue comes from tiny, durable crystals known as zircons. These crystals, some of which date back to 4.4 billion years ago, offer a glimpse into the Hadean eon, providing insights into the conditions and processes that shaped the early Earth.
Among these ancient zircons, a particular type called S-type zircons holds the key to understanding the emergence of plate tectonics. These zircons form in sedimentary rocks on land, which are then subducted—pushed into the Earth’s mantle—by the movement of tectonic plates. Later, these zircons resurface within metamorphic granites. Identifying S-type zircons, however, isn’t a simple task, as they lack a single defining characteristic and require a comprehensive analysis of trace minerals.
In this new study, published in the journal Nature Geoscience, researchers employed a machine-learning model to simplify the identification of S-type zircons. They first trained the model using data from 300 zircons of known origin, enabling it to distinguish between S-type and other types of zircons. Next, they tested the model’s accuracy by analyzing 74 more zircons, ensuring its ability to correctly classify them.
With the trained model at their disposal, the scientists then applied it to 971 new zircons from the Jack Hills of Australia, a region renowned for harboring some of the oldest zircons on Earth. The analysis revealed that 35% of these zircons were S-type, some dating back to 4.2 billion years ago. This finding suggests that plate tectonics was active during the Hadean eon, cycling rocks between the crust and the mantle. This discovery adds further weight to the idea that plate tectonics may have begun significantly earlier than previously thought.
While the new research provides compelling evidence for ancient tectonic activity, it’s not the first to hint at its early presence. Previous studies, examining melted rocks at high temperatures, suggested that the oldest continental crust formed through subduction, a process where one tectonic plate slides beneath another. Some studies even propose that plate tectonics was fully operational during the Hadean eon. However, the new study, while strengthening the case for early plate tectonics, likely won’t resolve all the controversy surrounding its origins.
As with any groundbreaking scientific discovery, this new research opens up new avenues for exploration. Future studies will delve deeper into the complexities of Earth’s early geological history, seeking to unravel the mysteries of plate tectonics and its influence on the evolution of our planet. The insights gained from these investigations will continue to refine our understanding of the processes that shaped the Earth we know and love today.
