Map of the Earth showing tectonic plates.
Hasterok et al. 2022/Earth-Sci.Rev.
Early Earth likely had no plate tectonics, but a solid outer crust with no tectonic activity covered the entire planet. After being broken up by convection currents in Earth’s mantle, the fragments of this stagnant crust formed the first tectonic plates.
Geologists are still not sure when this happened. Today plate tectonics is constantly recycling Earth’s crust and destroying the rock record. Rock formations older than 3 billion years are extremely rare and known only from a few remote localities.
So far, the oldest rocks formed by plate tectonics date back 2.5 billion years. Reconstructing the plate’s motion is possible only for the last 800 million years.
Analyzing olivine grains found in the 3.27-billion-year-old rocks, a new study found evidence suggesting that plate tectonics was already ongoing on Earth at the time.
The Barberton Greenstone Belt in Southern Africa formed at different times over a critical 800-million-year span from 4.15 to 3.3 billion years ago. As the name suggest, the Barberton Belt is a succession of quartzite, former sedimentary rocks, and greenish basaltic rocks, believed to be the remains of an ancient oceanic crust.
Olivine is a common mineral in basaltic rocks, rocks formed directly by crystallization from melts. Lead author Dongjian Ouyang, working as a geochemist for the Chinese Academy of Sciences, and his research team analyzed the content of oxygen and iron in the olivine to reconstruct the crystal’s origin.
Based on the chemical make-up, the researchers found that the olivine formed in rocks experiencing pressures and melting similar to modern subduction zones, suggesting the crust may have started moving. The study is specifically referring to a modern arc setting, when one plate with oceanic crust goes under another and the melting plate feeds volcanism on the surface.
Plate tectonics has, so far, only been observed on Earth, and may be essential to making a planet livable. The new results, together with previous research, suggests that something happened around 3.8 to 3.6 billion years ago, with Earth switching from a stagnant crust towards plate tectonics.
The full research paper,”Light oxygen isotopic composition in deep mantle reveals oceanic crust subduction before 3.3 billion years ago” was published in the journal Communications Earth & Environment and can be found online here.
The tectonic map by Hasterok et al. 2022 is published in the journal Earth-Science Reviews.
The animation of the drifting continents is part of a study by Merdith et al. 2021 published in the journal Earth-Science Reviews.
