Previous evidence for early plate motion came from geochemistry — rock compositions consistent with subduction, recycled crustal material appearing in unexpected places. The inference was indirect: if these chemical signatures exist, the crust must have been moving. A 2026 paper in Science by Harvard geoscientists adds a different kind of evidence: paleomagnetic data from the Pilbara Craton in Western Australia showing that a segment of Earth's surface drifted from 53° to 77° latitude and rotated clockwise by more than 90° over a span of 30 million years, roughly 3.5 billion years ago. Tens of centimeters per year — comparable to modern plate velocities.
The Pilbara rocks preserve magnetic minerals aligned with the Earth's field at the time they cooled. By sampling a stratigraphic sequence spanning 30 million years just after 3.5 billion years ago, the team tracked how the magnetic direction changed through the section. The shift is too large and too systematic to be explained by magnetic field wandering alone. The rock moved.
This matters because it distinguishes between two models that geochemistry alone couldn't separate. The chemical evidence is consistent with either plate-like motion (separate rigid pieces sliding past each other) or a drip-and-delaminate model (where heavy crustal material sinks locally without lateral movement). Only the first model produces systematic latitude drift and rotation. The paleomagnetic data rules out the stagnant lid.
What it doesn't resolve is whether 3.5-billion-year-old plate motion looked like modern plate tectonics — subduction zones, spreading ridges, transform faults — or something mechanically different that still produced lateral motion. The plates might have been thicker, driven by different forces, or organized differently. Motion is established. The type of motion is not.
The broader point is methodological: geochemistry tells you what happened to the material. Paleomagnetism tells you where the material was. You need both to reconstruct a dynamic system, because chemical transformation and physical displacement are independent observations. The same rock can be chemically altered without moving, or moved without being altered. The Pilbara gave both — and only the combination pins down the kinematics.