Plate Tectonics May Have Started Over 4 Billion Years Ago, New Study Suggests

Recent research suggests that the plate tectonics responsible for earthquakes, mountain formation, and continental shifts may have begun during Earth’s infancy, much earlier than previously believed. The study proposes that plate tectonics started over 4 billion years ago, shortly after Earth formed 4.5 billion years ago. During the Hadean era, Earth had a hot, ammonia-and-methane-rich atmosphere and enough water to form a global ocean as it cooled, leading to a solid outer crust.

Today, tectonic plates shape this crust by moving atop the more fluid mantle beneath. However, the exact onset of plate tectonics remains uncertain. Some scientists believe it began in the Hadean as soon as the crust solidified, while others argue it started around 3.2 billion years ago when notable geochemical changes occurred, or even more recently within the last couple of billion years.

Determining the start of plate tectonics is challenging due to the lack of rocks older than 4 billion years, leaving only tiny, resilient crystals called zircons as direct evidence. The oldest zircons date back 4.4 billion years, with S-type zircons indicating plate tectonics. These zircons form in sedimentary rocks, are pushed into the mantle, and then re-emerge in metamorphic granites.

Identifying S-type zircons involves examining multiple trace minerals. In a study published on July 8 in the journal PNAS, researchers utilized a machine-learning model to simplify this process. The model, trained with data from 300 known zircons, accurately identified S-type zircons among 74 samples. Applying the model to 971 zircons from the Jack Hills in Australia, where the oldest zircons are found, revealed that 35% were S-type, some dating back 4.2 billion years. This finding implies that plate tectonics were active during the Hadean, moving rocks between the crust and mantle.