New research indicates that plate tectonics may have been well underway on Earth more than 3.2 billion years ago, adding a new dimension to an ongoing debate about exactly when plate tectonics began influencing the early evolution of the planet. An analysis of lingering magnetism in rocks from the nearly 3.2 billion year-old Honeyeater Basalt of the East Pilbara Craton, a stable block of crust in Western Australia, provides strong evidence for a large change in the latitude of the block relative to the Earth's magnetic poles between 3.35 and 3.18 billion years ago. This evidence pushes back the date for the onset of modern plate tectonics to the late Paleoarchean time period, supporting the theory that changes in the crust have resulted from continuous, uniform processes similar to those we observe today - or at least that the crust has experienced intermittent switching between episodes of movement and immobility. Alec Brenner and colleagues note that these findings will contribute to scientists' understanding of how Earth's crust formed and how plate tectonics may have evolved on other planets. A defining feature of modern plate tectonics is the plates' gradual but steady horizontal motion. Beneath our feet, these titanic slabs of rock collide, pull apart and slide past each other, molding mountains and unleashing earthquakes that rattle the world above. But while plate tectonics is central to the evolution of Earth as we know it, scientists have been uncertain about when, exactly, this geologic process began. To determine whether the lithospheric plates experienced significant motion before the early Neoarchean period some 2.8 billion years ago, Brenner et al. extracted samples from a total of 235 magnetically oriented Honeyeater Basalt cores - igneous rocks that retain a record of Earth's magnetic field at the time of their crystallization. Since the researchers knew the ages of rocks that crystallized at different times within a single block of the crust, they were able to deduce changes in the block's latitude over millions of years. They found that this section of crust drifted at an average rate of at least 2.5 centimeters per year - a velocity comparable to plate motion rates observed today.
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