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Recent studies of the motion of the Pacific and North American plates show that plate tectonic motion averaged over several years equals that averaged over several million years, suggesting that the motion of tectonic plates is steady. Until recently, the only method of measuring the speed of one plate relative to another was through analysis of the magnetic anomalies above mid-ocean ridges. As two plates move apart over millions of years, the direction of the geomagnetic field is frozen into the new crust formed by cooling magma that fills the gap opened between the diverging plates. The geomagnetic field occasionally reverses polarity, and these reversals are frozen into the crust, causing highs and lows in the magnetic field sensed above the young seafloor at mid-ocean ridges. Rates of motion are deduced from the spacing between dated magnetic highs and lows.