New crust forms at the ridge axis and moves away in both directions, recording Earth's reversing magnetic field as symmetric stripes. A survey ship reads the stripes and recovers the spreading rate.
Half-rate is how fast each plate moves away from the axis. Full spreading rate is twice that. 1 mm per year equals 1 km per million years.
Vertical scale: ocean depth is exaggerated 25 times; the lithosphere cross-section is true scale (1 unit = 100 km). The ship is much larger than real.
Blue: measured anomaly (smoothed, with noise). Colour bar: true polarity of the crust (orange reversed, dark normal).
The estimator sees only the noisy profile and the known reversal timescale, not the set rate. The typical literature coefficient is about 350 m per √Myr (check it against a source).
New oceanic crust forms at the ridge and moves away at half-rate u. A point at distance x has age t = |x| / u. The basalt records the polarity of Earth's field when it cooled, so reversals appear as mirror-image stripes (Vine and Matthews, 1963). Matching stripe positions to the known reversal timescale gives u = x / t.
Cooling lithosphere thickens as L = 2.32 √(κ t) with diffusivity κ = 10⁻⁶ m²/s (about 13 km at 1 Myr and 130 km at 100 Myr), and as it cools and contracts the seafloor deepens: d = 2500 + 350 √t metres for young floor, levelling to about 6400 m for old floor (typical empirical values).
Simplifications: a steady, symmetric ridge with no transform faults, no sediments or hotspots, constant rate. The reversal record here is either synthetic (random intervals) or an approximate list of real ages for the last 5 Myr; it is illustrative, not the official geomagnetic polarity timescale. Anomaly amplitude (300 nT) and the 8 km smoothing of the magnetised layer are illustrative. Narrow chrons blur on slow ridges, as in nature. The rate estimator is a correlation fit, not manual stripe picking.
Everything shown is educational; ocean depth is vertically exaggerated and the ship is not to scale.