Seafloor Spreading & Magnetic Stripes

Ridge Controls

Half-spreading rate 5.0 cm/yr

Avg. reversal interval 300 kyr

Time speed 0.30 Myr/s

Vertical exaggeration 4.0×

Live Readout

Elapsed time

0.00 Myr

Ridge separation

0 km

Current polarity (new crust)

Normal

How it works

Molten mantle rock rises at the mid-ocean ridge axis, cools, and welds onto the trailing edges of two plates moving apart — new crust is added right at the ridge and rides outward on a rigid conveyor at the plate's spreading rate:

x(t) = ±v_half · (t − t_form)
v_half = 10 · v[cm/yr]  km per Myr  (½-rate; full rate = 2·v_half)

As each strip of basalt cools through the Curie temperature (~580°C for magnetite), it locks in the direction of Earth's magnetic field at that instant — thermoremanent magnetization. Earth's field flips polarity at irregular intervals (the geomagnetic polarity timescale), modeled here as a Poisson process with a tunable mean interval, so each stripe records whichever polarity was active when it froze:

P(next reversal within Δt) = 1 − e^(−Δt / τ),  τ = mean interval

Because both plates share the same ridge and the same field history, the stripe pattern is symmetric about the axis — the evidence Vine, Matthews and Morley used in 1963 to prove seafloor spreading and confirm plate tectonics. A fast ridge (e.g. the East Pacific Rise, ~8–9 cm/yr half-rate) stretches each reversal interval into a wide stripe; a slow ridge (e.g. the Mid-Atlantic Ridge, ~1–2 cm/yr) compresses the same interval into a narrow, choppy one — try both rates and watch the stripe widths change for an unchanged reversal history.

The seafloor also deepens with age as the lithosphere cools and contracts — the classic square-root-of-age relation from half-space cooling models:

depth(age) ≈ depth_ridge + k·√age[Myr]

giving the ridge its characteristic cross-sectional profile: shallow, warm, and volcanically active at the axis, deepening smoothly away from it. The vertical relief here is exaggerated for visibility (real ridge flanks slope only a few degrees).

Normal polarity crust Reversed polarity crust Ridge axis (active spreading)

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